Chordata Biology PDF

Chordata  The chordates are one of the most diverse and successful of all animal groups.  They include the birds and mammals  During the embryonic development of chordates, all of them pass through a stage called the pharyngula with these features: At this stage they all...

Chordata  The chordates are one of the most diverse and successful of all animal groups.  They include the birds and mammals  During the embryonic development of chordates, all of them pass through a stage called the pharyngula with these features: At this stage they all contain a: 1. A single, hollow nerve cord runs just beneath the dorsal surface of the animal. In vertebrates, the dorsal nerve cord differentiates into the brain and spinal cord. 2. A flexible rod, the notochord, forms on the dorsal side of the primitive gut in the early embryo and is present at some developmental stage in all chordates. 3. Gill slits, They are the paired openings in the wall of the pharynx and allow the water to be passed outside in the aquatic forms. In these forms they are lined with vascular filaments which forms gills for respiration, in the terrestrial forms like reptiles, birds and mammals. They are to be found only in early development and disappear in the adult stage. Gill clefts are also called gill slits or pharyngeal clefts. The gill clefts which do not bear gills are called visceral clefts. 4. Chordates have a postanal tail that extends beyond the anus, at least during their embryonic development. As adults, the nerve cord remains while the notochord is replaced by the vertebral column and all but one pair of pharyngeal slits are lost. This remaining pair forms the Eustachian tubes that connect the throat to the middle ear. 5. The blood flows backwards in the dorsal blood vessel and forwards in the ventral blood vessel, a case which is contrary to that in invertebrates. -1- -2- There are three subdivisions of the chordates: 1. Cephalochordata 2. Tunicata Nonvertebrate chordates have a notochord but no backbone (vertebral column ) and include Urochordata and Cephalochordata 3. Vertebrata (Craniata): The vast majority of chordates have a skull enclosing their brain, eyes, inner ear, etc.). All but one group of these (the cyclostomes) also convert their notochord into a vertebral column or backbone thus qualifying as vertebrates. -3-  I- Subphylum: Urochordata This group (also called Tunicata) includes animals known as ascidians (and commonly called sea squirts). They are  marine  sessile animals that  feed by filtering food particles from seawater taken in through one opening, or siphon, and squirted out the other.  United sexes (hermaphrodite).  colonial  Many adult tunicates secrete a tunic, a tough sac composed mainly of cellulose. The tunic surrounds the animal and gives the subphylum its name. Cellulose is a substance frequently found in the cell walls of plants and algae but is rarely found in animals. It is hard to see what makes these animals chordates. The adults have neither notochord nor a dorsal tubular nervous system. However, these animals disperse themselves with free-swimming tadpole like larvae that have  a dorsal tubular nervous system  notochord and  gill slits II- Subphylum: Cephalochordata The representative member of this tiny subphylum of so-called lancelets is a small (5 cm), marine, fishlike creature called amphioxus (on the right). (For years its -4- genus name was Amphioxus but that has now been replaced by the name Branchiostoma.) Amphioxus retains:  a dorsal nerve cord  notochord and  gill slits throughout its life. It is marine inhabitant of shallow water, like a small fish, as its elongated body is pointed at both ends and bears fins. As in almost all other chordates the sexes are separate but Males and females have no obvious external differences exist between them. The gonads, either testes or ovaries, lie in the ventral part of the lateral body wall in the pharyngeal region, and form a series of bulges inwards towards the atrium. They follow a metameric arrangement, without any genital ducts, and the ripe gametes are released by the rupture of the overlying tissue into the atrium, where they are carried to the outside through the atriopore and fertilization takes place externally in the surrounding water. -5-  T. S. in pharyngeal region of Amphioxus sp. T. S. in intestinal region of Amphioxus sp. T. S. in tail region of Amphioxus sp. -6-  Formation of vertebral column in vertebrata 1-The development of vertebral column in vertebrata starts by the appearance of mesodermal cells around the notochord forming a skeletogenous tissue which is more condensed at four places, two dorsal above the notochord, and two ventral below it. When these condensations are transformed into cartilage they give rise to two basidorsals and two basiventrals respectively. 