Classification of Chordates

Phylum Chordata

General Characters

(1) Aquatic, aerial or terrestrial. All free-living with no fully parasitic forms.

(2) Body small to large, bilaterally symmetrical and metamerically segmented.

(3) A post anal tail usually projects beyond the anus at some stage and may or may not persist in the adult.

(4) Exoskeleton often present well developed in most vertebrates.

(5) Body wall triploblastic with 3 germinal layers : ectoderm, mesoderm and endoderm.

(6) Coelomate animals having a true coelom, enterocoelic or schizocoelic in origin.

(7) A skeletal rod, the notochord, present at some stage in life cycle.

(8) A cartilaginous or bony, living and jointed endoskeleton present in the majority of members (vertebrates).

(9) Pharyngeal gill slits present at some stage may or may not be functional.

(10) Digestive system complete with digestive glands.

(11) Blood vascular system closed. Heart ventral with dorsal and ventral blood vessels. Hepatic portal system well developed.

(12) Excretory system comprising proto-or meso- or meta-nephric kidneys.

(13) Nerve cord dorsal and tubular. Anterior end usually enlarged to form brain.

(14) Sexes separate with rare exceptions.

Classification of chordata : Phylum chordata can be divided into two groups: Acrania (Protochordata) and Craniata (Euchordata) having contrasting characters.

Group A. Acrania (Protochordata) : (Gk. a = absent kranion = head,) or, (Gk. protos = first chorde = cord). All marine, small, Primitive or lower chordates. Lacking a head, a skull or cranium, a vertebral column, jaws and brain. About 2,000 species. The Acrania is divided into three subphyla: Hemichordata, Urochordata and Cephalochordata, chiefly on the character of notochord present.

Subphylum I. Hemichordata : (Gk. hemi = half chorde =cord). Body divided into 3 regions: Proboscis, collar and trunk. Notochord doubtful, short, confined to proboscis and non–homologous with that of chordates.

Class 1. Enteropneusta : (Gk. enteron = gut pneustos = breathed). Body large and worm-like. Gill slits numerous. Intestine straight. Acorn or tongue worms. 70 species. e.g. Balanoglossus, Saccoglossus.

Class 2. Pterobranchia : (Gk. pteron = feather branchion = gill). Body small and compact. Gill-slits one pair or none. Intestine U–shaped. Pterobranchs. 20 species. e.g. Cephalodiscus, Rhabdopleura.

Subphylum II. Urochordata or Tunicata : (Gk. oura = a tail) (L. chorda = cord). Notochord and nerve cord only in tadpole-like larva. Adult sac-like, often sessile and encased in a protective tunic. Tunicates.

Class 1. Ascidiacea : Sessile tunicates with scattered muscles in tunic. Solitary, colonial or compound. Gill-clefts numerous. Ascidians or sea squirts. 1,200 species. e.g. Herdmania, Ciona, Molgula.

$\bullet$ Retrograssive metamorphosis present in Herdmania.

Class 2. Thaliacea : Free-swimming or pelagic tunicates with circular muscles in tunic. Sometimes colonial. Salps or chain tunicates. 30 species. Salpa, Doliolum, Pyrosoma.

Subphylum III. Cephalochordata : (Gk. kephale = head) or (L. chorda = cord). Notochord and nerve cord present throughout life along entire length of body.

Class Leptocardii : Body fish-like, segmented with distinct myotomes and numerous gill-slits. Free swimming and burrowing. Lancelets. 30 species. e.g. Branchiostoma (= Amphioxus), Asymmetron.

Group B. Craniata (Euchordata) : Aquatic or terrestrial, usually large-sized, higher chordates or vertebrates with distinct head. Notochord is embryonic, in adult replaced by vertebral column. Jaws and brain protected by a skull or cranium. The Craniata includes a single subphylum, the vertebrata.

Subphylum IV. Vertebrata : (L. vertebratus = backbone). Notochord supplemented or replaced by a vertebral column or backbone composed of overlapping vertebrae. Body divisible into head, neck, trunk and tail. Usually dioecious. Vertebrates, largest chordate subphylum including about 46,500 species. The subphylum Vertebrata is divided into two divisions: Agnatha and Gnathostomata, with contrasting characters as follows

Division I. Agnatha : (Gk. a, not gnathos, jaw). Jaw less primitive fish-like vertebrates without true jaws and paired limbs.

Class 1. Ostracodermi. (Gk. ostrakon = shell derma = skin). Several extinct orders of ancient primitive heavily armoured, Palaeozoic, world&aposs first vertebrates, collectively called the ostracoderms. e.g. Caphalaspis, Drepanaspis.

