Comparative Anatomy of Vertebrates PDF

Comparative Anatomy of Vertebrates Part I (Skeletal, Respiratory & Circulatory Systems) Dr./ Hayam Eldawoudy Ali Saleh  Comparative Anatomy of Vertebrates Phylum: Chordata Sub phylum: Verte...

Comparative Anatomy of Vertebrates Part I (Skeletal, Respiratory & Circulatory Systems) Dr./ Hayam Eldawoudy Ali Saleh  Comparative Anatomy of Vertebrates Phylum: Chordata Sub phylum: Vertebrata (Craniata) D. Agnathostomata D.Gnathostomata 1- Class: Cyclostomata Ex. Petromyzon (Lamprey) Pisces Tetrapoda 1. Class: chondrichthyes (Cartilaginous fishes) Ex. Scyliorhinus (Scyllium) (Dog fish) 2- Class: Osteichthyes Anamniota Amniota (Bony fishes) 1-Class: Amphibia 1-Class: Reptilia Ex. Tilapia (Bolti) Ex. Bufo (Toad) Ex. Chalcides (the Lizard) 2- Class: Aves (Birds) Ex. Columba (the Pigeon) 3- Class: Mammalia Ex. Oryctolag (the Rabbit) Dr.\ Hayam Saleh 1  Comparative Anatomy of Vertebrates Introduction to the Comparative Anatomy of Vertebrates The comparative anatomy is to perform a comparative study of the anatomy of an organ in various groups of the vertebrates, to derive the evolutionary importance from it, and to know as to why an organ evolved the way it is present today. The evolution of an organ depends on the genetic and environmental factors, on which the natural selection operates to determine its existence or elimination. The subject of the relation between the form (anatomy) and function has taken the attention of the comparative anatomists at the end of the eighteenth century. This subject includes the concepts of homology and analogy, which are comprehensible only in the terms of the principle of evolution. (a) Homology Homology is an intrinsic similarity indicative of the same evolutionary origin. Homologous structures may seem different superficially but can be proved to be equivalent by any or all of the subsequent criteria: similarity of the anatomical construction, identical topographical relations to the body of the animal, similar track of the embryonic development, and similarity of the certain physiological function or mechanism. A famous example of homology is the wing of the bird, flipper of the seal, and foreleg of the cat; examination of these structures shows that they have a similar arrangement of the bones and muscles, have the same positional relation to the body, develop in the same way from a similar primordium, and work physiologically by the same mechanism. The requirement of including a particular physiological function or mechanism as a criterion will become obvious when one looks at such structures as the endocrine glands. Although these have a similar anatomical (histological) structure in all vertebrates, they frequently vary in position and in details of their embryonic origin in different vertebrates; but their particular function, which is the definitive test of their homology, remains the same throughout. Dr.\ Hayam Saleh 2  Comparative Anatomy of Vertebrates (b) Analogy Analogy is a similarity of general function or of superficial appearance that not related to similarity of intrinsic anatomical construction or of embryonic origin and development. An example of analogy is the scales of the fish and snakes, where the body of both animals are covered with scales for protection (similar general function), but the examination of the two types of scales shows that they are histologically dissimilar and differ in their way of embryonic origin. Analogous structures also differ in an accurate functional mechanism; where an insect leg and a cat leg perform the identical broad general function, that of walking, but the mechanism of walking is totally different in the two cases. When the analogous structures show an amazing similarity of the appearance, this is named convergence or parallelism. Such correspondences are usually related to living in the same environment, i.e., they are "adaptations." Not only parts of the animals but whole animals may look like each other clearly through living in the same environment, such as porpoises and fish. On the opposite hand, the animals closely related by descent may vary greatly in general appearance after long living in different environments, like seals and cats. This phenomenon is named divergence. Dr.\ Hayam Saleh 3  Comparative Anatomy of Vertebrates Subphylum: Vertebrata or Craniata The vertebrates belong to the subphylum Vertebrata of the phylum Chordata. A vertebrate may be defined as a special kind of chordate animal that has a cartilaginous or bony endoskeleton consisting of a cranium, housing a brain and a vertebral column through which the nerve cord passes. Their names are derived from the presence of serially arranged 'Vertebrae', which comprise a major part of their axial endoskeleton, the vertebral column, or the backbone. Another feature, that all vertebrates share as a common diagnostic character, is the elaboration of the anterior skeletal elements into a 'cranium' or skull' which houses various sense organs and a complex brain. This gives another name, the 'Craniata*, which is sometimes used for the group. Additionally, there are a series of skeletal structures (cartilages or bones) referred to as visceral arches, which are formed between the gill clefts for their support. In some vertebrates, certain of those visceral arches are modified to make jaws. Such vertebrates are grouped together to create the Gnathostomata, to tell apart them from the Agnatha which doesn't possess jaws. Craniata is split into two divisions: Agnatha and Gnathostomata.  General Characters of Subphylum Vertebrata or Craniata 1-The lower vertebrates are aquatic, while the higher vertebrates are predominantly terrestrial. 2-The body is bilaterally symmetrical and metamerically segmented. 3-The body is typically made of a head, trunk and postanal tail. A neck may also be present, especially in the terrestrial forms. 4-The trunk bears typically two pairs of lateral appendages; may be reduced or absent in some. 5-The skin or integument is made of an outer epidermis of stratified epithelium and an inner dermis of connective tissue; with many mucous glands in the aquatic species. The skin is covered by a protective exoskeleton comprising scales, feathers, hairs, claws, nails, horns, etc. Dr.\ Hayam Saleh 4  Comparative Anatomy of Vertebrates 6-The coelomic cavity is large and differentiated into a pericardial cavity and a perivisceral cavity. In mammals, the perivisceral cavity is further differentiated into a pleural cavity enclosing the lungs, and an abdominal cavity containing the other organs 7-The notochord is present in some stages of their life. It remains in some primitive vertebrate animals, while it disappears in the late embryonic stages of higher vertebrates to be replaced by a vertebral column, which may consist of cartilage or bone, depending on the type of animal. 8-A living jointed endoskeleton is composed of bone or cartilage or both. It includes the skull, vertebral column, girdles, and limb bones. 9-The digestive canal is more or less convoluted. The liver is massive and not tubular. The pharyngeal gill slits are not more than 7 pairs except in some cyclostomes. 10-The respiration in lower aquatic forms takes place by paired gills, but in terrestrial forms by the lungs. 11-The blood vascular system is closed. The heart is ventral, muscular, and contractile and consists of 2, 3, or 4 chambers. The blood plasma contains both white and red blood corpuscles, the latter containing the respiratory pigment, hemoglobin. 12-The excretion takes place by paired kidneys which is mesonephric or metanephric. 13-The anterior end of the dorsal nerve cord enlarges into a complex brain protected by the skull. The remaining nerve cord is forming the spinal cord which is surrounded and protected by the vertebrae. 10 to 12 pairs of cranial nerves are in the head. The spinal cord gives rise to paired spinal nerves, each of which arises from two roots (dorsal & ventral roots). The dorsal and ventral nerve roots are separate in lower vertebrates, like cyclostomes. In higher vertebrates, the two nerve roots are united to form a common spinal nerve. The dorsal nerve bears a swelling, called a spinal ganglion. There is a sympathetic nervous system. 14- Special sensory organs, including a pair of eyes, olfactory organs, and auditory organs, are derived from the brain. 15- An endocrine system of ductless glands scattered through the body regulates the body processes such as growth and reproduction. Dr.\ Hayam Saleh 5  Comparative Anatomy of Vertebrates 16- In most vertebrates, the sexes are separate, and the gonads are paired organs (mostly, there is one pair of the gonads, testes or ovaries). The gonads are compact bodies and their ducts are closely associated with those of the kidneys (particularly in males in which the male urinary ducts are also used to discharge the sperms), for this reason, the two systems, excretory and genital, together are called the urinogenital system. Also, the protochordate animals (e.g. Amphioxus) will be studied as an introduction to the study of vertebrates because they give the simple structural basis on which the most advanced vertebrate animals are built. Dr.\ Hayam Saleh 6  Comparative Anatomy of Vertebrates I- The Skeletal System (Endoskeleton) In vertebrates, the skeleton forms a framework of the body to which muscles are found attached. The skeleton is formed of cartilage or bone or of both cartilage and bone. The presence of an endoskeleton could be a characteristic of vertebrates, though many vertebrates have a dermal exoskeleton too. The skeleton is split into two parts: 1-The Exoskeleton (Dermal or integumentary bony skeleton) The dermal exoskeleton is composed of the bony plates that are embedded within the skin or are developed beneath it. The dermal bones of fishes comprise the scales and fin rays. It also forms the bony armor (plates and scutes) of turtles, crocodilians, and a few mammals (armadillos). Thus, it's somewhat artificial to create a distinction between the dermal exoskeleton and endoskeleton because the dermal bones share in the formation of some parts of the endoskeleton. The dermal elements of the vertebrates are remained in the skull and also the pectoral girdle. 2- The Endoskeleton It lies deep within the body and originates from mesenchyme. In the embryonic stage, it is composed of cartilage but in adults the cartilage is replaced by bones. If the bones are developed in situ of existing cartilages, so they are called cartilage or replacing bones, while if the bones are developed within the dermis, they are called dermal or membrane bones. Both the above formed bones are similar in the structure. The notochord forms the endoskeleton in Amphioxus, but in an exceedingly majority of living vertebrates, the notochord is present only in the embryonic stage and it is replaced largely by a vertebral column in the adult.  Functions of the Endoskeleton 1- The endoskeleton forms a framework which supports the body. 2- It maintains a specific shape of an animal. Dr.\ Hayam Saleh 7  Comparative Anatomy of Vertebrates 3- It provides protection for the delicate vital organs of the body. 4- It provides adequate surface for attachment of the muscles by means of tendons. 5-In bone marrow, the blood corpuscles are manufactured. 