2- From the basidorsals and basiventrals, skeletogenous cells pierce and invade the fibrous tissue surrounding the notochord. Gradually these cells increase in number and then they are transformed into cartilage giving rise to the centrum of the vertebra. During the formation of the centrum the notochord gradually decreases in size until it disappears when the centrum is fully formed. 3- The two basidorsals extend dorsally forming the neural arch enclosing the neural canal which includes the spinal cord, and carrying at its apex a dorsal process known as the neural spine. 4- In the trunk region the basiventrals extend laterally forming the transvers processes, but in the tail region they extend ventrally forming the haemal arch surrounding the caudal artery and vein, and carrying at its lower edge a ventral process known as the haemal spine. The vertebral column surrounds the spinal cord and follows posteriorly a peculiar skeletal structure, the skull, which is formed of a cranium surrounding the brain, and sense capsules surrounding the sense organs which are the paired eyes, olfactory organs and auditory organs. -7-  III - Sub-Phylum: VERTEBRATA Although Lampreys and hagfishes, never replaces their notochord with a vertebral column, and thus might seem not to qualify as vertebrates, they share a number of other features with other vertebrates and certainly should be classified with them. All the other members of the craniata convert their notochord into a vertebral column or "backbone" (even though in some it is made of cartilage not bone). The vertebrates are subdivided into the  jawless vertebrates (Agnatha) and the  jawed vertebrates (Gnathostomata) 1-SUPERCLASS :AGNATHOSTOMATA (Agnatha) 1- CLASS: CYCLOSTOMATA Eg: PETROMYZON (LAMPREY) Lampreys and hagfishes are the only jawless vertebrates to survive today. They both have a round mouth and for this reason are often referred to as cyclostomes. They are the most primitive of the vertebrates. General characters: 1. Body is slender, eel-like rounded with soft skin containing mucous glands but no scales. 2. The suctorial mouth is ventral and lies at the bottom of a suctorial disk called the oral or buccal funnel which is held open by a ring of cartilage. Around the oral funnel there are a number of papillae and horny teeth which are found also on the tongue. 3. Single nasal opening. 4. 7 gill slits. 5. There are 2 dorsal fins and one caudal fin, but no paired fins. 6. The muscles in the walls of the trunk and tail are segmented in E shaped arrangement. 7. The notochord persists as a well-developed structure in the adult. 8. The digestive system lacks a stomach and the intestine is provided with a -8-  fold typhlosole, two mesonephric kidneys. 9. The sexes are separate in the adult. However, the immature gonad is hermaphrodite, but later becomes male or female. There is a single gonad without genital ducts but eggs or sperms break out into the coelom and pass out through the urinogenital opening to the water where fertilization occurs externally. The eggs hatch out into larvae known as Ammocoetes which changes to the adult in about 8 years. -9-  T. S. in branchial region of Petromyzon sp T. S. in trunk region of Petromyzon sp. T. S. in tail region of Petromyzon sp. - 11 -  A comparison between Amphioxus and Petromyzon Amphioxus Petromyzon Anterior It extends as rostrum with the Mouth is suctorial, circular region mouth below the oral hood and anterior in position Skin Consists of a single layer, it is Consists of more than one transparent with no pigments. layer, it is pigmented. Myotomes V- shaped W- shaped Pharyngeal It includes the pharynx It includes the pharynx and region together with a cavity known respiratory tube which is as the atrium closed by a tongue provided with horny teeth Respiration Takes place in the pharynx Takes place in the where the pharynx wall is respiratory tube where perforated with gill slits. The water enters this tube then it co2 comes out form the passes to the gill pouches deoxygenated blood and which contain feathery gills diffuses in water and gets into and gaseous exchange takes the atrium then it passes place and water diffuses out through the atriopore with co2 through the gill slits Gill slits Open into the atrium Open to the exterior directly It has a very large number of It has 7 pairs of gill slits gill slits in the pharynx wall Liver Has liver diverticulum Has a well developed liver kidney It is pronephros It is Mesonephros gonads Have 26 pairs of gonads. Sexes Have one gonad. Sexes are are separate there is no separate there is no connecting ducts. They are connecting ducts. They are found in the pharyngeal found in the intestinal region region Fertilization External External Fins Has unpaired fins Has unpaired fins The dorsal fin isn’t divided The dorsal fin is divided into 2 lobes - 11 -  1I-SUPERCLASS: GNATHOSTOMATA Gnathostomata are the vertebrates which possess two jaws and two pairs of appendages. Olfactory organs and paired nostrils are present. They are divided