Class 2. Cyclostomata. (Gk. cyklos = circular stoma = mouth). Body eel-shaped, without scales, jaws and lateral fins. Mouth rounded and suctorial. Gills 5� pairs. Parasites and scavengers. 45 species. e.g. Lampreys (Petromyzon) and hagfishes (Myxine).

Division II. Gnathostomata : (Gk. gnathos = jaw stoma = mouth). Jawed vertebrates having true jaws and paired limbs.

For convenience, some taxonomists further divide Gnathostomata division into two super classes. All the fishes like aquatic gnathostomes are placed in the superclass Pisces, whereas all the four-footed terrestrial gnathostomes in the superclass Tetrapoda. Their contrasting features are as follows:

Superclass 1. Pisces

(1) Exclusively aquatic gnathostome vertebrates.

(2) Paired limbs, if present, as fins.

(3) Median fins present

(4) Skin usually moist and scaly

(5) Respiration aquatic, by gills

(6) Sense organs functional in water

(7) It consist of fishes only.

Superclass 2. Tetrapoda

(1) Aquatic or terrestrial. Some arboreal and aerial

(2) Paired pentadactyle limbs present

(3) Median fins absent

(4) Skin usually dry and cornified

(5) Respiration aerial, by lungs

(6) Sense organs functional in air.

(7) It consist of classes Amphibia, Reptilia, Aves and Mammals.

Superclass Pisces

Class 1. Chondrichthyes (The Cartilaginous Fishes)

(Gk. chondros = cartilage ichthys = fish)

General characters.

(1) Mostly marine and predaceous.

(2) Body fusiform or spindle shaped.

(3) Fins both median and paired, all supported by fin rays. Pelvic fins bear claspers in male. Tail heterocercal.

(4) Skin tough containing minute placoid scales and mucous glands.

(5) Endoskeleton entirely cartilaginous, without true bones. Notochord persistent. Vertebrae complete and separate. Pectoral and pelvic girdles present.

(6) Mouth ventral. Jaws present. Teeth are modified placoid scales. Stomach J-shaped. Intestine with spiral valve.

(7) Respiration by 5 to 7 pairs of gills. Gill-slits separate and uncovered (except, chimaeras). Operculum absent. No air bladder and lungs.

(8) Heart 2𠄼hambered (1 auricle and 1 ventricle). Sinus venosus and conus arteriosus present. Both renal and portal systems present. Temperature variable (poikilothermous or cold blooded or ectothermal animal.

(9) Kidneys mesonephric or opisthonephric. Excretion ureotelic. Cloaca present.

(10) Brain with large olfactory lobes and cerebellum. Cranial nerves 10 pairs.

(11) Olfactory sacs do not open into pharynx. Membranous labyrinth with 3 semicircular canals. Lateral line system present.

(12) Sexes separate. Gonads paired. Gonoducts open into cloaca. Fertilization internal. Oviparous or ovoviviparous. Eggs large, yolky. Cleavage meroblastic. Development direct, without metamorphosis.

Classification of Chondrichthyes

(a) Subclass I. Selachii : (Gk., selachos, a shark)

(1) Multiple gill slits on either side protected by individual skin flaps.

(2) A spiracle behind each eye.

(3) Cloaca present.

Examples : True sharks. Dogfishes (Scoliodon, Chiloscyllium, Mustelus, Carcharinus), spiny dogfish (Squalus) seven gilled shark (Heptanchus), Stegostoma, Sphyrna, Rhineodon. Skates and rays. Skate (Raja) Trygon, Torpedo, Myliobatis, Rhinobatus, Pristis.

First dorsal fin

$\bullet$ Zebra shark (Stegostoma) is the most beautiful fish in the sea.

(b) Subclass II. Holocephali : (Gk., holos, entire + kephale, head)

(1) Single gill opening on either side covered by a fleshy operculum.

(2) No spiracles, cloaca and scales.

(3) Jaws with tooth plates.

(4) Single nasal opening.

(5) Lateral line system with open groove.

Examples : Hydrolagus (= Chimaera).

Class 2. Osteichthyes–(The Bony fishes)

(Gk. osteon = bone ichtyes = fish)

General Characters

(1) Inhabit all sorts of water-fresh, brackish or salt warm or cold.

(2) Body spindle-shaped and streamlined.

(3) Fins both median and paired, supported by fin rays of cartilage or bone. Tail usually homocercal.

(4) Skin with may mucous glands, usually with embedded dermal scales of 3 types ganoid, cycloid or ctenoid. Some without scales. No placoid scales.

(5) Endoskeleton chiefly of bone. Cartilage in sturgeons and some other. Notochord replaced by distinct vertebrae Pelvic girdle usually small and simple or absent. Claspers absent.

