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PHYLUM CHORDATA EMMANUEL TAIWO IDOWU Ph.D. Department of Zoology, University of Lagos, Akoka, Nigeria [email protected]  INTRODUCTION Phylum Chordata is the largest of the deuterostome phyla. It is the highest and the most important phylum comprising a vast variety of livi...

PHYLUM CHORDATA EMMANUEL TAIWO IDOWU Ph.D. Department of Zoology, University of Lagos, Akoka, Nigeria [email protected]  INTRODUCTION Phylum Chordata is the largest of the deuterostome phyla. It is the highest and the most important phylum comprising a vast variety of living and extinct animals including man himself. Comprise: fishes, amphibians, reptiles, birds and mammals.  INTRODUCTION Phylum Chordata are among the most successful in the Animal Kingdom. They have adapted themselves to more modes of existence than any other group including arthropods. They are found in the sea, fresh water, in the air and on all parts of land from poles to the equator. All lower chordates are marine, fishes are aquatic and higher chordates are predominantly terrestrial. Chordates are interesting because they illustrate so well the broad biological principles of evolution, development and relationship. Three Fundamental Chordate Characters All chordates possess three outstanding unique characteristics at some stage of their life history. A dorsal hollow or tubular nerve cord A longitudinal supporting rod-like notochord A series of pharyngeal gill slits DIAGRAM SHOWING FUNDERMENTAL CHORDATE CHARACTERS  Dorsal Hollow Nerve Cord  The central nervous system of chordates is present dorsally in the body.  It is in form of a longitudinal hollow tubular nerve chord lying just above the notochord and extending lengthwise in the body.  The nerve cord serves for the integration and coordination of the body activities.  In vertebrates, the anterior region of nerve cord is specialized to form a cerebral vesicle or brain which is enclosed by a protective or cartilaginous cranium.  The posterior part of the nerve cord becomes the spinal cord and protected within the vertebral column.  Notochord  The notochord is an elongated rod-like flexible structure extending the length of the body.  It is present immediately beneath the nerve cord and just above digestive canal.  The notochord is like prime diagnostic features of the Phylum Chordata which derives it name from it.  Protochordates have typical notochord, in adult vertebrates, it is surrounded or replaced by vertebral column (internal skeleton support).  Pharyngeal Gill Slits  In all the chordates at some stage of their life history, a series of paired lateral gill clefts or gill slits perforated through pharyngeal wall of the gut behind the mouth.  They are variously termed as pharyngeal, branchial and visceral clefts or pouches.  They serve primarily for the passage of water from the pharynx to outside, thus bathing the gills for respiration.  The water current secondarily aids on filter feeding by retaining food particles in the pharynx.  In protochordates and lower aquatic vertebrates, the gill slits are functional throughout life. But in higher vertebrates, they become modified in the adults with the acquisition of pulmonary respiration  Advancement of Chordata over other Phyla 1. Living Endoskeleton With the exception of echinoderms and a few others, only chordates possess a living endoskeleton. It grows in size with the rest of the body so that there is no compulsion to shed it periodically to allow for growth like the non-living chitinous exoskeleton of the non-chordate phyla. 2. Efficient Respiration The gills in aquatic chordates and lungs in terrestrial forms efficient organs of respiration. 