into two groups : Group: Pisces-fishes. This group includes aquatic vertebrates which possess paired fins. The skin is covered over with scales and they respire by gills. The living fishes are divided into two classes: Chondrichthyes and osteoichthyes. Group: Tetrapoda It is a group of vertebrates which have two pairs of limbs. They respire by lungs, the skeleton is bony. They are divided into four classes : Amphibia. Reptilia, Aves and Mammals. Class: Chondrichthyes The Class Chondrichthyes includes sharks & dogfish. forms. Example: The dog fish (Scyliorhinus canicula). General characters: 1. Body is spindle-shaped, bilaterally compressed and about 30 cms long. The skin is grayish in colour with dark brown spots and is covered with placoid scales. Each consists of a bony basal plate, with a spine in the center, directed backward and is composed of dentine covered with a hard enamel like dentine. The method of embryonic development of the scale and its dentinal nature indicate that the placoid scales are homologous with the vertebrate teeth. 2. The fins are either un paired (median) or paired (lateral). The unpaired fins are 2 median dorsal fins one behind the other, the ventral or anal fin - 12 -  posterior to the cloacal aperture and a caudal fin around the upturned tail, it is heterocercal. The paired fins are 2 pectoral situated one either side of the trunk, and the 2 pelvic fins, one on each side of the cloacal aperture. The pelvic fins in the male poses rod like structure known as the claspers. All the fins are supported by fin rays. 3. The mouth is crescent - shaped slit on the ventral surface of the head surrounded by the upper and lower jaws which bears several rows of small pointed teeth directed backward. The teeth are modified placoid scales. 4. In front of the mouth, there are 2 nostrils each is communicated with the mouth by one or a-nasal groove. 5. The eyes are found on either dorsal side of the head, each is guarded by two eye-lids. 6. Posterior to each eye lie a small circular opening, spiracle which opens into the pharynx. The spiracle is followed by 5 elongated gill slits which open also in the pharynx. Respiration is by gills which are situated in the gill pouches separated by gill septa. The gill lamellae are highly vascular folds of mucous membrane arranged on both sides of each gill septum. 7. The endoskeleton is entirely cartilaginous, made up of a chondrocranium which houses the brain and auditory organs and partially surrounds the eyes and olfactory organs; a vertebral column made of separate vertebrae; appendicular skeleton and visceral skeleton. 8. The digestive system begins with the mouth followed by the wide pharynx into which open the spiracle and gill slits. The pharynx leads into short wide oesophagus which opens in the U shaped stomach with a wide cardiac limb and a narrower pyloric limb. The intestine follows the stomach and it begins with a short duodenum and a wide ileum within which is a spiral valve which prevents a too rapid passage of food and allows increased - 13 -  absorption. The last portion of the intestine is a short narrow rectum which opens into the cloaca. There are 2 digestive glands I. e. the bilobed liver with gall bladder and the pancreas. 9. The circulatory system is composed of 4 chambered S shaped heart; sinus venosus, auricle, ventricle and conus arteriosus. The heart contains venous blood only and is pumped in it through the ventral aorta and hence into the afferent branchial vessels to be oxygenated in the capillaries of the gills and then passes into the afferent branchial arteries which carry it to the dorsal aorta. There are also hepatic and renal portal systems. 10. The brain is more highly developed. It consists of 2 remarkably large olfactory lobes, 2 cerebral hemispheres, 2 optic lobes, a prominent cerebellum, a medulla oblongata. There are 10 pairs of cranial nerves which are distributed largely to the head regions. Surrounding the spinal cord are the neural arches of the vertebrae along the spinal cord a pair of spinal nerves with united dorsal and ventral roots is distributed to each body segment. 11. There is a lateral line sensory system made up of canals which extend along the side of the trunk and tail and cover the head region. 