(6) Mouth terminal or sub terminal. Jaws usually with teeth. Cloaca lacking, anus present.

(7) Respiration by 4 pairs of gill on body gill arches, covered by a common operculum on either side.

(8) An air (swim) bladder often present with or without duct connected to pharynx. Lung-like in some (Dipnoi).

(9) Ventral heart 2-chambered (1 auricle + 1 ventricle). Sinus venosus and conus arteriosus present. Aortic arches 4 pairs. Erythrocytes oval, nucleated. Temperature variable (poikilothermous).

(10) Adult kidneys mesonephric. Excretion ureotelic.

(11) Brain with very small olfactory lobes, small cerebrum and well developed optic lobes and cerebellum. Cranial nerves 10 pairs.

(12) Well developed lateral line system. Internal ear with 3 semicircular canals.

(13) Sexes separate. Gonads paired. Fertilization usually external. Mostly oviparous, rarely ovoviviparous or viviparous. Eggs minute to 12 mm. Cleavage meroblastic. Development direct, rarely with metamorphosis.

Classification of Osteichthyes

(a) Subclass I. Sarcopterygii : (Gk., sarcos = fleshy pterygium = fin)

(1) Paired fins leg-like or lobed. With a fleshy, bony central axis covered by scales.

(2) Dorsal fins 2. Caudal fin heterocercal with an epichordal lobe.

(3) Olfactory sacs usually connected to mouth cavity by internal nostrils or choanae, hence the previous name of subclass, choanichthyes (Gk. choana = funnel ichthyes = fish).

(4) Popularly called fleshy or lobe-finned, or air breathing fish. Divided into 2 superorders or orders: Crossopterygii and Dipnoi.

Order 1. Crossopterygii : (Gk. crossoi = a fringe pteryx = fin)

(1) Paired fins lobate. Caudal fin 3–lobed.

(2) Premaxillae and maxillae present.

(3) Internal nares present or absent. Spiracles present.

(4) Air bladder vestigial.

Example : Primitive fleshy-finned extinct fishes. Single living genus Latimeria.

Order 2. Dipnoi : (Gk. di = double pnoe = breathing)

(1) Median fins continuous to form diphycercal tail.

(2) Premaxillae and maxillae absent.

(3) Internal nares present and spiracles absent.

(4) Air bladder single or paired, lung-like

Examples : Lung fishes. Only 3 living genera : Epiceratodus (Neoceratodus), Protopterus and Lepidosiren

(b) Subclass II. Actinopterygii : (Gr. actis = ray pteryx = fin)

(1) Paired fins thin, broad, without fleshy basal lobes, and supported by dermal fin rays.

(2) One dorsal fin, may be divided.

(3) Caudal fin without epichordal lobe.

(4) Olfactory sacs not connected to mouth cavity.

(5) Popularly called ray-finned fishes. Divided into 3 infraclasses or superorders: Chondrostei, Holostei and Teleostei.

Superorder A. Chondrostei : (Gk. chondros = cartilage osteon = bone)

(1) Mouth opening large.

(2) Scales usually ganoid.

(3) Tail fin heterocercal.

(4) Primitive ray-finned fish or cartilaginous ganoids.

Examples : Acepenser (Sturgeon), Polyodon (paddlefish)

Superorder B. Holostei : (Gk. holos = entire osteon = bone)

(1) Mouth opening small.

(2) Ganoid or cycloid scales.

(3) Tail fin heterocercal.

(4) Intermediate ray-finned fish, transitional between Chondrostei and Teleostei

Examples : Lepisosteus (garpike)

Superorder C. Teleostei – (Gk. teleos = complete osteon = bone)

(1) Mouth opening terminal, small.

(2) Scales cycloid, ctenoid or absent.

(3) Tail fin mostly homocercal.

(4) A hydrostatic swim bladder usually present.

(5) Advanced or modern ray-finned fishes

Examples – Harpodon, Cyprinus, Labeo rohita, Catla, Botia, Carassius, Clarius, Heteropneustes or Saccobranchus, Wallago, Mystus, Electrophorus, Anguilla, Muraena (moray) Hemirhamphus (half beak), Belone (garfish), Hippocampus (sea horse), Syngnathus, Fistularia Ophiocephalus or channa Amphipbnous, Symbranchus. Mastacembelus, Macrognathus, Pterois, Pleuronectes, Synaptura, Solea, Echeneis or Remora, Tetrodon

$\bullet$ Hippocampus (Sea horse) shows parental care. On the belly of male is a brood pauch for incubating eggs.

$\bullet$ Remora (Echeneis) has modified dorsal fin into a sucker. It attaches to the body of shark, whales, etc. (commensalism).