3. Efficient Circulation The circulatory system of the chordates is well developed, and blood flows freely in the respiratory organs ensuring rapid exchange of gases. Moreover the blood-vascular system forms an important medium for several vital activities of the body. 4. Centralized Nervous System The invertebrate phyla shows a growing tendency of centralization of nervous system, reaching its culmination in the high chordates. The sensory systems is also modified accordingly  Characters Common to Chordates and Higher Non-chordates 1. Axiation: The body in both has a distinct polar axis. The anterior end is differentiated into a cephalic region or head that usually precedes in locomotion. The opposite or posterior end forms a tail in most cases. This longitudinal axis of the body running from head to tail is known as the antero-posterior axis. 2. Bilateral symmetry: Due to existence of longitudinal antero-posterior axis, the body of all chordates and most higher non-chordates (Annelida, Arthropoda) exhibits bilateral symmetry 3. Triploblastic condition: invertebrates above the level of coelenterates and all chordates are triploblastic animals. 4. Coelom: A secondary body cavity or true coelom exist between the body wall and the digestive tube, and it is lined on all sides by mesoderm. 5. Metamerism: Metamerism is a condition in which the body is composed of a linear series of similar body segments called metameres or somites. 6. Organ- systems: In an organ system, several organs work together for the same function such as digestion, circulation, respiration, etc.  Major Subdivision of Phylum Chordata The phylum chordate is first conveniently separated into 3 or 4 primary subdivision called subphyla, based on the character on notochord. Subphylum 1. Hemichordata (or Adelochordata) 2. Urochordata (or Tunicata) 3. Cephalochordata 4. Vertebrata  CLASSIFICATION Hemichordata (half cord) have long been traditionally considered to be the lowest chordates. But recent workers consider the so-called notochord of hermichordates not a true notochord but a Stomochord. Hence Hemichordates are removed from the chordates treated as an independent invertebrate phylum. Subphylum Urochordata ( a tail cord) includes 3 classes (Larvacea, Ascidiacea and Thaliacea) Subphylum Cephalochordata (head cord) includes a single class (Leptocardii) Subphylum Vertebrata (backbone) is subdivided into a classes (Ostracodermi, Cyclostomata, Placodermi, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves and Mammalia)  Classification Protochordata and Euchordata The first two subphyla under phylum chordate (Urochordata and Cephalochordata) are all marine, relatively small and without a vertebral column or backbone. They are often collectively referred as the non-vertebrate or invertebrate chordates or protochordates (first cord) The third subphylum vertebrata provided with a vertebral column is regarded to be more advanced and belongs to the Subdivision Euchordata of the phylum chordata. The protochordates are known as lower chordates while vertebrates or the Euchordates as the higher chordates. Acrania and Craniata The protochordates subphyla lack a head and a cranium, so they are known as Acrania (absent head) On the other hand, the subphylum vertebrata (Subdivision Euchordata) has a distinct head and a cranium and is also called Craniata.  Classification Agnatha (lack true jaws) and paired appendages Agnathans or agnathostomes include a small number of primitive by highly specialized fish-like forms, the extinct ostracoderms and modern cyclostomes. Gnathostomata (true jaws and paired appendages) All other vertebrates have true jaws and paired appendages and are called Gnathostomata. The basic division of Gnathostomata recognizes two super classes: Pisces and Tetrapoda  Classification The superclass: Pisces includes all the fishes which are strictly aquatic forms with paired fins. The superclass: Tetrapoda (four foot) is formed by four legged land vertebrates including : amphibians, reptiles, birds and mammals. Anamnia and Amniota Another method of grouping the vertebrates based on pattern of embryological development. It is based on the presence of a special membrane, the amnion, that holds the developing embryo in a reservoir of fluid, and permits the laying of eggs on lard. The animals that possess it belong to the Amniota which includes the classes Reptilia, Aves and Mammalia. The animal without thin membrane are Anamniota including Cytostomes, Fishes and Amphibians. Sometimes, the anamniotes are referred to as the lower vertebrates, and the amniotes as higher vertebrates Class Polychaeta  Subphylum Hemichordata (Balanoglossus) Habitat: Exclusively marine, solitary or colonial, mostly tubicolous. Body shape: Soft, fragile, vermiform, unsegmented. Body