6-The ear ossicles help in hearing, 7- The tracheal rings and the ribs aid in breathing.  Subdivisions of the endoskeleton The endoskeleton may be split into the following parts: A- An axial skeleton that includes the notochord or vertebral column, skull, ribs, and sternum. B- An appendicular skeleton that includes the pectoral and pelvic girdles and the skeleton of paired fins and limbs and also, the skeleton of unpaired (median) fins of fishes. These two types are included in somatic skeleton. The visceral skeleton forms a support of jaws and gills in fishes, but in higher forms, it contributes to the skull and hyoid and is included in the axial skeleton. The bones or other rigid structures are connected together by ligaments and connected to the muscles by means of tendons. Also, there are joints within the endoskeleton so that movement of endoskeletal parts is possible. In the embryonic stage of all vertebrates, the endoskeleton is initially, cartilaginous, which is replaced by bone in most adult vertebrates. In cyclostomes and elasmobranchs, the skeleton does not cross beyond the cartilage stage, but in higher vertebrates, the endoskeleton is largely bony. Dr.\ Hayam Saleh 8  Comparative Anatomy of Vertebrates A-The Axial Skeleton 1-Notochord or Vertebral Column  The notochord The notochord is the primitive axial skeleton present in all chordate embryos. It is a stiff rod with tapering ends extending from the tip of the snout to the tail end. It lies mid- dorsally above the gut and beneath the nerve cord. In the early stages of development, the notochord is composed of large alternate fibrous and gelatinous vacuolated cells filled with fluid. The notochord is surrounded by a fibrous sheath of connective tissue. The fibers of the sheaths are fibrous and elastic in nature and surround the large vacuolated cells of the notochord. The notochord cells create pressure against the sheaths, due to presence of vacuoles filled with fluid.  Protochordates and cyclostomes In protochordates (Amphioxus) and cyclostomes (lamprey), the axial skeleton is primarily the notochord, which continues throughout lifetime of the animal. In lampreys, there are two pairs of small cartilaginous elements, termed arcualia, on either side of the spinal cord in each body segment. They do not meet above the spinal cord to form a neural spine. Also, the centrum is lacking. Dr.\ Hayam Saleh 9  Comparative Anatomy of Vertebrates Dr.\ Hayam Saleh 11  Comparative Anatomy of Vertebrates  The vertebrae In fishes and higher animals, the notochord is later enveloped by cartilaginous or bony rings called vertebrae. It is virtually abolished in tetrapods, and it is replaced by a vertebral column. The vertebral column is formed of a metameric series of vertebrae extending longitudinally from the posterior end of the skull to the end of the tail. The vertebrae are diverse in various animals, and also, there are differences among the vertebrae even in the same vertebral column, but all vertebrae conform to a basic plan. Generally, fish vertebrae are divided only into two types, the trunk or preanals and the caudal or postanals. In most tetrapods, the vertebral column has five regions: cervical, thoracic, lumbar, sacral and caudal, and there are several vertebrae in each region. In Amphibians, there are only a single cervical (atlas) and one sacral (9th) vertebra. Basic structure of a vertebra: A typical vertebra is made of a cylindrical body or centrum which encloses or replaces the embryonic notochord. Above the centrum, there are the neural arches surrounding a neural canal through which the spinal cord passes. The neural arches extend dorsally and unite to form a neural spine. In the caudal region of fishes, below the centrum there are the haemal arches surrounding a haemal canal through which the caudal artery and vein pass. Also, the haemal arches extend below and unite to form a haemal spine.  Types of the processes: Various kinds of projections or apophyses originate from a vertebra. They include: (i) Zygapophyses, existing over the anterior and posterior faces of the neural arches, are articulations between the successive vertebrae. Prezygapophyses are anterior projections facing upwards, while postzygapophyses are posterior projections facing downwards. (ii) Transverse processes: the lateral transverse process originates from the centrum or from the base of the neural arch. (iii) Diapophyses arise from the extremity of the transverse process, for attachment of the upper head (tuberculum) of a bicipital (two-headed) rib. Dr.\ Hayam Saleh 11  Comparative Anatomy of Vertebrates (iv) Parapophyses originate laterally from the centrum, for attachment of the ventral head (capitulum) of a bicipital rib. (v) Basapophyses are ventrolateral processes of the centrum, and they are remnants of the haemal arch or articulate with the haemal arch when present. (vi) Hypapophysis is a mid-ventral projection from the centrum.  Types of centra and vertebrae The centra of the vertebrae are placed end to end in a row, and the shape of the ends of the centra is of importance for articulation. There are five principal shapes of the centra. 1- Amphicoelous: The centrum of each vertebra is concave at both ends. This is the most primitive type. It is found in nearly all fishes and some urodeles (a few salamandere & caecilians), and the primitive reptiles. 2- Procoelous: The centrum of each vertebra is concave at the anterior end and convex at the posterior end. It is found in anurans (frogs) and most living reptiles. 3- Opisthocoelous: The centrum of each vertebra is convex at the anterior end, and concave posteriorly. Most salamanders are of the opisthocoelous type. 4- Heterocoelous: The vertebra has a saddle-shaped centrum at both ends. The anterior end is convex dorsoventrally and concave sideways, while the posterior end is opposite of the anterior end. This is the most specialized vertebra. It is found in birds, (especially in the neck region), to provide high flexibility to the neck movement. 5- Acoelous: The centrum of each vertebra is flat at both ends. This type is found in mammals. Dr.\ Hayam Saleh 12  Comparative Anatomy of Vertebrates Dr.\ Hayam Saleh 13  Comparative Anatomy of Vertebrates  The vertebral column of fishes The vertebral column of fishes contains only two types of vertebrae, trunk vertebrae and caudal vertebrae. The vertebrae are of the amphicoelous type. The cartilaginous fishes have totally cartilaginous vertebrae, while in teleosts, the vertebrae become fully ossified. In fishes, the caudal vertebrae are characterized by the presence of haemal arches and haemal spines that are developed from the basiventrals. The trunk vertebrae do not have any haemal arches and their places are the basapophyses, which are articulated with the ribs. Generally, the vertebrae of fishes are connected to each other by contact of their vertebral bodies, since the zygapophyses are absent. The successive vertebrae are interconnected together by collagenous and elastic ligaments which facilitate the lateral undulation of the body. Fish can't move their heads independent of their bodies, because the first vertebra and the skull are united together by cartilage or non-elastic connective tissue. Dr.\ Hayam Saleh 14  Comparative Anatomy of Vertebrates Dr.\ Hayam Saleh 15  Comparative Anatomy of Vertebrates  The vertebral column in Tetrapoda With the advent of life on land, the tetrapoda underwent many changes. One of these changes involved a regional specialization in the vertebral column. For a movement of the head, the anterior trunk vertebrae, called cervical vertebrae, had miniature ribs. With the formation of the limbs for use on land, the last few trunk vertebrae, called sacrals, were modulated for the articulation with the pelvic girdle, where, these vertebrae bear short strong ribs for bracing the pelvic girdle against the vertebral column. In modern tetrapods, the number of caudal vertebrae is much reduced and highly variable. Toward the end of the tail in all tetrapods, the vertebrae become progressively smaller until finally the vertebrae are reduced to small cylindrical centra only without any arches or processes. Generally, the vertebral column of tetrapods is divided into cervical, trunk (dorsal), sacral and caudal vertebrae. In many reptiles and all birds and mammals, the trunk vertebrae are further differentiated into thoracic vertebrae having long ribs and lumbar vertebrae with reduced or no ribs. (i) The vertebral column of Amphibia Apodans (limbless amphibians) have as many as 250 or more vertebrae and some urodeles have 100. The anurans (Frogs) have the shortest vertebral column among vertebrates. The vertebral column is composed of nine vertebrae and a terminal rod-like structure called the urostyle. Vertebrae are procoelous and caudal region is absent. They have a short neck with a single cervical vertebra (atlas). The atlas lacks the transverse processes and prezygapophyses, and its cranial (anterior) end bears two concave facets that articulate with the two occipital condyles of the skull and allows little head movement. Also, its centrum is reduced. The cervical vertebra is followed by seven trunk vertebrae (vertebrae from 2-8) which possess all typical processes. There is a single sacral vertebra (9th vertebra) which connected to the pelvic girdle by its broad transverse process, which is used for protection Dr.\ Hayam Saleh 16  Comparative Anatomy of Vertebrates of the back. Urostyle is a section of unsegmented vertebral column probably derived from separate caudals of early anurans. The caudal vertebrae are found only in tailed forms and all caudal vertebrae, except the first, bear haemal arches with haemal spines. Dr.\ Hayam Saleh 17  Comparative Anatomy of Vertebrates  The vertebral column in Amniotes (Reptiles, Birds and Mammals) Amniotes have a larger number of cervical vertebrae than amphibians, which provides them with a long flexible neck. The first two vertebrae are modified to permit considerably more independent movement of the head. The first vertebrae or atlas is ring-like because most of its centrum is missing. On its anterior end are one or two condylar facets for articulation with the single occipital condyle of reptiles and birds or with the two of mammals. These are condyloid joints in which the skull rocks as in nodding ‟yes‟. In all amniotes, except snakes, the centrum of the atlas has become the odontoid process of the second vertebra or axis. The process projects forward to rest on the floor of the atlas. There are no zygapophyses between the atlas and axis, especially in the mammals. The skull and atlas rock as a unit on the odontoid process, which is the axis of rotation. Rocking is facilitated by the reduction or absence of prezygapophyses on the atlas. (ii) The vertebral column of Reptiles The reptiles exhibit great varieties of differentiation of the vertebral column, but the cervical vertebrae are well developed and the first two cervical vertebrae are modified into atlas and axis. The vertebral column in crocodilians and most lizards is differentiated into cervical, thoracic, lumbar, sacral, and caudal regions. In snakes and limbless lizards, there is no specialization in the vertebrae and the vertebral column is divided only into two types of vertebrae, the precaudal and caudal vertebrae. Snakes have the longest columns with as many as 400 or more vertebrae, and the precaudal vertebrae all, except the first vertebra, carry ribs. In snakes and some lizards, in addition to zygapophyses, all vertebrae (especially precaudal vertebrae) carry a peg-like process called zygosphene on the anterior side, which fits into a cavity called zygantrum on the posterior side of the anterior vertebra, making the attachment between vertebrae very strong. Dr.