Class Amphibia – The vertebrates with Dual life

(Gk. Amphi = both bios = Life)

General characters

(1) Aquatic or semi aquatic (freshwater), air and water breathing, carnivorous, cold𠄻looded, oviparous, tetrapod vertebrates.

(2) Head distinct, trunk elongated. Neck and tail may be present or absent.

(3) Limbs usually 2 pairs (tetrapod), some limb less Toes 4-5 (pentadactyle) or less. Paired fins absent. Median fins, if present, without fin rays.

(4) Skin soft, moist and glandular. Pigment cells (chromatophores) present.

(5) Exoskeleton absent. Digits claw less. Some with concealed dermal scales.

(6) Endoskeleton mostly bony. Notochord does not persist. Skull with 2 occipital condyles. i.e. Dicondylic skull.

(7) Mouth large. Upper or both jaws with small homodont teeth. Tongue often protrusible. Alimentary canal terminates into cloaca.

(8) Respiration by lungs, skin and mouth lining. Larvae with external gills which may persist in some aquatic adults.

(9) Heart 3𠄼hambered (2 auricles + 1 ventricle). Sinus venosus present. Aortic arches 1-3 pairs. Renal and hepatic portal systems well developed Erythrocytes large, oval and nucleated. Body temperature variable (poikilothermous).

(10) Kidneys mesonephric. Urinary bladder large. Urinary ducts open into cloaca. Excretion ureotelic.

(11) Brain poorly developed. Cranial nerves 10 pairs.

(12) Nostrils connected to buccal cavity. Middle ear with a single rod-like ossicle, columella. Larval forms and some aquatic adults with lateral line system.

(13) Sexes separate. Male without copulatory organ Gonoducts open into cloaca. Fertilization mostly external. Females mostly oviparous.

(14) Development indirect. Cleavage holoblastic but unequal. No extra𠄾mbryonic membranes. Larva a tadpole which metamorphoses into adult.

Classification of Amphibia : The living amphibians belong to only 2,500 species, a very much smaller number than that of other principal classes of vertebrates. Ranging from mid-Palaeozoic (Devonian) to early Mesozoic (Triassic). They dominated the World during Carboniferous, but most of them have become extinct since long. The classification most generally followed nowadays was provided by G. Kingsley Noble (1924).

(a) Subclass I. Stegocephalia (Extinct) : Limbs pentadactyle. Skin with scales and bony plates. Skull with a solid bony roof leaving openings for eyes and nostrils. Permian to Triassic.

Order 1. Labyrinthodontia : Oldest known tetrapods called stem Amphibia. Carboniferous to Triassic.

Example : Eryops.

Order 2. Phyllospondyli : Small salamander-like. Carboniferous to permian.

Example : Branchiosaurs (Ichthyostega).

Order 3. Lepospondyli ­– Small salamander or eel-like.Carboniferous to Permian. 

Examples – Diplocaulus, Lysorophus.

(b) Subclass II. Lissamphibia (living) : Modern Amphibia lacking dermal bony skeleton. Teeth small, simple.

Order 1. Gymnophiona or Apoda : (Gk. gymnos = naked ophioneos = serpet-like).

(1) Limb less, blind, elongated worm like, burrowing tropical forms known as caecilians or blind worms. (2) Tail short or absent, cloaca terminal.

(3) In some dermal scales embedded in skin which is transversely wrinkled.

(4) Skull compact, roofed with bone.

(5) Limb girdle absent.

(6) Males have protrusible copulatory organs,

Examples : Ichthyophis, Uroaeoryphlus, Rhinatremam, Typhlonectes.

$\bullet$ Ichthyophis is a Limb less amphibian showing parental care. It has no tongue.

Order 2. Urodela or Caudata : (Gk. Ura = tail delos = visible) or (L. = cauda = tail). (1) Lizard-like amphi- bians with a distinct tail.

(2) Limbs 2 pairs, usually weak, almost equal.

(3) Skin devoid of scales and tympanum.

(4) Gills permanent or lost in adult.

(5) Males without copulatory organs.

(6) Larvae aquatic, adult-like, with teeth.

(7) It mainly includes Newts and Salamanders.

Examples : Cryptobranchus, Megalobatrachus, Ambystoma, Salamandra, Desmognathus, Amphiuma, Plethodon. Siren, Pseudobranchus, Triturus, Necturus.

$\bullet$ Larva of Ambystoma is axolotl. It shows neoteny or paedogenesis.

$\bullet$ The main difference between gymnophiona and urodela is that urodela have smooth moist skin.

Order 3. Salientia or Anura : (L., saliens = leaping ) or (Gk., an = without nura = tail)

(1) Specialized amphibia without tail in adults.