division: Body divided into Proboscis, collar and trunk. Coelom: Enterocoelum, divided into protocoel, mesocoel and metacoel corresponding to the body division. Body wall. Single cell layer. Epidermis with mucous gland. No dermis. Alimentary Canal: Digestive tube complete, straight or U-shaped. Foregut give rise to a hollow buccal diverticulum into proboscis. Respiration: Through body surface. Dorso-lateral pharyngeal gill slits. Ciliary feeders Blood Circulatory System: Simple and open including dorsal heart and two longitudinal vessels, one dorsal, one ventral. Excretion: Excretion by single proboscis gland or glomerulus connected to blood vessels. Nervous System: Primitive, consisting mainly of subepidermal nerve plexus. Dorsal collar nerve hollow. Reproduction: Mainly sexual. Sexes usually separate. Fertilization external in water. Development direct or indirect with a free swimming tornaria larva. Balanoglossus Balanoglossus Balanoglossus Classification of Hemichordata  Subphylum Urochordata (Tunicata) Habitat: Exclusively marine and cosmopolitan. Sedentary (fixed), some pelagic or free swimming. Solitary or colonial (aggregate in groups) Size: 0.25 to 250 mm (shape and colour variable) Adult Stage: Body degenerate, sac-like, unsegmented, without paired appendages and usually without tail. Body covered by a protective tunic or test composed largely of tunicine (C6 H10 O5)n, similar to cellulose. A terminal branchial aperture and a dorsal atrial aperture usually present. Coelom absent. Notochord present in larva tail hence named Urochordata. Alimentary Canal: Complete, pharynx (branchial sac) large with endostyle and two to several pairs of gills-slits. Ciliary feeder. Respiration: Through test and gill slits Blood Vascular System: Blood vascular system is open, Heart simple, tubular and ventral Excretion: Excretion by neural gland, pyloric gland and nephrocytes Nervous System: Dorsal tubular nerve cord only in larval stage, reduced to a single dorsal nerve ganglion in adults. Reproduction: Mostly hermaphrodites, fertilization is cross and external. Development indirect including a free swimming tailed larva with basic chordate characters. Metamorphosis retrogressive. Asexual reproduction by budding common  Subphylum Urochordata (Tunicata) Class I. Ascidiacea:- Tunic with scattered muscles. Many pharyngeal gill-slits Solitary or Colonial, Marine. Size variable, Tail absent in adult. Test well developed, Thick and permanent, no muscle bands present in it. Atrium opens dorsally by atriopore Pharynx large with many permanent gills. Excretion by subneural gland. Sex united, hermaphrodite, protogynous. Asexual reproduction is common Larva developed, undergo retrogressive metamorphosis. Brain reduced to dorsal ganglion. Examples Ascidia metula, Ciona intestinalis, Phallusia inigra (Sea Squirt) Clavelina - Herdmania - (Brown Sea squirt) Styela clava  Subphylum Urochordata (Tunicata) Class - II - Thaliacea Adults free living, solitary or colonial. Pelagic Size variable Thin tests, permanent with circular muscle bands Atriopore is posterior Pharynx with 2 to many gills, opens in atrium Sex united, larva present or absent. Budding common Alternation of generation present, polymorphism present. Examples Pyrosoma atlaticum Doliolum spp Salpa maggiore  Subphylum Urochordata (Tunicata) Class - III - Larvacea Solitary, Free swimming Pelagic Tail, notochord, nerve cord, brain in adults Test temporary, jelly like, periodically shed. Atrium and atriopore absent or reduced. Gill slits 1 or 2 Sex united, no metamorphosis, larva shows paedogenesis. Examples Oikopleura Appendicularia Kowalevskia Ciona Ciona  Subphylum Cephalochordata General Characters All members in shallow marine water. Sedentary, remain buried, nocturnal. 5 to 8 cm, fish like, bilaterally symmetrical, Almost transparent. Metamerism present, enterocoelous.  Subphylum Cephalochordata General Characters 1. Paired appendage absent, median fin present. 2. Exoskeleton absent. 3. Epidermis of single layer, below it are muscle bands arranged in ‘>’ shape called myotomes. 4. Head absent. Body have trunk and tail. 5. Notochord persistent. Present from Rostrum to tail. 6. Alimentary canal complete. Pharynx large with many gill slits that opens in atrium. Filter feeder. 