12. The excretory system is composed of 2 kidneys each is formed of 2 portions. - 14 - - 15 - - 16 -  VERTEBRATES TEETH True teeth in vertebrates are integumentary structures homologous with the placoid scales. There are three parts of typical teeth, a crown that projects beyond the gum, roots which are embedded in a socket or alveolus of the jaw, and a neck which is the transitional region between the crown and roots. Inside the hollow tooth is the pulp cavity harboring blood vessels and nerves that enter through a passage way usually remaining open at the base. The growing teeth of Rodents continue to grow because their opening at the base is unobstructed throughout life, and additional tooth material is added from below. In higher vertebrates the pulp cavity becomes so constricted that after certain size the growth ceases. The solid part of the teeth is made up of three types of material. The dentine, a tissue similar to bone and traversed by regular arranged dentinal canals. The dentine is protected by a layer of enamel which is penetrated by minute canals in lower vertebrates, but solid in higher forms. A bone-like substance, the cement, fixes the tooth firmly in the socket of the jaw, and occurs outside the dentine around the roots of the tooth. The cement may extend over the crown as in Ungulates. The number of teeth is indefinite in the lower vertebrates, but in Mammals the number becomes reduced and limited. In most of the lower vertebrates the teeth are being constantly replaced, as successions are numerous and continuous - 17 - throughout life. Such a condition is called polyphyodont dentition and occurs in Dogfishes, in which diminishing rows of teeth are arranged behind the line of active teeth, and they replace the active ones as they get worn out. In Mammals, on the other hand, there are only two successions, and thus the condition is called diphyodont dentition. First there is a milk (deciduous) dentition, which usually erupts after birth, and it is replaced by permanent dentition. A few Mammals develop only the first set of teeth, and this is the monophyodont dentition. There are different modes of attachment of the teeth to the jaws in different vertebrates. In the simplest condition, the acrodont dentition found in fishes and Amphibians, the teeth are rootless, and are attached to the edge of the jaw or other skeletal foundation by fibrous connective. In some Amphibians and Reptiles the teeth are attached to the base and also to the inner side of the jaw bone. This method of attachment is called pleurodont dentition. In some Reptiles and Mammals the teeth may be rooted in bony sockets or alveoli, a condition which is called thecodont, and is considered to be the most efficient mode of attachment of teeth. In fishes the dentition is homodont as all teeth are of the same shape, in contrast to the heterodont dentition of Mammals where teeth are different in shape. In Mammals there are four types of teeth, the incisors or front teeth, the canines, and the cheek teeth made up of premolars and molars. The number of teeth and of those of each kind is important in systematic work, and to express this a dental formula has been derived for each species. As the number of teeth in the two sides is the same, only one side is represent in the formula, while teeth of upper and lower jaws are represented as fractions. Dental formula is: 2 x Incisors. Canines. Premolars. Molars Dental formula of Man is: 2 x I 2/2, C 1/1, P 2/2, M 3/3 =32 Dental formula of Dog is: 2 x I 3/3, C 1/1, P 4/4, M 2/3 = 42 - 18 - Dental formula of Rabbit: 2 x I 2/1, C 0/0, P 3/2, M 3/3 =28 Class: Osteichthyes - Bony Fishes Example: The Nile Boiti, Tilapia nilotica General Characters: 1. The body is bilaterally compressed, composed of head, trunk and tail. Its color varies from pale silvery grey to dark olive. 2. In the head, the mouth is small terminal opening surrounded by 2 jaws, each bears several rows of small teeth (some bony fishes are toothless). The eyes are large without eyelids. In front of the eyes there is 2 small nostrils. On each side of the head there is an operculum covering the gills. 3. The trunk has 2 paired fins (pectoral and pelvic) and unpaired fins i. e. the dorsal, anal, ventral and the homocrocal caudal fin. The fins are supported with fin rays. 4. The skin is covered by dermal scales of the cycloid type. These are nearly circular thin bony discs embedded in the dermis and covered by thin epidermis. These scales are in concentric rings which are indicative of years of growth. In other bony fishes, there may be two other types of scales i. e. the ctenoid and the ganoid scales. The ctenoid scales are very similar to the cycloid except that the outer edges have many small spines. The ganoid type is somewhat square and has a heavy covering of an enamel-like material called ganoid. 5. The endoskeleton is more or less bony. The skull is bony and replaced the primitive cartilaginous skull. It is composed of a great number of bones. Many of these bones have replaced the cartilage and called replacing bones, others are covering bones. The visceral skeleton is composed of 7 - 19 -  paired arches more or less modified. The first or mandibular arch forms the jaws, the second or hyoid arch is modified as a support for the gill covered and the others are the 5 gill arches. The vertebral column is formed of bony vertebrae with amplicoelous centra. Remains of notochord are still present. The appendicular skeleton is formed of the pectoral and the pelvic girdles and the bones which support the unpaired fins. 6. The digestive system