\ Hayam Saleh 18  Comparative Anatomy of Vertebrates In Chelonia (turtles), there is no lumbar vertebrae, so there are only cervical, thoracic, sacral and caudal vertebrae. All thoracic vertebrae including their ribs, two sacral vertebrae and first caudal vertebra are fused with the carapace, so, the cervical and caudal vertebrae except the first are the only movable vertebrae in the turtles. The caudal vertebrae of the lizards deserve mention. Many lizards, when captured by the tail break off to the end distal to the site of the capture and scurry away. The tail then regenerates. This autotomy, in self-defence, is implemented by a zone of soft tissue that divides each tail vertebrae into cephalic and caudal sections. In all living reptiles, sacral vertebrae are always two and fused together to provide a strong articulating joint with the pelvic girdle. The caudal vertebrae in reptiles and other amniotes have small Y-shaped chevron bones on the ventral side of their centra. They are formed from the hypocentra and are not homologous with haemal arches of lower forms (fishes) in which haemal arches are formed from basiventrals. All types of centra are found in reptiles. They are amphicoclous, some have opisthoeoclous vertebrae and most lizards, snakes and crocodilians have procoelous vertebrae. Sacral vertebrae of crocodilians are acelous. Dr.\ Hayam Saleh 19  Comparative Anatomy of Vertebrates Dr.\ Hayam Saleh 21  Comparative Anatomy of Vertebrates (iii) The vertebral column of Birds The chief characteristic of the vertebral column is its rigidity (due to fusion of the bones). The rigidity is an advantage in flight. The vertebral column has cervical, thoracic, lumbar, sacral, and caudal regions. The cervical region is very mobile but the other regions are capable of little movement. In the cervical region the number of vertebrae is variable from 8 to 25, and the first two cervical vertebrae are modified into atlas and axis. The cervical vertebrae have saddle- shaped articular surfaces (heterocelous) that permit great freedom of movement. They bear short ribs, and ventrally each vertebra has a hypapophysis. Transverse processes are pierced by vertebrarterial foramen, which appears clearly in the remaining cervical vertebrae after the axis. The thoracic vertebrae are more or less fused together, though some are free, and their centra are produced below into prominent hypapophyses (also lumbar vertebrae have hypapophyses). They have large centra and broad neural spines and transverse processes. Each thoracic vertebra has two additional processes, diapophysis and parapophysis, which articulate with the bicipital ribs on each side. The posterior thoracic, lumbar, sacral, and the first few caudal vertebrae are all fused together to form a single synsacrum, which becomes fused with the pelvic girdle. The number of vertebrae making up the synsacrum varies from 13 to 20 in different birds. The synsacrum and its fusion with the pelvic girdle provide a rigid framework valuable in flight and also provide support for bipedal locomotion. The number of caudal vertebrae is small, the first few after the synsacrum are free but the posterior ones are fused to form a pygostyle, which supports the tail feathers used for steering in flight. Dr.\ Hayam Saleh 21  Comparative Anatomy of Vertebrates Dr.\ Hayam Saleh 22  Comparative Anatomy of Vertebrates Dr.\ Hayam Saleh 23  Comparative Anatomy of Vertebrates (iv) The vertebral column of Mammals The vertebral column has cervical, thoracic, lumbar, sacral, and caudal regions. An outstanding feature is the presence of 7 cervical vertebrae in most mammals even the long necked giraffe and neckless whale have seven cervical vertebrae. It is the length of the centra, not their number. The sole exception, are some mammals (Edentata) which have 6 to 9 cervical vertebrae. The first two cervical vertebrae are modified into atlas and axis as in amniotes. The atlas has large transverse processes and its centrum is much reduced. There are no zygapophyses between the atlas and axis and there are no prezygapophyses on the atlas. Ribs are fused to cervical vertebrae. There being a vertebrarterlal canal between the two heads of the rib. The thoracic vertebrae have long neural spines, and each vertebra has two specialized processes, diapophysis and parapophysis, which articulate with bicipital ribs. The usual number of thoracic vertebrae is 12-13, although the whale has only 9 and the sloth, 24. The lumbar vertebrae (five or more in number) are longer than others and their centra are very broad and flattened. They have prominent transverse processes directed forward. Lumbar vertebrae have a pair of additional long processes called metapophysis for the attachment of the muscles. The variable number of the sacral vertebrae is usually fused together: [3 in dog, 4 in rabbit, 5 in horse and 5 in man]. There is no sacrum in whales & dolphins because of a reduced pelvic girdle. The caudal vertebrae are variable in number according to length of the tail: [16-18 in rabbit, 20-23 in dog, 15-21 in horse, and 3-4 in man which fused to form a single coccyx or tail bone)]. Usually the most posterior caudal vertebrae are reduced to only centra, whereas the more anterior ones possess all the parts. The vertebrae are acoelous, [but in Perissodactyla the cervical vertebrae are opisthocoelous]. Between the centra of adjacent vertebrae, there are cartilaginous intervertebral discs. These discs are made of fibrocartilage with a remnant of the notochord in the center, called nucleus pulposus. Dr.\ Hayam Saleh 24  Comparative Anatomy of Vertebrates Dr.\ Hayam Saleh 25  Comparative Anatomy of Vertebrates 2 -The Ribs The ribs are cartilaginous or bony structures, which are either fused to or articulated with vertebrae. Typically, there is one pair of ribs to each vertebra. In some fishes, there are two sets of ribs with each vertebra in the trunk region. The dorsal ribs separate the epaxial and hypaxial musculature of each segment and grow out in the horizontal septum. The ventral ribs form along the lining of the coelomic cavity. They are serially homologous with the haemal arches of the caudal region.  Ribs in Fishes With the exception of the agnathans that have no ribs at all, most fishes have either dorsal or ventral ribs, but some have both kinds on the same vertebra, [e.g., Polypterus (primarily cartilaginous fish) and some teleosts (e.g., Perch)]. In elasmobranchs only dorsal ribs are present, while most teleosts have only ventral ribs. A few fishes (skates, chimaeras & sea horses), have no ribs.  Ribs in Tetrapoda There is one pair of ribs to each vertebra in tetrapoda. It is now believed that, they represent the ventral ribs of fishes. Although, the tetrapod rib and ventral fish rib share a similar position, they are not homologous structures (the tetrapod rib appears to be homologous with the dorsal rib of fishes). All ribs are short, except those in the thorax, in tetrapods. Most tetrapod ribs are bicipital. The ribs of the thoracic region unite ventrally with the sternum and dorsally articulate with the vertebrae by two heads (bicipital). In these two- headed or bicipital ribs the tuberculum or upper head articulates with a process on extremity of a transverse process called diapophysis, while the capitulum or lower head articulates with a process of the centrum called parapophysis. In the cervical region where ribs are fused with the veretebrae, the space between the two heads of the rib is a vertebrarterial canal through which a nerve and blood vessels pass. Ribs in amphibians are either present or absent. All ribs of anurans and urodeles are either highly reduced or absent (that’s right: no ribs). By contrast, the ribs of the limbless Dr.\ Hayam Saleh 26  Comparative Anatomy of Vertebrates apodans are quite long and they are present in associated with most vertebrae and as in snakes, they play a vital role in locomotion. The presence of short ribs probably is correlated with the mechanism of breathing in amphibians. Obviously, anurans don‟t employ costal ventilation in breathing; they instead rely on buccal pumping. Typical thoracic ribs of amniotes consist of two parts, a costal (vertebral) rib articulates with the vertebrae, and a distal sternal rib articulates with the sternum. Sternal ribs are ossified in birds but remain cartilaginous in mammals. In turtles, the ribs are expanded and fused with the dermal plates of the carapace. The snakes have long curved ribs beginning at the second vertebra and continuing far into the tail and form a tube to protect the internal organs. There is no sternum for ribs to attach to, but the ribs extend to the ventral scales to which they are attached by muscles. These ribs participate in locomotion. In birds and some lizards, thoracic ribs bear broad uncinate processes on the vertebral ribs. These processes project from the posterier aspect of the anterior ribs and overlap the next ribs in a line. They provide birds with a sturdy (strong) thoracic skeleton for the attachment of the powerful muscles required for flight and respiration. Mammalian ribs articulate with the vertebrae and also articulate differently with the sternum, and can be either true ribs (directly attached to the sternum via costal cartilages, false ribs (attached indirectly to the sternum through the costal cartilages of the other ribs and floating ribs (do not attach to the sternum).  Gastralia (abdominal or ventral ribs) They resemble ribs, but they are not true ribs because they are actually unique dermal bone elements (exoskeletal structures), while true ribs are endoskeletal structures. They are present in the ventral region of the abdomen and do not articulate with the vertebrae but act as an accessory skeletal system for muscle attachment and support for the abdomen. Gastralia are found in some reptiles (e.g., crocodilians). Dr.\ Hayam Saleh 27  Comparative Anatomy of Vertebrates Dr.\ Hayam Saleh 28  Comparative Anatomy of Vertebrates Dr.\ Hayam Saleh 29  Comparative Anatomy of Vertebrates 3-The Sternum The sternum is an elongated structure in the midvental region of the thorax. It is absent in fishes but occurs in tetrapoda. Some tetrapoda such as some amphibians, snakes, turtles and limbless lizards have no sternum. In tetrapoda the sternum articulates with the pectoral girdle. In amniotes (but not in amphibians) it also articulates with ribs to form a thoracic basket which is protective and aids in breathing. 1. Pisces. Fish lack a sternum as do the first amphibians. 2. Amphibia. The sternum is absent in some amphibians. It is well differentiated only in anurans, but the homology of such a sternum with that of amniotes is doubtful. 3. Reptilia. Reptiles are the first tetrapods which have a true sternum attached with the ribs, but the sternum is absent in the turtles, snakes, and most limbless lizards. 