7. Respiration through general surface. No special organs for respiration. 8. Circulatory system well developed, closed, heart absent. 9. Hepatic portal system well developed. 10. Excretion by protonephridia with solenocyte. Hatscheks nephridia at the roof of oral hood, Brown’s funnels on the sides of pharynx and renal rapilla at the floor of atrial cavity also helps in excretion. 11. Nerve cord dorsal, tubular, no ganglion in it. Two cranial and many spinal nerves present. 12. Sex separate, Gonads numerous, metamerically arranged. Gonoduct absent. 13. Asexual reproduction absent. 14. Free swimming larva present  Subphylum Cephalochordata Classification This subphylum have only one class - Leptocardi. It has 2 genus Branchiostoma and Asymetron.  Branchiostoma have 30 species. It is also known as amphioxus or sea lancelets.  These are considered as blue print of phylum chordata as have all the primary  chordate characters even in the adult stage. Only median fins present and are supported by fin ray boxes of connective tissue.  Myotomes : Blocks of striated muscles.  Buccal cavity has a wheel-organ (also called Muller’s organ or rotatory organ). It  develops circular water currents. Excretion is by protonephridia having groups of flame cells called solenocytes  Branchiostoma Branchiostoma Branchiostoma  General Comparison of the three Protochordata Sub Phyla Subphylum-III - Subphylum-I - Hemichordata Subphylum-II - Urochordata S.No. Characters Cephalochordata (Balanoglossus) (Herdmania) (Branhiostoma) 1. Distribution Marine, Worldwide Marine, Worldwide Marine, cosmopolitan Solitary or colonial. Solitary or colonial. Solitary, burrowing and 2. Habits and habitat Mostly tubicolous Fixed or free-swimming free swimming Elongated, cylindrical, Fish-like, laterally 3. Shape Degenerate. bag-Iike wormlike compressed Unsegmented without head Segmented without distinct 4. Body divisions Head, neck, trunk and tail head 5. Postanal tail Absent Absent Present 6. Fins Absent Absent Median Fins 7. Test Absent Present Absent Enterocoelous, divisible into Absent, replaced by atrial 8. Coelom Reduced by atrial cavity three body regions cavity 9. Atrial cavity Absent Greatly developed Well developed Absent, instead stomochord Only in larval tail. absent in Persistent, throughout life, 10. Notochord present adult rod like Metamerically segmented 11. Muscles Unsegmented Unsegmented as myotomes  General Comparison of three Protochordata SubPhylum 12. Digestive tube Complete, straight or coiled Complete, coiled Complete, Straight 13. Oral hood Absent Absent Present Large, complex. with 2 to Large, simple with two Large, complex with several pairs 14. Pharyngeal apparatus several pairs of lateral gill dorsal linear rows of gill slits of lateral gill slits slits 15. Feeding mechanism Ciliary or filter Ciliary Ciliary Absent, Respiration through Absent, Respiration through Absent. Respiration through body 16. Respiratory organs general surface body surface surface Well developed muscular 17. Heart A dorsal heart present Heart absent heart Neural gland, pyloric gland 18. Excretory organels Glomerulus in prob Protonephridia. & nephrocytes 19. Nervous system Intraepidermal below & outside epidermis below & outside epidermis 20. Sexes Separate United Separate Numerous pairs metamerical 21. Gonads One to several pair One pair arranged 22. Gonoducts Absent Present Absent Direct or indirect, with or Indirect with retrograde 23. Development Indirect with metamorphosis without metamorphosis  Subphylum Vertebrata General Characters (1) Lower vertebrates - aquatic, higher vertebrates - predominantly terrestrial. (2) Body medium to large, bilaterally symmetrical and metamerically segmented. (3) Body typically made of head, trunk and a postanal tail. A neck may also be present, especially in the terrestrial forms. (4) Trunk bears typically two pairs of jointed, lateral appendages; may be reduced or absent in some; serve for support, locomotion and other special functions. (5) Body covering or integument is a stratified epithelium made of an outer epidermis and an inner dermis; with many mucous glands in aquatic species. (6) Skin covered by a protective exoskeleton comprising