begins by the mouth which is surrounded by 2 jaws covered by many conical teeth. There is a small tongue which projects from the floor of the mouth cavity and it functions chiefly as a sensory organ (tactile). The mouth cavity is followed by the pharynx which forms with the mouth a buccopharyngeal cavity. In the pharynx there are 4 pairs of gills. The oesophagus is short and pierces the transverse septum to open into the stomach. The stomach is of the caecal type; its cardiac portion extends backwards into a blind sac called the coecum in which food is stored, while the pyloric portion is small and passes into the intestine which is a long coiled tube ends at the anus. The liver is large bilobed gland with the left lobe much larger than the right. There is a gall-bladder and bile duct which opens at the beginning of the intestine very close to the pylorus. The pancreas is diffused in the adipose tissue of the mesenteries and inside the liver. 7. The respiratory system is composed of 4 pairs of gills supported by the gill arches. Each gill arch bears a double row of gill filaments which are supplied with numerous capillaries. At anterior border of each gill arch there are 3 rows of short teeth-like processes called gill-rakers which protect the delicate filaments from the silt (mud) material which they strain from the respiratory water current. 8. Air bladder or swim bladder: It is found in all bony fishes except a few - 21 -  bottom forms. It originates as a diverticulum from the ventral side of the pharynx. In some forms, the connection persists and these fishes are described as physostomous fishes e. g. Clarias lazera, while in others as in the Bolti, this connection is lost. The air bladder is thin-walled elongated sac lying in the dorsal part of the body below the vertebral column. It is filled with gas and is a hydrostatic organ or "Float" in certain fishes. The gas bladder decreases the specific gravity making the body of the fish equal in weight to the amount of water it displaces. The amount of gas within the air bladder depends upon the pressure of the surrounding water and in some way; it is regulated by the fish according to depth. If a fish is suddenly brought to the surface from a great depth, the air bladder which was under considerable pressure suddenly expands forcing the stomach out of the mouth. In physoclystic fishes, gases are mostly secreted into the bladder by a special anterior gland, red gland, with its remarkable network of blood vessels. A posterior oval gland of the air bladder has the powder to absorb these gases to lessen its size. Other functions have been ascribed to the air bladder as being an organ of sound production, a sense organ or may aid in respiration in some fishes. 9. Circulatory system: The heart lies in the pericardial cavity, it is composed of 3 chambers namely a sinus venosus, an auricle and a muscular ventricle. The ventral aorta swells up and proceeds forward. Below the gills, the ventral aorta gives rise to 4 pairs of afferent branchial arteries which carry the blood to the gills where gaseous exchange takes place. From the gills, the efferent branchial arteries of each side join to form the lateral dorsal aorta. The 2 lateral dorsal aortas unite together anteriorly and posteriorly forming the cephalic circle. The median dorsal aorta proceeds posteriorly distributing the blood throughout the body. - 21 -  The venous blood returns to the sinus venosus. Blood contains nucleated red corpuscles and amoeboid white corpuscles. 10. Excretory system: Two kidneys lie just beneath the vertebral column in the abdominal cavity. They are thin elongated bodies and mostly fused along their entire length. Posteriorly there comes out of each kidney a thin short tube, the mesonephric duct. The 2 ducts unite to form common ducts which rise to a small urinary bladder before it opens on the urinogenital papilla. 11. Nervous system: This in bolti differs from that of the dog fish in many respects. The two olfactory lobes in bolti are relatively small, while the optic lobes and the cerebellum are much larger than those of the dogfish. 12. The principal sense organs are: a) Cutaneous sense organs: The lateral line contains sensory cells which serve to detect vibrations in water and pressure stimuli. b) Olfactory sacs. c) Ear. d) Eyes. 13. The reproductive system: The sexes are separate. Two elongated gonads in the body cavity and each is connected to the anterior body wall by a thin strand. The gonads become very narrow posteriorly forming a hollow duct I. e. vas deferens in the male or oviduct in the female. - 22 - - 23 - - 24 -  TETRAPODA They are four-footed vertebrates. They possess two pairs of limbs, with skeleton based upon a common schematic plan known as the pentadactyle limb skeleton, as the limb terminates with five digits (fingers or toes). The pectoral and pelvic girdles are also built up on the same generalized plan. Tetrapoda includes four classes: Class 1 Amphibia (Anamniota in which the embryo does not develop within amniotic embryonic membrane), Class 2 Reptilia, Class 3 Aves & Class 4 Mammalia (Amniota in which the embryo develop in amniotic embryonic membrane). CLASS AMPHIBIA General Characters: 1- They generally start their life as tadpoles which are fish-like larval stages, breathing by gills, and swimming with median fins. 2- In many cases, as in toads and frogs, they gradually develop into lung-breathing. 