4. Aves. In birds, the sternum is completely ossified. The mid-ventral portion of the sternum is drawn out into a carina or keel in flying birds for insertion of large flight muscles, it is called a carinate sternum (a massively enlarged sternum). Extinct flying reptiles have a carinate sternum. 5. Mammalia. The sternum is made of a series of bones fixed together and arranged in a row lying mid-ventrally in the thorax. It is composed of a series of bony segments called sternbrae, [the first sternbra is called manubrium, and the last sternbrae, xiphisternum, is long and has a terminal expanded xiphoid cartilage. The ribs are joined to the sternum in such a way that each rib articulates with two sternbrae (i.e., ribs articulate with the sternum at the place of articulation of the sternbrae together, except the first pair of ribs which articulate with the manubrium at its middle)]. Bats have a carinate sternum. Dr.\ Hayam Saleh 31  Comparative Anatomy of Vertebrates Dr.\ Hayam Saleh 31  Comparative Anatomy of Vertebrates 4 -The Skull The skeletal framework of a vertebrate head is called a skull. The skull of vertebrates is derived from three different embryonic components. 1. Neurocranium: It includes brain case or cranium and capsules of olfactory, optic, and otic sense organs. 2. Splanchnocranium: It is derived from the visceral or pharyngeal skeleton which is cartilaginous but becomes largely replaced or invested by bones in higher forms. It provides support to the gills and forms the jaws and suspensorium in gnathostomes. 3. Dermatocranium: It develops in the dermis and consists of dermal bones that overlie the neurocranium and splanchnocranium and completes the protective cover of the brain and jaws in bony fishes and tetrapoda. A dermatocranium is absent in cyclostomes, elasmobranchs, and a few bony fishes because the entire skull is cartilaginous. 1. The neurocranium In all vertebrates the skull in embryonic stage is formed from cartilage and called chondrocranium. In higher vertebrates, bones become incorporated with the skull as development progresses, to convert the chondrocranium of the embryo into osteocranium in the adult.  Development of Chondrocranium. After formation of the central nervous system and notochord in the embryo, the mesenchyme cells form a membranous covering around the brain and the anterior part of the notochord. Cartilage is formed in this membrane. It gives rise to a pair of cartilaginous plates, the parachordals lying below the midbrain and hindbrain and anterior part of the notochord. In front of the parachordals forms a pair of curved cartilaginous rods, the trabecular or prechordals lying below the anterior part of the brain. At the same time cartilaginous capsules arise around the developing sense organs. They are a pair of olfactory capsules around the organs of smell, a pair of optic capsules around the eyes, and a pair of auditory or otic capsules around the auditory organs (internal Dr.\ Hayam Saleh 32  Comparative Anatomy of Vertebrates ears). The parachordals grow larger and fuse in the middle line forming a basilar plate leaving a small opening or basicranial fenestra. The two prechordals or trabeculae grow towards each other and fuse in the midline to form an ethmoid plate. The ethmoid plate and the basilar plate grow towards each other and fuse to form a single basal plate forming the floor of the brain. A large opening, the hypophyseal fenestra is present in the basal plate, which lodges the, pituitary gland. The olfactory and otic capsules join the basal plate to form a cartilaginous chondrocranium. The optic capsules are sometime cartilaginous but more usually are fibrous, and never fuse with the chondrocranium but remain free and form the sclerotic of the eye, thus, they permit mobility of the eyes. The floor of the brain, thus formed by the basal plate, begins to grow upwards on the sides. In elasmobranchs, side walls grow and meet above the brain to enclose it in a brain-box or neurocranium. A few openings are left uncovered for the cranial nerves and blood vessels. But in most other vertebrates there is no roof of cartilage, and the neurocranium forms a supporting floor for the brain but does not encase it. Only the posterior or occipital Dr.\ Hayam Saleh 33  Comparative Anatomy of Vertebrates region gets roofed over by cartilage, and the rest of the brainbox has only a dorsal membrane. Later on, membrane or dermal bones form the roof of the brain. The formation of chondrocranium leaves a large opening or foramen magnum behind through which the spinal cord emerges. Dr.\ Hayam Saleh 34  Comparative Anatomy of Vertebrates 2. The splanchnocranium (The visceral arches) The Visceral arches are pieces of cartilages or bones that support the pharyngeal region of the vertebrates and also help attach the jaws with the skull. A visceral skeleton is formed partly from the neural crest cells and from splanchnic mesoderm around the pharynx between gill-clefts for their support. Typically, there are 7 pairs of visceral arches in fishes (especially sharks), though this number varies and modifies in different groups of vertebrates depending upon the presence or absence of gills and type of jaw suspension. The typical visceral arches (in sharks) include seven visceral arches: the first visceral arch is the mandibular arch, the second is the hyoid arch, the remaining ones are five branchial arches because they generally support gills in lower aquatic vertebrates. In all vertebrates, except Agnatha, the mandibular arch forms jaws for supporting the mouth. This arch on either side is divided into a dorsal palatopterygoquadrate cartilage and a ventral Meckel's cartilage (mandibular cartilage). The palatopterygoquadrate or palatoquadrate forms the upper jaw, while the Meckel's cartilage forms the lower jaw. The palatoquadrate and Meckel's cartilage bear the teeth. The hyoid arch on either side consists of hyomandibular, ceratohyal and basihyal. The hyomandibular cartilage support and connect jaws to chondrocranium beneath the auditory region. The remaining five branchial arches form support for the gills. [Each branchial arch typically consists of 4 pieces of cartilages, namely, pharyngobranchial, epibranchial, certobranchial and hypobranchial. In addition to a series of mid-ventral cartilages, basibranchials in the pharyngeal floor. The branchial arches support the gills in fishes and some amphibians. As vertebrates became increasingly specialized for life on land, the ancestral branchial skeleton underwent substantial adaptive modifications. Some previously functional parts were deleted, and those that persisted perform new and sometimes surprising functions. For example, the hyobranchial apparatus supports gills in larval salamanders and the tongue in Dr.\ Hayam Saleh 35  Comparative Anatomy of Vertebrates metamorphosed adults. In anurans, however, vocalization is possible because of modifications of the hyobranchial apparatus to form laryngeal cartilages. Dr.\ Hayam Saleh 36  Comparative Anatomy of Vertebrates  Hyoid Apparatus The hyoid or hyoid apparatus is a ventral derivative of the splanchnocranium behind the jaws. In fishes, it supports the floor of the mouth. Elements of the hyoid apparatus are derived from the ventral parts of the hyoid arch and from parts of the first few branchial arches. In larval and paedomorphic amphibians, the branchial archs persist but form a reduced hyobranchial apparatus that supports the floor of the mouth and the functional gills. In adults, the gills and the associated part of the hyoid apparatus are lost, although elements persist within the floor of the mouth usually to support the tongue. Typically, the hyoid apparatus includes a main body, the corpus, and extensions, the cornua (“horns”). In many mammals, including humans, the distal end of the hyoid horn fuses with the otic region of the braincase to form the styloid process. The hyoid apparatus is a plate or rod of cartilage or bone, situated in the throat and having projecting processes extending to the otic region. It supports the tongue and larynx, serves for muscle attachment (some muscles of the larynx, and muscles that aid in swallowing), and in amphibians it plays an essential role in buccal respiration. The larynx or voice box is a chamber at the top of the trachea whose walls are supported by cartilages derived from the gill arches. The exact composition of the hyoid apparatus and the laryngeal cartilages varies in different vertebrates.  The jaw suspension or suspensoria. The method by which the visceral arches (upper and lower jaws) are suspended or attached from the neurocranium is known as jaw suspension or suspensorium. The suspensoria are of four or five principal types: 1. Autodiastylic: In the autodiastylic suspension, the jaws are attached only by ligaments to the neurocranium. The hyoid arch does not support the jaws but remain completely free as the posterior branchial arches. e.g., Early bony fishes (acanthodians). 2. Amphistylic: In this type, the upper jaw (palatoquadrate) has basal and otic processes which are Dr.\ Hayam Saleh 37  Comparative Anatomy of Vertebrates attached to the neurocranium. Besides this, hyomandibular of the hyoid arch is also attached to the neurocranium. Thus, it is a double suspension in which both the mandibular and hyoid arches are attached to the neurocranium. This type of suspensorium is found in primitive sharks and some osteichthyans. 3. Hyostylic: In this suspension, both jaws are suspended from the hyomandibular which is attached to the otic region of the neurocranium. Thus, only hyoid arch binds both the jaws with the cranium and, hence, it is called hyostylic. The upper jaw is loosely articulated with the cranium by ligaments. It is found in most elasmobranchs and bony fishes. Hyostylic suspension provides the jaws a wider movement and helps in swallowing larger preys. 4. Autostylic: In this type of suspension, the upper jaw is completely fused by its processes to the neurocranium and the lower jaw is suspended from the upper jaw. Also, the quadrate of the upper jaw articulates with the articular of the lower jaw. It is found in lungfishes and tetrapods except mammals. In lungfishes, the hyoid arch is independent and not attached to the skull. Also, in tetrapods, the hyomandibular does not take part in suspensorium, but becomes modified into columella or stapes of middle ear, for transmitting sound waves. In many tetrapods, the quadrate usually becomes an immovable part of the auditory region of the skull, a condition termed monimostylic suspension (quadrate firmly fused with the skull). But in some tetrapods, notably lizards, snakes, and birds, the quadrate is not firmly fused with the skull and is movable, a condition termed streptostylic suspension, permitting a wide gape for swallowing large prey. 5. Craniostylic suspension: This type of jaw suspension is characteristic of mammals, and some consider it as modification of the autostylic type. In this type, the upper jaw fuses throughout its entire length with the cranium, and hyomandibular forms stapes of the middle ear bones. The quadrate and articular also modify Dr.