scales, feathers, hairs, claws, nails, horns, etc. (7) Coelom is large, nearly always developed as a schizocoel, and largely filled with the visceral systems. (8) Notochord stops short beneath the forebrain, invested by cartilage or bone or replaced by a vertebral column. (9) A living jointed endoskeleton of bone or cartilage or both, including skull, vertebral column, girdles and limb bones. (10) Many muscles are attached to the endoskeleton for motion and locomotion. (11) Digestive canal more or less convoluted. Liver massive not tubular. Pharyngeal gill-slits not more than 7 pairs except in some cyclostomes. (12) Respiration in lower aquatic forms by paired gills; in terrestrial forms by lungs.  Subphylum Vertebrata (13) Blood vascular system closed. Heart ventral, muscular contractile and consists of 2, 3 or 4 chambers. Blood plasma contains both white and red corpuscles, the latter containing the respiratory pigment, haemoglobin. (14) Excretion by paired kidneys, mesonephric or metanephric, segmental or non- segmental, and discharging through ducts into cloacal or anal region. (15) Anterior end of dorsal nerve cord enlarges into a complex brain, protected by skull. Remaining nerve cord forming the spinal cord surrounded and protected by vertebrae. 10 to 12 pairs of cranial nerves in the head. The ventral nerve is efferent or motor in nature, it carries nerve impulse from the central nervous system to the effector organ. While, the dorsal nerve is mixed. It bears a swelling called spinal ganglion. (16) Special sensory organs include a pair of eyes and a pair of auditory organs, derived in part from brain. (17) An endocrine system of ductless glands scattered through body, regulating body processes, growth and reproduction. (18) Sexes separate. Gonads paired discharging sex cells through ducts opening into or near the anus. (19) Development direct or indirect. There is never a typical invaginate gastrula. Mesoderm arises as paired longitudinal bands, which subsequently become segmented  Phylogeny or Evolutionary History of Vertebrates The evolutionary history and relationship between groups of organisms is termed phylogeny. The concept of phylogeny is used to place animal groups in proper evolutionary sequence. Thus, in phylogenetic order, fishes came before reptiles, and reptiles before the birds and mammals. The results are represented in a phylogenetic tree that provides a visual output of relationships based on shared or divergent physical and genetic characteristics. A phylogenetic tree makes a good model to record the evolutionary history of vertebrates. The vertical dimension or height of the tree represent geological time. The diversification into group is represented by branches. The width of each branch indicates the relative abundance of each group in time. Geological Time Scale Phylogeny or Evolutionary History of Vertebrates  Phylogeny or Evolutionary History of Vertebrates Cambrian and Ordovician periods. The first fossils of vertebrates were found in the rocks of the Ordovician period in the form of the Ostracoderms. These were small jawless, bony, fish-like forms related to the cyclostomes that lived some 480 million years ago. This shows that their chordate ancestors must have existed much before, in the late Cambrian. The scarcity of early vertebrate fossils is probably due to the fact that they evolved mainly in freshwater and did not have as much chance to become fossilized as marine forms did. The Ostracodermi became extinct but some Cyclostomata (modern lampreys and hagfishes) are still with us.  Phylogeny or Evolutionary History of Vertebrates Silurian and Devonian periods. Some fossil fish are found in the Silurian period, but far more are present in the succeeding Devonian period which is known as the Age of the Fishes. Ostracoderms were jawless fish, but during Devonian the first jawed fish, the placoderms, arose. The placoderms became extinct without leaving living representatives. It is likely that early placoderms were ancestors of cartilaginous and bony fishes, both of which have true jaws, but we do not have any direct evidence. Contrary to the former belief, cartilaginous fishes (Chondrichthyes) did not give rise to bony fishes (Osteichthyes).  