(2) Hind limbs usually adapted for leaping and swimming.

(3) Adults without gills or gill openings.

(4) Eyelids well-formed. Tympanum present.

(5) Skin loosely-fitting, scale less mandible toothless.

(6) Pectoral girdle bony. Ribs absent or reduced. Vertebral column very small of 5𠄹 pre sacral vertebrae and a slender urostyle.

(7) Fertilization always external.

(8) Fully metamorphosed without neotenic forms.

(9) It mainly includes frogs and toads.

Examples : Alytes, Bombinator, Discoglossus, Pipa, Xenopus, Pelobates, Scaphiopus, Bufo, Rhinoderma, Dendrobates, Hyla, Gastrotheca, Rana, Polypedates or Rhacophorus.

$\bullet$ Bufo is a poisonaus amphibian.

$\bullet$ Bombinator is famous for warning colouration.

$\bullet$ Xenopus is used as a test in diagnosis of human pregnancy.

Class Reptilia –The creeping vertebrates

(L. reptare = to creep)

General Characters

(1) Predominantly terrestrial, creeping or burrowing, mostly carnivorous, air𠄻reathing, cold𠄻looded, oviparous and tetrapodal vertebrates.

(2) Body bilaterally symmetrical and divisible into 4 regions-head, neck, trunk and tail.

(3) Limbs 2 pairs, pentadactyle. Digits provided with horny claws. However, limbs absent in a few lizards and all snakes.

(4) Exoskeleton of horny epidermal scales, shields, plates and scutes.

(5) Skin dry, cornified and devoid of glands.

(6) Mouth terminal. Jaws bear simple conical teeth. In turtles teeth replaced by horny beaks.

(7) Alimentary canal terminates into a cloacal aperture.

(8) Endoskeleton bony. Skull with one occipital condyle (monocondylar). A characteristic T–shaped inter clavicle present.

(9) Heart usually 3𠄼hambered, 4𠄼hambered in crocodiles. Sinus venosus reduced. 2 systemic arches present. Red blood corpuscles oval and nucleated. Cold𠄻looded.

(10) Respiration by lungs throughout life.

(11) Kidney metanephric. Excretion uricotelic.

(12) Brain with better development of cerebrum than in Amphibia. Cranial nerves 12 pairs.

(13) Lateral line system absent. Jacobson&aposs organs present in the roof of mouth.

(14) Sexes separate. Male usually with a muscular copulatory organ.

(15) Fertilization internal. Mostly oviparous. Large yolky meroblastic eggs covered with leathery shells, always laid on land. Embryonic membranes (amnion, chorion, yolk sac and allantois) appear during development. No metamorphosis. Young resemble adults.

(16) Parental care usually absent

Classification of Reptilia : According to Bogert, there are more than 7,000 living and several extinct species of reptiles, grouped into approximately 16 orders of which only 4 are living.

(a) Subclass I Anapsida – Primitive reptiles with a solid skull roof. No temporal openings.

Order 1. Chelonia or Testudinata : (Gk. chelone = turtle) or (L. testudo = turtle)

(1) Body short, broad and oval.

(2) Limbs clawed and or webbed, paddle-like.

(3) Body encased in a firm shell of dorsal carapace and ventral plastron, made of dermal bony plates. Thoracic vertebrae and ribs usually fused to carapace.

(4) Skull anapsid, with a single nasal opening and without a parietal Foramen. Quadrate is immovable.

(5) No sternum is found.

(6) Teeth absent. Jaws with horny sheaths.

(7) Cloacal aperture a longitudinal slit.

(8) Heart incompletely 4𠄼hambered with a partly divided ventricle.

(9) Copulatory organ single and simple.

(10) About 400 species of marine turtles, freshwater terrapins and terrestrial tortoises.

Examples : Chelone, Chrysemys, Testudo, Trionyx, Dermochelys.

(b) Subclass II Euryapsida (extinct) : Skull with a single dorso–lateral temporal opening on either side bounded below by postorbital and squamosal bones.

(c) Subclass III Parapsida (extinct) : Skull with a single dorso–lateral temporal opening on either side bounded below by the supra temporal and post frontal bones.

(d) Subclass IV Synapsida (extinct) : Skull with a single lateral temporal opening on either side bounded above by the postorbital and squamosal bones.

(e) Subclass V Diapsida (Living) : Skull with two temporal openings on either side separated by the bar of postorbital and squamosal bones.

Order 2. Rhynchocephalia : (L. rhynchos = snout Gk. kephale = head)

(1) Body small, elongated, lizard-like.

(2) Skull diapsid. Parietal foramen with vestigeal pineal eye present. Quadrate is fixed.