\ Hayam Saleh 38  Comparative Anatomy of Vertebrates into incus and malleus respectively. Thus, squamosal of skull and dentary of lower jaw articulate with each other and both are dermal bones. This suspension is distinguished as craniostylic (i.e., dentary articulates with the cranium by squamosal). Dr.\ Hayam Saleh 39  Comparative Anatomy of Vertebrates  Development of Osteocranium In all vertebrates the skull (neurocranium) in the embryonic stage is formed from cartilage and called chondrocranium. The formation of the skull stops at the cartilaginous stage in cyclostomes and Chondrichthyes. In higher vertebrates (Teloests and tetrapods), bones become incorporated with the skull as development progresses in one of two ways, to convert the chondrocranium of the embryo into osteocranium: A- The embryonic cartilage are converted into bone and the resulting bones called replacing or cartilaginous bones (endochondral and perichondral bones), B- New bones are added to the skull as development progresses and these bones are called dermal or membrane bones (covering bones). A-Replacing or cartilaginous bones In Teloests and tetrapods, however, the embryonic cartilaginous neurocranium is replaced partially or wholly by endochondral bone as development progress. The process of endochondral ossification within the neurocranium occurs at numerous separate ossification centers. The specific numbers of centers varies among species, but four regional groups are universal. These groups include, occipital, sphenoid, ethmoid and otic, in addition to the replacing bones of the mandibular arch. 1- Occipital centers The cartilage surrounding the foramen magnum may be replaced by as many as four bones which include: one basioccipital, one supraoccipital and two exoccipital. In some mammals (e.g., human), all four occipital elements usually fuse to form a single occipital bone. The neurocranium (skull) of tetrapods articulates with the first vertebrae via one or two occipital condyles which arise as projection on the basioccipital and exoccipital. Modern amphibians and mammals have two occipital condyles, while living reptiles and birds have a single occipital condyle. Dr.\ Hayam Saleh 41  Comparative Anatomy of Vertebrates 2- Sphenoid centers The sphenoid bones are present in the floor of the skull anterior to the basioccipital and include basisphenoid bone, presphenoid bone and laterosphenoid bone. The laterosphenoid bone forms the lateral ossification centers of the sphenoid region and consistes of the orbitosphenoid and alisphenoid bones. The sphenoid elements in mammals (basisphenoid, presphenoid and alisphenoid) may remain separate or unite to form a single sphenoid bone in some mammals (e.g., Human). 3- Ethmoid centers The ethmoid region lies anterior to the sphenoid and includes the ethmoid plate and olfactory capsules. The ethmoid tends to remain cartilaginous in tetrapods from amphibians to mammals. In amniotes, ossification centers are chiefly mesethmoid bones which developed in the internasal septum. Also an ectethmoid bone develops in the lateral walls of the nasal passageway. One or more scroll-like bones, turbinal bones (nasal conchae), arise from the walls of the nasal passage (ectethmoid) of most reptiles, birds and mammals (higher vertebrates). They are best developed among mammals having a good sense of smell. The scroll-like bones increases the surface area of the olfactory epithelia. In mammals, the two ectethmoids form together a cribriform plate which are perforated by olfactory foramina that transmit bundles of the olfactory nerve fibers from the olfactory epithelium to the brain. 4- Otic centers The cartilaginous otic capsule is replaced by several bones with such names as prootic, opisthotic and epiotic. In birds and mammals, the prootic, opisthotic and epiotic all unite to form a single periotic bone. 5- Cartilage bones in the visceral arches (a) In the mandibular arch: The caudal end of the cartilaginous palatoquadrate or pterygoquadrate undergoes ossification to become the quadrate bone. The principal bone ossified in Meckel's cartilage in vertebrates is the articular bone in its rear end; an anterior Dr.\ Hayam Saleh 41  Comparative Anatomy of Vertebrates mentomeckelian bone is common in amphibians. The articular articulates with the quadrate to form the suspension of the lower jaw, and this quadrate-articular type of suspension (autostylic) is characteristic of tetrapods except mammals, where both bones are reduced and have become bones of the, middle ear. The remaining parts of both palatoquadrate and Meckel's cartilages disappear and their places become invested by several membranes bones. (b) In the hyoid and branchial arches: The dorsal part of the hyoid arch is the hyomandibular cartilage of elasmobranchs and ossifies into a bone, the hyomandibula in bony fishes. This element may function as a suspensor of the lower jaw (hyostylic type). In tetrapods the hyomandihula, at least in part, becomes the columella of the ear. The rest of the hyoid arch in tetrapods associates with the remaining gill arches, much reduced, to form the hyobranchial skeleton. This consists of the hyoid apparatus and the cartilages of the larynx. Note: No replacement bones develop above the brain. Dr.\ Hayam Saleh 42  Comparative Anatomy of Vertebrates B- Dermal or membrane bones (Dermatocranium) The dermal bones appear in the head region of bony fishes as scales. These dermal scales are actually parts of the exoskeleton which sink inwards and fuse with the roof of the chondrocranium to complete a protective envelope around the brain. Living cyclostomes and cartilaginous fishes lack a dermal skeleton. Higher vertebrates show a greater degree of fusion of dermal bones. The dermal bones, in Labyrinthodont (extinct amphibians), include bones that form most of the roof of the skull and palate of most vertebrates. The major dermal bones are grouped into several series for purposes of description: orbital, cheek, temporal, facial, vault, palatal and mandibular series.  The orbital series consists of jugal, lacrimal, prefrontal, postfrontal and postorbital.  The cheek series comprises squamosal and quadratojugal. intertemporal, supratemporal and tabular bones form the temporal series.  The facial series has the teeth bearing bones premaxilla and maxilla as well as the nasals.  The frontals and parietals that are quite enlarged in mammals and the post-parietal form the vault series.  The palatal series (primary or soft palate): The primary palate is the roof of the oropharyngeal cavity of fishes and of the oral cavity of basal tetrapods. In sharks, it is cartilaginous, being the floor of the neurocranium on which the brain rests. In bony vertebrates, the dermal bones of the primary palate cover much of the roof of the mouth. These bones includes, paired vomers, palatines, ectopterygoids, and pterygoids. In bony fishes and lower tetrapods, there is also an unpaired medial dermal bone, the parasphenoid.  The mandibular series (Dermal bones of the upper and lower jaws): The upper jaw is fused to the skull in tetrapods, and hence the dermal bones which sheath the remaining of the palatoquadrate cartilages become the marginal bones of the skull. These bones are the premaxilla and maxilla, jugal and quadratojugal. Also, the remaining of the Mecke’s cartilage becomes sheathed in membrane bones forms the mandibular series. Laterally, the Dr.\ Hayam Saleh 43  Comparative Anatomy of Vertebrates wall of this series includes the tooth-bearing dentary and one or two splenials, the angular and the surangular. Many of these bones meet the prearticular and one or several coronoids to complete the mandibular. Left and right mandibles usually meet anteriorly at the midline in a mandibular symphysis.  No dermal bones occur in connection with the hyoid or branchial arches. Dr.\ Hayam Saleh 44  Comparative Anatomy of Vertebrates The skull in various classes of vertebrates In most vertebrates, neurocranium, dermatocranium and splanchnocranium are the same, but still there are differences in form and detailed structure of the skull. In agnathans and elasmobranchs, the chondrocranium does not ossify and the neurocranium remains cartilaginous throughout life. Also, there are some species of fish (chondrosteans and dipnoans) that also retain a cartilagenous neurocranium throughout life. It ossifies at one or more sites in bony vertebrates. The neurocrania of teleosts, apodans, and amniotes are well ossified. Those of dipnoans, basal ray fins, and most amphibians possess considerable cartilage.  The skull in Agnatha 1- Skull of Cyclostomata In cyclostomes, the head skeleton is complex, very peculiar, and quite unlike that of other vertebrates. In lampreys, the skull is cartilaginous and very incomplete. The base and side walls of the cranium are also present, but the cartilaginous roof is lacking and the brain is protected dorsally only by tough membranous fibrocartilage. [The chondrocranium includes large cartilages which form an incomplete cartilaginous box that partially enclose the brain and sense organs, and others that extend anteriorly to form: a ring of annular cartilage surrounding the buccal funnel, and a median lingual cartilage supporting the tongue. Also, extending posterior and connected to the chondrocranium is a network of cartilages forming the branchial basket which supports the pharyngeal region. The cartilage at the posterior end of the branchial basket is the pericardial cartilage that enclosing the heart and pericardium.] There are three main differences between the pharyngeal skeleton of lampreys and that found in fishes. One is that, the structure is a connected network, rather than relatively separate and articulated arches. Also, jaws are lacking, so that, the branchial basket of lampreys is not homologous with the visceral arches of fishes. The teeth of lampreys are analogous in function, but are different from other toothed vertebrates in their structure and development. Lamprey teeth are cornified epidermal Dr.\ Hayam Saleh 45  Comparative Anatomy of Vertebrates (ectodermal) structures, but the teeth of other vertebrates are dermal (mesodermal) structures. All vertebrates, with the exception of agnathans have jaws and form the group Gnathostomes ("jaw mouth").  The skull in Gnathostomata 2- Skull of Chondrichthyes The cranium is cartilaginous and consists of a single structure forming a case around the brain. The floor and sides of the cranium are complete, but its roof possesses a large gap, in the olfactory region, known as the anterior fontanelle. [The most anterior part of the cranium includes a forward plate of cartilage, the rostrum, and capsules to enclose the olfactory organs. Behind these are the orbits, and then an additional pair of capsules (otic capsules) enclosing the structure of the inner ear. Both the olfactory and otic capsules are incorporated into the skull, but the optic capsules remain free in the orbit (Page 33). The posterior or occipital region of the cranium is Dr.