3-In toads and frogs, the body is short and broad, and the hind limbs are longer than the fore limbs. 4-The gills of the tadpoles may be retained throughout life, but the adults normally possess functional lungs. 5-The nostrils, through which air enters, open into the mouth. 6-The median fins, frequently present in both larvae and adults, are without fin rays. 7-The skin is generally colored, and devoid of any exoskeleton. When the paired limbs are present, they have distinct digits. The skull has two occipital condyles. - 25 - 8-There are ten pairs of cranial nerves. The heart is formed of five chambers which are a sinus venosus, two auricles, a ventricle and truncus arteriosus. 9-The gut ends in a cloaca into which the ducts from the kidneys and reproductive organs open. 10-The ova are small, numerous, usually pigmented and with rather heavy yolk. They are almost laid in water, and there is usually metamorphosis in development. 11- The throat of the male toad is much darker than that of the female, due to the presence of the darker vocal sacs in males only. In toad, there are three types of respiration which are buccal respiration, cutaneous respiration and pulmonary respiration. The heart is surrounded by the pericardium, and consists of the sinus venosus, right and left auricles, ventricle and truncus arteriosus. The class: Reptilia This class includes Lizards, Chameleons, Vipers, Tortoises, Turtles, Alligators and Crocodiles. These are generally known as creeping animals, according to the presence of relatively short paired limbs, which are attached on the sides of the body in a way that can hardly lift the body off the ground. The reptilian body is divided into regions which are the head, neck, trunk and tail. Two pairs of moderately developed pentadactyle limbs are usually present in Reptiles. These limbs are entirely absent in some lizards and in all snakes, and - 26 - are modified into paddles in turtles. The body is covered by a peculiar type of scales which are known as horny scales. They are produced by the skin, and actually represent thickened areas of the horny layer of the epidermis. This scaly layer is not permanent, but, it is thrown away at intervals, and a new scaly layer is formed below the old one. In snakes the scaly layer of the skin is thrown as one piece, while in lizards it is thrown in small pieces. Some reptiles possess not only horny scales, but also bony scales, which are found inside the dermis. In tortoises and turtles these bony scales become greatly enlarged and fused together forming a bony case which surrounds the body. The reptilian skin is described as a dry skin according to the presence of only one gland, Scent gland, producing odors for attracting the opposite sex. The horny scales covering the head are usually so regular as to be of importance in classification, and are known as head shields. In reptiles, there is a single occipital condyle at the posterior portion of the skull. The reptilian heart consists of a sinus venosus, right and left auricles, and a ventricle which is either incompletely divided by a ventricular septum into right and left chambers. In all reptilia, except order crocodilia, the ventricular septum does not completely divide the ventricle into two chambers. Fertilization is internal, and the eggs are markedly rich in yolk. In most reptiles the eggs are laid enclosed in hard shells (oviparous), and pass through a period of incubation before they hatch, but in some lizards and snakes the eggs complete their development inside the body of the female, and the young are born alive (ovaviviparous). Types of hatching  Oviparous  Viviparous - 27 -   Ovaviviparous CLASS AVES – Birds COLUMBA - PIGEON This class comprises the birds which are vertebrates adapted for aerial life. The body is divided into head, neck, trunk and tail, and external as well as internal structures are modified to suit the process of flight. - 28 -  There are several exoskeleton structures, which are the feathers covering the greater part of the body, the horny scales covering the feet, the claws found at the tips of the digits, and the horny beaks covering the bony beaks. All of these structures are epidermal in origin. The fore limbs are modified into wings which are used for flight. They are moved by the chest muscles which are very well developed. The hind limbs, by means of which they move on land, are attached to the body much forward, thus