(3) Vertebrae amphicoelous or biconcave. Numerous abdominal ribs present.         

(4) Teeth acrodont. Cloacal aperture transverse.

(5) Heart incompletely 4𠄼hambered.

(6) No copulatory organ is male.

Example : Represented by a single living species, the “Tuatara” or Sphenodon punctatum of New Zealand.

$\bullet$ Sphenodon is referred to a “living fossil” because it has retained many primitive characteristics of fossil or stem reptiles.

Order 3. Squamata : (L. squama = scale or squamatus = scaly)

(1) Advanced, small to medium, elongated.

(2) Skull diapsid. Quadrate movable.

(3) Vertebrae procoelous. Ribs single – headed.

(4) Heart incompletely 4𠄼hambered.

(5) Cloacal aperture is transverse.

(6) It includes snakes and lizards.

Examples : Phrynosoma, Draco Hemidactylus, Heloderma, Chameleon, Ophisaurus, Anguis, Rhineura, Barkudia Geko, Iguana, Varanus komodoeusis, Python, Typhlops, Eryx johuii, Naja, Bungarus caeruleus, Dryophis, Vipera russelli. Hydrophis, Enhydrina, Crolatus, Ancistrodon, Micrurus, Lachesis, etc.

Order 4. Crocodilia : (Gk. krokodeilos = Crocodile)

(1) Skin thick with scales bony plates and scutes.

(2) Skull diapsid. Quadrate immovable. No parietal foramen. A pseudopalate present. Pineal gland absent.

(3) Ribs bicephalous. Abdominal ribs present.

(4) Heart completely 4𠄼hambered.

(5) Cloacal aperture is a longitudinal slit.

Examples : Crocodylus, Gavialis, Alligator

$\bullet$ The lung cavity of crocodile is separated from rest of body cavity by a muscular diaphragm, analogous to that of mammals.

Class Aves – The Birds

(L. avis = bird) or (Gk. ornis = bird)

General Characters

(1) Feather-clad, air-breathing, warm-blooded, oviparous, bipedal flying vertebrates.

(2) Limbs are two pairs. Forelimbs are modified as wings for flying. Hind limbs or legs are large, and variously adapted for walking, running scratching, perching, food capturing, swimming or wading, etc.

(3) Exoskeleton is epidermal and horny.

(4) Skin is dry and devoid of glands except the oil or preen gland at the root of tail.

(5) Pectoral muscles of flight are well developed.       

(6) Skull smooth and monocondylic, bearing a single occipital condyle. Cranium large and dome-like. Sutures indistinct.

(7) Vertebral column short. Centra of vertebrae heterocoelous (saddle-shaped).

(8) Sternum large, usually with a vertical, mid ventral keel for attachment of large flight muscles.

(9) Ribs double-headed (bicephalous) and bear posteriorly directed uncinate processes.

(10) Both clavicles and single inter clavicle fused to form a V–shaped bone, called furcula or wishbone or merry-thought bone.

(11) Heart completely 4𠄼hambered. There are neither sinus venosus or truncus arteriosus. Only right aortic (systemic) arch persists adult. Renal portal system vestigial. Blood corpuscles nucleated.

(12) Birds are the first vertebrates to have warm blood. Body temperature is regulated (homoiothermous).

(13) Respiration by compact, spongy, nondistensible lungs continuous with thin air-sacs.

(14) Larynx without vocal cords. A sound box or syrinx, producing voice, lies at or near the junction of trachea and bronchi.

(15) Kidneys metanephric and 3–lobed. Uterus open into cloaca. Urinary bladder absent. Birds are urecotelic. Excretory substance of urates eliminated with faeces.        

(16) Sexes separate. Sexual dimorphism is well marked in some birds like peacock and parrot.

(17) Fertilization internal, preceded by copulation and courtship. Females oviparous.

(18) Eggs develop by external incubation. Cleavage discoidal, meroblastic. Development direct, Extra-embryonic membranes (amnion, chorion, allantois and yolk-sac) present (Amniota).

(19) Parental care is well marked.

Classification of Aves : Birds show less diversification than any other group of vertebrate animals. 25 to 30 avian orders are recognized depending on the taxonomist. Class Aves is first divided into two subclasses.

(a) Sub-Class I. Archaeornithes : (Gk. archios = ancient ornithos = bird)

(1) Extinct, archaic, Jurassic birds of Mesozoic Age, about 155 million years ago.

(2) Wings primitive, with little power of flight.

(3) Vertebrae amphicoelous.

(4) Sternum without a keel.

(5) Thoracic ribs slender, without uncinate processes. In Archacopteryx beak in toothed.