\ Hayam Saleh 46  Comparative Anatomy of Vertebrates perforated behind by a large median opening, the foramen magnum. On both sides of the foramen magnum are found two articular surfaces known as the occipital condyles, which articulate with the vertebral column. A number of foramina for nerves and blood vessel perforate the skull wall.] The palate in sharks is cartilaginous, being the floor of the neurocranium on which the brain rests. The dermatocranium is absent, reflecting a loss of all bone from the skeleton. Elasmobranchs contain full set of cartilaginous visceral arches (7 pairs) and the arrangement of these arches is close to the typical pattern, as they have 5 pairs of functional gills. The upper jaw is loosely attached to the skull and jaw suspension is of the hyostylic type. 3- Skull of Osteichthyes Althougth, bony fishes have a bony endoskeleton, some bony fishes (lower ganoids) retain many cartilages in their skulls, and there are a number of dermal bones, presumably derived from scales that have sunk beneath the surface, that cover the root of the skull. The skulls of bony fish are compressed laterally. In higher bony fishes (teleost), the skull is well ossified and composed of a large number of bones, and the dermal bones are numerous, forming armour around the skull. The primary palate consists of the same bones that are described in primitive tetrapod skull (labyrinthodonts), and there is also an unpaired medial dermal bone, the parasphenoid. The visceral skeleton resembles that of sharks except that bone is added. [The teeth are usually present on the premaxillae and maxillae (as well as on many bones forming the palate), but in teleosts, maxillae may be toothless]. The quadrate remains as the site of articulation of the cranium with the articular of the lower jaw in most bony fishes and all nonmammalian tetrapods. The hyoid arch of bony fish undergoes extensive ossification and modified for the movement of the operculum and functioning of the lower jaw. Also, the branchial arches ossify and the last branchial arch shows sign of degeneration as the number of gills is reduced to 4 pairs. The jaw suspension in bony fishes may be either hyostylic (Holostei and Teleostei) or Dr.\ Hayam Saleh 47  Comparative Anatomy of Vertebrates autostylic (Dipno (lungfishes). The opercular bones The operculum is a flap of tissue that arises as an outgrowth of the hyoid arch and extends cauded over the gill slites in bony fishes. An operculum is absent in most cartilaginous fishes (elasmobranchs). The operculum is a series of bones that serves as a support structure and a protective covering for the gills, and also used for respiration and feeding. Most of the opercular bones complete the dermatocranium laterally. Dr.\ Hayam Saleh 48  Comparative Anatomy of Vertebrates  The skull in tetrapoda In movement from fishes to tetrapods, several general changes in the skull may be noted: 1. A gradual reduction in the number of the separate bony elements by elimination and fusion - there may be as many as 180 bones in a fish‟s skull, whereas the human skull contains only 28. 2. Autostylic method of jaw attachment - creates changes in articulation of the jaw and evolution of the secondary or hard palate. The primary (true or soft) palate is still present with modifications in all tetrapods, but in those that also develop a secondary palate. 3. Shift in gas exchange mechanism from gills to lungs - requires the evolution of a pair of internal nostrils that pass from the external nares to the lungs. 4. Pulmonary respiration creates shift in the function of the visceral arches - no longer used to support gills and are often modified to perform different functions. In nonmammalian tetrapods, the hyomandibular ceases to be involved in jaw suspension and instead becomes a columella (or stapes in mammals) within the middle ear. The remainder of the second arch skeleton and that of the third, and sometimes part of the fourth arches gives rise to horns of the hyoid apparatus. Part of the fourth arch and perhaps the fifth contribute to the skeleton of the larynx. The other arches become the auditory ossicles or other cartilages. 5. Movement of the dermatocranium from a close relationship with the integument to a deeper position in the head where it articulates more closely with the chondrocranium. So, roofing bones and chondrocranium become more tightly associated. 6. The opercular series of bones covering the gills are typically lost. Along with this, the pectoral girdle loses its attachment to the back of the skull. 4- Skull of Amphibia Modification in the skull of Amphibians is correlated with the shift from water to land. There are fewer bones and much more embryonic cartilage in the skull of modern amphibians. General characteristics of the amphibian skull are strong deviations from the Dr.\ Hayam Saleh 49  Comparative Anatomy of Vertebrates generalized tetrapod skull (fossil ancestors, labyrinthodonts).The skull is flattened, and has fewer bony elements than the skull of bony fishes. The skull of tadpole (is the larval stage of an amphibian, particularly that of a frog or toad.) consists entirely of cartilage and resemble that of the dogfish. Anurans (Frogs) have quite lightly built skulls which may be important for their jumping mode of locomotion. The neurocranium is incomplete dorsally and much of it remains cartilaginous in adult anurans. The only replacement bones in their skulls are a single sphenethmoid and paired prootics and exoccipitals, each of the latter bearing an occipital condyle. The dermatocranium is quite incomplete: a nasal bone is present, but only a paired composite frontoparietal remains of the roofing bones. The bones that surrounded the orbit of early amphibians have been lost in anuran. Also missing are the primitive bones of temporal region, only the squamosal and sometimes a quadratojugal remain in this region. In anurans, the posterior ends of the embryonic palatoquadrate cartilages may remain as quadrate cartilages, or they may ossify to become quadrate bones. Also, the part of Meckel's cartilage which in most vertebrates ossifies, forming the articular bone, remains unossified in the Frog. [In most Amphibia, teeth are generally present on both jaws and sometimes they occur on some palatal bones, vomers and palatines. Some anurans (e.g. true toads) lose the upper and lower jaw teeth.] In anurans and a few urodeles, large palatal vacuities (enormous spaces or holes) have evolved beneath the orbit, reducing the palatines and the pterygoids. The single parasphenoid has expanded to form a large plate that has crowded other palatal bones. The splanchnocranium, a major component of the fish skull, is reduced in modern amphibians, and the hyomandibula plays no role in jaw suspension. Larval gill-bearing amphibians have visceral arches that support gills. During metamorphosis, changes occur that result in a pharyngeal skeleton being converted in the span of a few days to one characteristic of animals that live on land and breathe air. Among its new roles is serving as an anchorage for the muscular tetrapod tongue. Those amphibians (salamanders) which Dr.\ Hayam Saleh 51  Comparative Anatomy of Vertebrates remain aquatic as adults retain an essentially fishlike branchial skeleton throughout life, except that the number of gill-bearing arches is fewer than in fishes. Dr.\ Hayam Saleh 51  Comparative Anatomy of Vertebrates 5- Skull of Reptiles The skull of stem reptiles were little changed from those of labyrinthodonts and some primitive features are still present in living reptiles. Among these are a well-ossified neurocranium (except, nasoethmoidal region), a single occipital condyle, and a large complement of membrane bones. Major changes from stem reptile skulls included the appearance of temporal fossae and development of a partial or complete secondary palate. Also, between the two orbits, there is an interorbital septum in all reptiles, except snakes. In turtles only: nasal, lacrimal, and ectopterygoid are absent from the skull. Also, postfrontal is absent in some turtles. In the occipital region of all reptiles, all four occipital bones surround the foramen magnum, and there is a single occipital condyle for articulation with the first vertebra, atlas. In the otic region, the prootic, epiotic, and opisthotic bones remain separate, and none of the otic bones are on the surface, being overlaid by the squamosal. In some reptiles, in addition to the primary palate, a new secondary palate is formed. In crocodilians, shelf-like palatal processes of the premaxillae and maxillae, palatines and pterygoids bones grow and meet in the midline to form a long bony shelf, this is the secondary palate. This new palate is the floor of the nasal passageway and divides the primitive oral cavity into separate oral and nasal passageways, thereby displacing the internal nares far to the rear. In other reptiles, not all the palatal processes reach the midline, so secondary palate is incomplete. Secondary palate is well developed in crocodiles, less developed in turtles and nearly absent in others. The lower jaw in each half has one cartilage bone (articular) and five dermal bones (dentary, angular, supraangular, coronoid and splenial). In most reptiles, the rami (both halves) of the mandible (lower jaw) are fused together at their anterior ends by a suture, but they are completely fused in turtles. In snakes, the rami of the lower jaw are connected together by an elastic ligament, and snakes are capable of being widely separated from one another. Also, the premaxilla is single and small, and as a rule, connected with the maxilla only by a ligament. So, the snake can swallow preys bigger than its head size. Several snake lineages have evolved venom which is typically delivered by specialized teeth called Dr.\ Hayam Saleh 52  Comparative Anatomy of Vertebrates fangs located on the maxilla. The remaining visceral arches are modified, the hyoid arch forms a columella of the middle ear from its hyomandibular, the lower part of the hyoid arch and third and fourth visceral arches form the hyoid apparatus that also extends forward to support the tongue. The remaining arches form cartilages of the larynx and trachea. The class Reptilia has been divided into four principles groups based on characteristics of the temporal region (area behind each eye) of the skull. Behind the orbits are one or two temporal fossae (except Chelonia) for muscles of the jaws needed to operate the lower jaw of amniotes. These groups include: 1-Anapsida: It has a solid roof of the skull and the temporal region is covered completely by bone that is not pierced by temporal openings. Only the necessary openings that include the nares, orbits and pineal foramen, are present. Today, among living amniotes only turtles are anapsid. 2- Euryapsida (Extinct): Skull with a single pair of temporal openings (one on either side) bounded below by an upper temporal arch formed by postorbital and squamosal bones. 3- Synapsida: The synapsid skull has a single pair of temporal openings bordered above by an upper temporal bar formed by postorbital and squamosal bones. It also bordered below by a lower temporal arch formed by jugal and quadratojugal bones. It is found in mammalian ancestors. This synapsid skull was transmitted to mammals. 4- Diapsida: The skull is characterized by two pairs of temporal openings separated by a bar of postorbital and squamosal bones. The lower temporal opening bordered below by the jugal and quadratojugal bones. Crocodilians, together with Sphenodon, and squamates (lizards and snakes) represent the surviving reptiles with a diapsid skull. Dr.