enabling the bird to be well balanced on land. Many trunk and tail vertebrae, as well as some bones of the fore and hind limbs are fused, giving compound, compact bones more suitable for flight. The bones are generally provided with air spaces, the presence of which lessens the weight of the body. The two clavicles unite to form a furcula, and the sternum has a ventral keel in flying birds. The trunk carries anteriorly the fore limbs or wings, and posteriorly the hind limbs or legs. The hand possesses only three fingers, representing the first, second and third. The foot is provided with four toes, which are the first, second, third and forth. The first toe is directed backwards, while the other toes are directed forwards. The first finger consists of one phalanx, the second of two phalanges, and the third of one phalanx. The digital formula is: I:II:III:IV:V 12100 Four toes are present, representing the first, second, third and fourth, and the digital formula is: I: II: III: IV: V. 2 3 4 5 0 At the posterior end of the trunk, on its ventral surface, is a large, transverse cloacal aperture. The tail is short, and carries an oil gland secreting oily substance used in lubricating the feathers. The feathers are of three principal types, which are the contour feathers - 29 - occuring all over the body, the filoplumes lying at the bases of the contour feathers, and the down feathers which cover the body of the young. The skull is produced into upper and lower beaks carrying no teeth, and posteriorly it carries a single occipital condyle. The skin possesses a single gland lying dorsally at the base of the tail, and known as oil gland. The alimentary canal includes a crop which is a wide sac used for temporary storage of food, and the stomach consists of two parts, the proventriculus which secretes the gastric juices, and the gizzard. The gizzard has thick walls, and small cavity. This cavity contains small stones, which are swallowed by the bird, to help the gizzard in grinding up the food. The heart consists of four chambers, which are two auricles, and two ventricles. There is only one aortic arch which is the right one. There is a special organ of voice, the lower larynx, lying at the base of the trachea. This syrinx is not present in any other class of vertebrata The lungs are connected with air sacs, These are nine in number, two cervical, two anterior thoracic and two posterior thoracic, two abdominal and a single interclavicular between the lungs. The air sacs are connected with the spaces inside the bones. Such arrangement increases the efficiency of respiration, and adds to the lightness of the bird. The urinary bladder is absent, and excretory materials are semisolid. In female, there is a single left ovary, and left oviduct, the right elements are usually atrophied. Fertilization is internal, and the eggs are large, rich in yolk, and enclosed by hard calcareous shells. - 31 - - 31 -. - 32 - Digital formula = 1: 2: 1 Digital formula = 2: 3: 4: 5 - 33 -  CLASS: MAMMALIA The name of this class is derived from the word Mamma, which means breast, and refers to the mammary glands on females that supply milk for suckling the young. The body is divided into head, neck, trunk and, in many cases, a tail. Two pairs of limbs are always present, except in most aquatic Mammals, where the hind limbs are completely missing, or represented by rudiments found between the muscles of the body wall and not visible externally. The limbs terminate in five digits, modified for walking, running, climbing, burrowing, swimming or flying. The digits are provided with homy claws, nails or hooves, and in many cases with fleshy pads. The skin is covered with hair which is not found in any other class of vertebrata. It possesses sweat glands, sebaceous glands, scent glands and mammary glands. The ear consists of external ear or pinna, middle ear and internal ear. The skull has two occipital condyles. The lower jaw is reduced to a single bone, the dentary. While the teeth in fishes. Amphibians and Reptiles are similar, and thus dentition is described as homodont, those of Mammals are different, and thus dentition is described as heterodont. Four different kinds of teeth are present, which are the incisors, canines, premolars and molars. Mastication of food takes place in the mouth cavity, where it is mixed with saliva helping in swallowing and digestion. There is always a muscular septum known as diaphragm, separating the thoracic cavity including the heart and lungs, from the abdominal cavity including all other viscera. - 34 -  The heart is composed of two auricles and two ventricles, and there is a single aortic arch which is the left one. In most Mammals testes are found in scrotal sac, lying outside the abdomen. The ovaries are small, and eggs are generally with no yolk. Fertilization is internal, and the embryo in most cases develops inside the uterus till birth. - 35 - - 36 - - 37 -