This sub-classes includes a single order

Order Archaeopterygiformes : Example : Archaeopteryx lithographica, from Jurassic or Bavaria, Germany one specimen lying in the British museum, London, the other lying in the Berlin.

(b) Sub-class II. Neornithes : (Gk. neos = modern ornithos = Birds)

(1) Modern as well as extinct post-Jurassic birds.

(2) Wings usually well-developed and adapted for flight, with few exceptions.

(3) Teeth absent except in some fossil birds.

(4) Vertebrae heterocoelous in living forms.

(5) Sternum usually with a keel.

(6) Thoracic ribs usually with uncinate processes.

(7) Abdominal ribs absent

This sub-class is divisible into 4 super-orders:

Super-order 1. Odontognathae : (Gk. odontos = teeth)

(1) Extinct, Upper Cretaceous birds.             

(2) Jaws bear teeth, “so advantageous for catching fish”.

Order 1. Hesperornithiformes

Example – Hesperornis, Enaliornis, Baptornis, etc.,

Order 2. Ichthyornithiformes

Examples – Ichthyornis, Apatornis.

Super-order 2. Palaeognathae or Ratitae : (Gk. palaios = old gnathos = jaw) or (L. ratis = raft).

(1) Modern big-sized, flightless, running birds, without teeth.

(2) Wings vestigial or rudimentary feathers devoid of interlocking mechanism.

(3) Rectrices absent or irregularly arranged.

(4) Oil gland is absent, except in Tinamus and Kiwi.

(5) Skull is dromaeognathous or palaeognathous that is, vomer is large and broad and interpolated between palatines.

(6) Sternal keel vestigial, absent or flat, raft-like.

(7) Uncinate processes are vestigial or absent.

(8) Clavicles are small or absent.

(9) Pectoral muscles poorly developed.

(10) Syrinx is absent

The flightless birds or ratites are not represented in India. They are grouped in 7 orders as follows

Order 1. Struthioniformes : (Gk. struthio = ostrich + form)

(1) Legs strongly developed, each with two toes (3rd and 4th) with stunted nails.

(2) Pubes form a ventral symphysis.

Examples : True ostriches (Struthio camelus) of Africa and western Asia (Arabia).

Order 2. Rheiformes : (Gk. Rhea = mother of Zeus + form)

Examples : American ostriches or common rhea (Rhea americana) represented by two species in South American pampas Darwin&aposs rhea (Pteroncemia pennata).

Order 3. Casuariformes

Examples : Cassowaries (Casuarius) of australia, and New Guinea and Emus (Dromaius novaehollandiae) of New Zealand.

Order 4. Apterygiformes

Examples : Kiwis (Apteryx) of New Zealand.

Order 5. Dinornithiformes

Examples : Moas (Dinornis maximus) of New Zealand

Order 6. Aepyornithiformes

Examples : Giant Elephant-birds (Aepyornis titan) Mulleornis of Africa and Madagascar.

Order 7. Tinamiformes

Examples : Tinamou (Tinamus), Eudromia

Super-order 3. Impennae

Orderਁ. Sphenisciformes

Examples : Penguins (Aptenodytes) Southern Hemisphere.

Super order 4. Neognathae

(1) Most modern, usually small-sized. Flying birds.

(2) Wings well-developed feathers with interlocking mechanism.

(3) Rectrices present and arranged regularly.

(4) Pterylae are regular.

(5) Oil gland is present.

(6) Skull is neognathous, that is, vomer is short allowing palatines to meet.

(7) Sternum with a well-developed keel.

(8) Uncinate processes are present.

(9) Pygostyle is present

The super-order Neognathae includes several orders. For the sake of study they may be grouped into at least 6 homogeneous ecological groups, as follows :

Group A. Arboreal Birds : Under this group may be placed the majority of birds spending most of their lives in and around shrubs and trees.

Order 1. Passeriformes : (L. passer = sparrow + form)

This is the largest of all the bird orders including half the known species. Feet are adapted for perching, while beaks are adapted for cutting.

Examples : Passer domesticus, Corvus splendens, common myna (Acridotheres ttristis).

Order 2. Piciformes : (L. picus = wood pecker + form) It includes woodpeckers, toucans, sap-suckers and their allies.

Examples : Yellow fronted pied woodpecker (Dendrocopos mahrattensis).

Order 3. Columbiformes : (L. = columba = dove + form) It includes doves and pigeons.

Examples : Blue rock pigeon (Columba livia). Green pigeon (Crocopus), extinct dodo (Raphus).

Order 4. Psittaciformes : (L. = psitacus = parrot + form)

It includes parrots, parakeets, cockatoos, macaws, love-birds, etc., denizens of the equatorial jungles. Examples : Large Indian parakeet (Psittacula eupatria), green parrot (psittacula krameri).