\ Hayam Saleh 53  Comparative Anatomy of Vertebrates Dr.\ Hayam Saleh 54  Comparative Anatomy of Vertebrates In the crocodilian skull, both temporal bars are present, and the skull is firm, without any evidence of cranial kinesis. Loss of the lower temporal bar or both bars produces the modified diapsid skull of squamates. Modern lizards (lacertilia) have lost the lower arch and snakes (Ophidia) have lost both arches. Loss of the lower bar or both bars has left a cavernous void in the posterolateral walls of the squamate skulls and resulting in the quadrate becomes loosely attached on each side. Thus, there is a freely movable quadrate articulating with the lower jaw, such a suspension is known as streptostylic. In turtles and crocodiles, quadrate is firmly fused (not movable) with the skull and a movable palate are absent.  Cranial kinesis (kinetism): Cranial kinesis refers to movement within the skull. Reduction (in birds) or loss (in squamata) of the temporal arches, along with the acquisition of intracranial joints facilitated cranial kinesis, the movement of one section of a skull independent of others (i.e., these animals raise the upper jaw and palate as a unit independent of neurocranium when they open their mouth). Kinetism enable some squamates, especially vipers, to open their mouth wide enough to swallow prey larger than their own head. Dr.\ Hayam Saleh 55  Comparative Anatomy of Vertebrates Dr.\ Hayam Saleh 56  Comparative Anatomy of Vertebrates 6- Skull of Aves In birds, the skull is made on the same plan as in reptiles and it resembles the skull of lizards. The bird skull differs in being larger and very light due to the pneumatic bones. Some of the roofing bones have been lost, and the remaining dermal bones are much thinner and fused together, so that there are practically no sutures. Individual bones are identified clearly in the skull of a ratite (a non-flying bird) or a nestling. There is a large, arched cranium due to a greater development of the brain which lies in the posterior part. The skull represents a modified diapsid pattern, the upper temporal bar (arch between the superior and inferior temporal fossae) having been lost. The resulting fossa broadly opens to the rear and also merges with the orbit in front, so the orbits are immense. The two orbits are separated by a thin interorbital septum, thus, pushing the brain case posteriorly. A preorbital fossa is separated from the orbit by the lacrimal bone and from the nasal opening by the nasal bone. In the roof of the skull, the nasals, frontals and parietals bones are present. The nasal is small, while the frontal and parietal are very large and form most roof of the skull and also they extend below to contribute in the formation of the lateral wall of the skull. Like turtles and some dinosaurs, birds are toothless, and their jaws are covered by keratinized sheaths. There is a large pointed beak formed by the large premaxillae and dentaries. The maxillae are small in size. The infratemporal arch (jugal and quadratojugal bones) is intact but very slender, and there is a freely movable quadrate articulating with the lower jaw, thus the suspension is streptostylic. The foramen magnum is shifted to the ventral side and it is surrounded by all four occipital bones and there is a single occipital condyle (monocondylic). The three otic bones (Prootic, epiotic and opisthotic) are fused together to form a single periotic bone (petrosal bone). In the ethmoid region, the mesethmoid bone contributes in the formation of the anterior portion of the interorbital septum. The primary palate resembles that of squamates except that the ectopterygoids have been lost. The two vomers may be separate, but they often fuse together to form an unpaired vomer lies at the ventral midline of the skull. The secondary palate is very Dr.\ Hayam Saleh 57  Comparative Anatomy of Vertebrates incomplete, nearly absent, so that, the borders of the palatal fissure are fleshy palatal folds. The Meckel's cartilage is much reduced, it has one cartilage bone (articular) and four dermal bones that include the dentary, splenials, angular and surangular, but the coronoid bone is absent. Like reptiles, hyomandibular forms the columella of the middle ear, the rest of the hyoid arch, along with the first branchial arch forms the hyoid apparatus which supporting the tongue. Dr.\ Hayam Saleh 58  Comparative Anatomy of Vertebrates Dr.\ Hayam Saleh 59  Comparative Anatomy of Vertebrates 7- Skull of Mammals The skull of mammals represents a highly modified synapsid pattern. There is much variation in the skulls of different mammals, yet there are certain common features. Major features that differentiate mammalian skull from those of other amniotes are emergence of the dentary as the sole bone of the lower jaw, an altered site of articulation of the lower jaw with the braincase, alteration in the secondary palate, and the presence of the three bones (ear ossicles) in the middle ear cavity. Due to the formation of a large brain (especially enlargement of cerebral hemispheres) the cranium is very large having expanded dorsally and laterally. The cavity of the cranium is closed in front by a cribriform plate. In the evolution of the skull from fishes to mammals there has been a considerable reduction in the number of skull bones by loss and fusion. Various dermal elements are lost in therian mammals, including the prefrontal, postorbital, postfrontal, quadratojugal, and supratemporal. In mammals, all four occipital bones surround the foramen magnum and supraoccipital is the largest one. These four occipital elements may be separate, but usually they fuse to form a single occipital bone as in human. Skull is dicondylic, having 2 occipital condyles. In the roof of the skull, nasals, frontals, parietals and interparietals are present: nasals are small in primates, while frontals and parietals are large and form nearly the entire posterior valut of the cranium. Also frontals and parietals extend below and contribute with alisphenoid and orbitospenoid in the formation of the lateral wall of the orbit. Various degrees of fusion occur in the sphenoidal area. The presphenoid, orbitosphenoids, basisphenoid and alisphenoids may be separate or all fused into a single bone in some mammals (e.g., human). The upper jaw consists of premaxilla, maxilla and jugal, while quadratojugal and quadrate are absent. Both premaxilla and maxilla bear teeth. A zygomatic arch, on either side of the skull, is formed by the zygomatic process of squamosal and the zygomatic process of maxilla and jugal. It forms the ventral margin of the orbit and the temporal Dr.\ Hayam Saleh 61  Comparative Anatomy of Vertebrates fossa. The postorbital process of the frontal partially or completely separate the orbit from the temporal fossa (in most primates the separation is complete). Otic bones (Prootic, epiotic and opisthotic) are fused to form a single periotic bone which forms a swollen tympanic bulla with tympanic bone. Mammals possess a secondary palate (hard palate) that completely separates the rostral part of the nasal and oral cavities, and extends all the way to the pharynx. The hard palate is formed from the inward growth of bony processes of the premaxilla, maxilla, and palatine that meet at the midline as a bony platform. In some mammals the pterygoids also contribute to the bony palate. This hard palate and its fleshy continuation effectively separate the food chamber below from the respiratory passage above. The secondary palate enables mammals feed while breathing at the same time. Caudal to the secondary or hard palate is a membranous primary or soft palate, which reduced to a median unpaired vomer lies at the base of the nasal septum. The palatines contribute to the posterior part of the secondary palate and the lateral walls of the nasopharynx. The pterygoids are reduced to small, winglike, pterygoid processes in mammals. The parasphenoid and ectopterygoid have been lost. The lower jaw in each half is made of a single bone, the dentary, which has three processes. The dentary articulates by its condyloid process with a glenoid fossa (maxillary groove) in the squamosal rather than with the quadrate. The coronoid and the angular processes of the dentary serve as the insertion site for the muscles. The dentary also bears the teeth. The condition of the teeth in mammals is heterodont, where teeth are modified for different functions: incisors used for nipping, canines for grasping or tearing, and premolars and molars used for cutting and grinding. In all other vertebrates the condition of the teeth is homodont (all teeth are similar in the shape and function). The tympanic bulla encloses the tympanic cavity of the middle ear which contains 3 ear ossicles (malleus, incus and stapes). Palatoquadrate (quadrate) breaks into alisphenoid and incus, the former becomes part of the skull and the latter joins the ear ossicles. Meckel‟s cartilage (articular) modifies into malleus and hyomandibular modifies into stapes of the Dr.\ Hayam Saleh 61  Comparative Anatomy of Vertebrates middle ear cavity. The major part of the hyoid arch forms a hyoid apparatus which supports the tongue and laryngeal muscles. The remaining visceral arches form the thyroid, epiglottis, arytenoids, cricoid in the larynx and also the rings of the trachea. [Larynx of mammals evolved from the fourth and fifth visceral arches. Thyroid cartilage is a modification of 4th and 5th visceral arches while arytenoid and cricoid cartilages are modified fifth visceral arch]. Dr.\ Hayam Saleh 62  Comparative Anatomy of Vertebrates Dr.\ Hayam Saleh 63  Comparative Anatomy of Vertebrates B-The Appendicular Skeleton The appendicular skeleton consists of the skeletal elements of pectoral and pelvic girdles, and of the appendages (fins or limbs). The appendages are median and paired. The median appendages are found in fishes and aquatic tetrapoda, while the paired appendages are found in all vertebrates except cyclostomes. In fishes the paired appendages are pectoral and pelvic fins, but in tetrapoda they are fore- and hind limbs. Some vertebrates have no appendicular skeleton (e.g., agnathans, apodans, snakes, and some lizards), and in others it is much reduced. The paired appendages articulate with the axial skeleton by means of girdles. The girdles and paired limbs of tetrapoda are remarkably similar and have homologous parts.  Skeleton of the pectoral girdle: The pectoral girdles articulate with the pectoral fins or forelimbs. They consist of membrane and replacement bones (in bony vertebrates). In early fishes, which exemplifies the basic pattern of all pectoral girdles, the pectoral girdle had 3 replacement bones (coracoid, scapula, and suprascapula) and a series of dermal bones (clavicle, cleithrum, supracleithrum, and post-temporal). At the junction of scapula and coracoid, a depression or concavity is formed. It is known as glenoid cavity into which head of humerus of the forelimb articulates. Dr.