Phylogeny or Evolutionary History of Vertebrates Carboniferous period. In late Devonian or early Carboniferous period, the lobe- finned bony fishes (Crossopterygii) gave rise to labyrinthodonts or primitive stem Amphibia. They were the first vertebrates to walk on land. The Amphibia became abundant and mutated in many directions during Carboniferous usually known as the Age of Amphibians.  Phylogeny or Evolutionary History of Vertebrates Mesozoic era. In In the early Carboniferous, the very primitive amphibians also gave rise to the primitive reptiles. They reached their peak and great abundance during Mesozoic era, which is aptly known as the Age of Reptiles. They included among others the famed dinosaurs, the ichthyosaurs, and the pterodactyls or flying reptiles. They dominated the world for nearly 130 million years, until the end of the Mesozoic when most of them suddenly became extinct. While only 4 orders of reptiles are living today, as many as 10 orders are represented by their numerous fossil remains. The ancestral mammals were derived from the primitive reptiles during Triassic period. The first birds also appeared in the late Jurassic period and one of their fossils, the Archaeopteryx, had both reptilian as well as avian characteristics.  Phylogeny or Evolutionary History of Vertebrates Cenozoic era. Following the decline of the reptiles during the late Mesozoic, both birds and mammals started flourishing. The mammals became the most diversified of all animals during Cenozoic era which is also called the Age of Mammals. This era started nearly 70 million years ago and is divided into two periods: Tertiary and Quaternary. After reaching their peaks in Tertiary, the mammals are steadily declining in numbers of species even though man, a member of this group, is considered to be the dominant form of life on earth today. Phylogeny or Evolutionary History of Vertebrates Phylogeny or Evolutionary History of Vertebrates  Comparison of Lower Vertebrata and Higher Vertebrata S/N Lower Vertebrata Higher Vertebrata (Anamniota) (Amniota) 1. Lower vertebrates comprise the Higher vertebrates include the classes classes Cyclostomata, Reptilia, Aves and Mammalia forming the Chondrichthyes, Osteichthyes and group Amniota. Amphibia, forming the group Anamniota. 2. Predominantly aquatic Predominantly terrestrial 3. Body typically consists of three Neck usually present so that body typically parts: head, trunk and tail, Neck includes four divisions usually absent. 4. Appendages are 2 pairs of fins or Fins never present. Usually 2 pairs of limbs. pentadactyle limbs 5. Exoskeleton absent or dermal Exoskeleton includes epidermal scales, scales present feathers, hairs, claws, horns etc. 6. Pharyngeal gill clefts mostly persist Gill clefts disappear in the adult. throughout life. 7. Notochord usually persists, Notochord disappears in the adult, becomes invested by cartilage and replaced by a series of bony vertebrata segmented forming the vertebral column.  Comparison of Lower Vertebrata and Higher Vertebrata 8. Endoskeleton mostly cartilaginous. Endoskeleton mostly bony 9. Heart made of 2 or 3 chambers Heart of 4 chambers except in most reptiles. 10. Poikilothermous or cold blooded Homoiothermous or warm blooded except reptiles 11. Respiration usually by gills Respiration by lungs 12 Kidneys is mesonephric Kidneys is metanephric 13 Cranial nerves 10 pairs Cranial nerves 12 pairs 14 Male without copulatory organ Male with a copulatory organ except in most birds 15 Fertilization external Fertilization internal 16 Development includes Development indirect without metamorphosis metamorphosis 17 Amnion does not appear during A special membrane called amnion development hence Anamniota. present during development hence the name Amniota  References Young, J.Z. (1981). The Life of Vertebrates. Third edition. Oxford University Press, Walton Street, Oxfordox2 6DP. pp 645. Kardong, K.V. (1994). Vertebrates Comparative Anatomy, Function, Evolution. Fifth edition. McGRAW Hill International edition. pp779. Koptal, R.L. (2006). Modern Textbook of Zoology Vertebrates. Third edition. Capital offset Press, New Delhi, India. pp 861. Yoloye, V.L.A. (1985). Undergraduate Lecture notes on Vertebrate Zoology, University of Ilorin, Kwara State. unpublished.