\ Hayam Saleh 64  Comparative Anatomy of Vertebrates Cartilaginous fishes: Cartilaginous fishes have only the endoskeletal components that do not ossify, and have none of the dermal components. Bony fishes: The pectoral girdles of living bony fishes have reduced coracoid and scapula (replacement bone), but large cleithrum and supracleithrum (dermal bone). A posttemporal bone (dermal) connects the supracleithrum to the skull. The dermal bones predominate in the pectoral girdle of bony fishes, whereas the replacement bones predominate in tetrapods. Tetrapods: Early tetrapods had pectoral girdle similar to those of early bony fishes, but lost posttemporal and acquired interclavicle (which still occurs in several amniotes, e.g., alligator, birds, and monotremes). Tetrapods either need clavicles, coracoids or both to brace the scapula against the sternum (as in birds). Scapula is present in all tetrapods with vestiges of the anterior limbs, e.g., turtles & birds & mammals. In mammals, the scapular spine divides the scapula into supraspinous and infraspinous fossae, where arm muscles originate. The clavicle is present in most mammals, and it is large in mammals with strong forelimbs used in digging, climbing, or flying, but it is a vestigial splinter in cats. Dr.\ Hayam Saleh 65  Comparative Anatomy of Vertebrates  Skeleton of the pelvic girdle The pelvic girdles articulate with the pelvic fins or hindlimbs. They consist of replacement bones only, and there are no dermal bones in the pelvic girdles of fishes or tetrapods. In fishes: The pelvic girdle consists of 2 cartilaginous or bony plates (ischiopubic plates) that articulate with the pelvic fins. In tetrapods: The pelvic girdle has three cartilage bones in each half. They are the ilium, ischium and pubis. All three bones participate in the formation of an acetabulum cavity, into which the head of femur of the hind limb articulates. A cotyloid or acetabular bone often forms a part of the acetabulum in mammals. Generally a large obturator foramen lies between the ischium and pubis. It allows for passage of nerves and muscle tissue. The two halves of the pelvic girdle often meet midventrally at the pubic or ischiac symphysis. In most tetrapoda the pelvic girdle is attached to the axial skeleton through the ilia articulating with the sacral vertebrae. Dr.\ Hayam Saleh 66  Comparative Anatomy of Vertebrates Frogs & toads: The ilium is elongated and extended from the sacral vertebra to the end of urostyle. With very long ilia, the pelvic girdle forms a long lever for transferring the force from the hindlimbs to the vertebral column in jumping. Reptiles: The structure of the pelvic girdles of reptiles is correlated with their diverse body structure and with their mode of locomotion. In most reptiles, the pubis is directed away from the ischium, resulting in a triradiate girdle. An epipubic and hypoischial bone frequently develops in association with the pelvic girdle of reptiles. A few snakes (e.g., boas) retain vestiges of a pelvic girdle and limb skeleton. Birds: In birds the ilium and ischium expanded to accommodate musculature needed for bipedal locomotion. The girdle is braced against lumbar and sacral vertebrae and united with the synsacrum. The pubic bones are typically reduced (long but thin), and there is no ischial or pubic symphysis. Absence of these provides a wide outlet for laying eggs. Mammals: In mammals the ilium, ischium, and pubis ankylose early in postnatal life to form a left and right innominate (coxal) bone.  Skeleton of the fins Fins are one of the most distinguishing features of a fish and they have several different forms. Two types of fins are found in most of the fish: median and paired fins. Median fins are single in number which runs down the mid-line of the body. In fishes, median fins are dorsal, caudal and anal fins, while paired fins are pectoral and pelvic which are arranged in pairs homologous to human arms and legs. All jawed fishes (except eels) have pectoral & pelvic fins, which are corresponding to the fore- and hindlimbs of the tetrapods. Of the median fins, the caudal fin is well developed in most fishes because it plays the most important role in forward propulsion during swimming. Three main types of caudal fins are found in fishes. 1- The heterocercal caudal fin: heterocercal tail has the vertebral column bent and reaches up to the tip of the more prominent dorsal lobe, thus making the caudal fin strongly asymmetrical. It is typical of modern Elasmobranchs and some lower bony fishes. Dr.\ Hayam Saleh 67  Comparative Anatomy of Vertebrates 2- The diphycercal type: It is the most primitive type. The vertebral column extends up to the tip of the tail and divides the caudal fin into two equal valves. It is found in cyclostomes and the living dipnoans (lungfishes). 3- The homocercal tail: This is the advanced and most common type of caudal fin. The terminal part of the vertebral column is bent upwards, and become greatly reduced, so that it does not reach the posterior limit of the fin. There is no apparent dorsal lobe, but the ventral lobe divides into two equal superficial parts, so that the caudal fin appears symmetrical externaly, but internally it is asymmetrical. This type is common in higher bony fishes (Teleosts). Dr.\ Hayam Saleh 68  Comparative Anatomy of Vertebrates  Skeleton of the limbs Starting with amphibians, Tetrapods typically have 4 limbs. However, some tetrapods have lost one or both pairs and, in others, one pair is modified as arms, wings, or paddles. Some vertebrates lack both pairs of the limbs, such as caecilians (apodans), most snakes and snake-like lizards. Some vertebrates have forelimbs only, such as manatees & dugongs, dolphins, cetaceans (vestigial elements may be embedded in body wall) and sirens (salamander). The limbs of tetrapods are pentadactyle (five fingered). They are used not only for locomotion but also to support the weight of the body above the ground. Thus, they are provided with joints. They are evolved from the paired fins of primitive fish as an adaptation to locomotion on land. The Pentadactyle or Tetrapod limb: The pentadactyle limb is built on the same basic fundamental plan in all vertebrates. Each limb has three parts, the upper arm or thigh containing one long bone, the forearm or shank containing two long bones, and the hand or foot, which contains a number of small bones. The upper arm (or thigh), has a single humerus (or femur). The humerus joins the pectoral girdle at the glenoid cavity, while the femur joins the pelvic girdle at the acetabulum cavity. The forearm (or shank) contains two parallel bones, the radius and ulna (or tibia and fibula). The radius and tibia are lateral, while ulna and fibula are medial. Between the upper arm and forearm is an elbow joint (or knee joint). The hand or foot has three divisions, a carpus or wrist (tarsus or ankle), metacarpus or palm (metatarsus or sole), and digits forming fingers or toes. The carpus or tarsus in the primitive condition has 10 bones in 3 rows, the first row has a radiale (or tibiale), an intermedium, and an ulnare (or fibulare), in the second row are generally two centralis, while the third row has five distal carpals (or distal tarsals). The metacarpus (or metatarsus) has five long metacarpal' (or metatarsals). Dr.\ Hayam Saleh 69  Comparative Anatomy of Vertebrates The digits are generally five in number or pentadactyle, with the first digit on preaxial side and the fifth on the postaxial side. Each digit has a linear row of phalanges; they are 2, 3, 4, 5, 4 beginning from the first to the last. Dr.\ Hayam Saleh 71  Comparative Anatomy of Vertebrates II- The Respiratory System Every cell in a living organism consumes oxygen )O2) during the oxidation of substances resulting in the release of heat and energy and the production of carbon dioxide (CO2). This CO2 acts as a poison for protoplasm unless removed from the body. According to a simple definition, the term respiration means the intake of O 2 and getting rid of CO2 by a living organism. The system designed for the exchange of gases (O2 and CO2) between the organism and its environment is termed the respiratory system. Major modifications in the design of the respiratory organs have occurred during animal evolution to optimize the diffusion of important gases. The rate of passive diffusion between an organism and its environment depends on several factors. One is surface area. The greater the available surface area, the greater the opportunity for molecules to move across an epithelial surface. For instance, the gas exchange organs of vertebrates are highly subdivided to increase the surface available to transfer gases between air and blood. Another factor is distance. The greater the distance, the longer it will take for molecules to reach their destinations. Thick tissues slow diffusion, and thin barriers aid the process. The thin walls of the respiratory organs reduce the distance between the environment and the blood. A third factor is resistance to diffusion by the tissue barrier itself. The moist skin of living amphibians facilitates gas transfer. In contrast to this situation, the skin of most mammals is cornified and thick, a feature that slows gas diffusion with the environment. One of the most important factors affecting the diffusion rate is the difference in partial pressures across the exchange surface. The gills of most fishes experience a high partial pressure of oxygen relative to the blood; therefore, oxygen diffuses across the gills into the blood. Occasionally a fish living in warm and stagnant water may encounter a partial pressure of oxygen in the water below that of its blood. Under these unusual conditions, oxygen may actually diffuse in the reverse direction, and the fish is in danger of losing oxygen to the water! Dr.\ Hayam Saleh 71  Comparative Anatomy of Vertebrates In most fish gills, ventilation (breathing) is unidirectional. Water enters the buccal cavity through the mouth, passes across the row of gills known as the gill curtain, and exits flowing in one direction only (Fig.1a). Lung ventilation, however, is usually bidirectional (tidal), with air entering and exiting through the same channels (Fig.1b). A fresh breath of air that is inhaled into the lungs mixes with spent air and is exhaled. Vertebrates that live in aqueous environments most often encounter too little oxygen, termed hypoxia, partly because water is already low in dissolved oxygen. For this reason, most organs that supplement respiration are found among aquatic rather than strictly terrestrial animals. Fig. (1): Unidirectional and bidirectional flow. (a) In fishes and many aquatic amphibians, water movement is unidirectional because water flows through the mouth, across the gill curtain, and out the lateral gill chamber. (b) In many air-breathing vertebrates, air flows into the respiratory organ and then reverses its direction to exit along the same route, creating a bidirectional or tidal flow. Dr.\ Hayam Saleh 72  Comparative Anatomy of Vertebrates  Respiratory Organs The exchange of oxygen and carbon dioxide occurs in two places, i.e., in the respiratory organs and in tissues. Also, the exchange of O2 and CO2 in an organism takes place in two locations: internal and external. During internal respiration, also termed cellular or tissue respiration, gaseous exchange occurs between blood and tissues or cells of the body. During external respiration, gaseous exchange takes place between blood and the external environment (e.g., in aerial respiration within the lungs and in aquatic respiration within water and gills surface). The body structures w