Group B. Terrestrial Birds : These birds are perfectly able to fly but spend most of their time walking or running on ground. Order 5. Galliformes : (L. gallus = a cock + form) It includes gamebirds notable for their palatability, massive scratching feet, short and powerful flight and largely graminivorous diet.

Examples : Red jungle fowl (Gallus), peafowl (Pavo cristatus). 

Order 6. Cuculiformes : (L. = cuculus = cuckoo + form) It includes cuckoos and their allies.

Examples : Cuckoo (Cuculus canorus), Koel (Eudynamis scolopaeous), Crow-pheasant (Centropus sinensis).

Group C. Swimming and Diving Birds

Order 7. Anseriformes : (L. anser = goose + form) Aquatic birds such as geese, swans and ducks belong to this order.

Examples : Wild duck or mallard (Anas), common teal (Nettion crecca), bar-headed goose (Anser indica)

Order 8. Coraciiformes : (Gk. korax = crow or raven + form) It includes kingfishers and their allies. Examples : White breasted kingfisher (Halcyon smyrnensis), pied kingfisher (Ceryle rudis), bee eater (Hoopoe).

Order 9. Gaviformes : (L. gavia = sea mew + form) It includes marine birds, called loons (gavia) represented by only four species.

Order 10. Podicipediformes or Colymbiformes (Gk. kolymbos = diving bird).

It includes grebes (Podicipes), often called divers because of their habits.

Order 11. Procellariformes : (L. Procella = a tempest + form). It includes tube-nosed, long and oily winged seabirds such as albatrosses (Diomedea), Petrels (Procellaria), shearwaters.

Order 12. Pelecaniformes : (L. pelicanus = pelican + form) It includes pelicans, darters, gannets and cormorants.

Examples : Pelicans (Pelecanus), little cormorant (Phalacrocorax niger)

Group D. Shore Birds and Wading Birds

These aquatic birds seldom swim or dive beneath the water to any great extent.

Order 13. Charadriiformes : (NL. charadrius = genus of plovers + form) This order includes a rather diverse group of water frequenting shore birds characterized by long wading legs, webbed toes and mudprobing beaks.

Examples : Red wattled lapwing (Lobivanellus indicus)

Order 14. Ciconiiformes : (L. ciconia = a stork + form)

It includes long–legged, marshy wading birds with long snake-like neck and javelin or pincer-like beak for piercing their aquatic prey.

Examples : Cattle egret (Bubulcus ibis), heron (Ardea herodias), spoonbil (Platalea leucorodia), strok (Ciconia), flamingo (Phonicopterus).

Order 15. Gruiformes : (L. grus = crane + form) It includes crane-like wading birds with long legs and partially webbed feet.

Examples : Common coot (Fulica atra),

Group E. Birds of Prey

Order 16. Falconiformes : (L. falco = falcon + form)

The diumal birds of prey with sharp hooked beaks and strong curved claws.

Class-Mammalia

(L. mamma = breast)

General characters

(1) Hair-clad, mostly terrestrial, air-breathing, warm blooded, viviparous, tetrapod vertebrates.

(2) Limbs 2 pairs, pentadactyle, each with 5 or fewer digits. Hind limbs absent in cetaceans and sirenians.

(3) Exoskeleton includes lifeless, horny, epidermal hairs, spines, scales, claws, nails, hoofs, horns, bony dermal plates, etc.

(4) Skin richly glandular containing sweat, sebaceous (oil) and sometimes scent glands in both the sexes. Females also have mammary glands with teats producing milk for suckling the young.

(5) Endoskeleton thoroughly ossified. Skull dicondylic having 2 occipital condyles. Cranium large. A single zygomatic arch present. Pterygoids small, scale-like. Otic bones fused into periotic which forms tympanic bulla with tympanic. Each half of lower jaw made of a single bone, the dentary, articulating with squamosal of skull Vertebrae with terminal epiphyses and flat centra (acoelous). Cervical vertebrae usually 7. Ribs bicephalous. Coracoid vestigial.

(6) Teeth are of several types (heterodont), borne is sockets (thecodont) and represented by two sets (diphyodont).

(7) Respiration always by lungs (pulmonary). Glottis protected by a fleshy and cartilaginous epiglottis. Larynx contains vocal cords.

(8) Heart 4-chambered with double circulation.

(9) Kidneys metanephric.

(10) Brain highly evolved. Both cerebrum and cerebellum large and convoluted. Optic lobes small and 4 in number called corpora quadrigemina. Corpus callosum present connecting both cerebral hemispheres. Cranial nerves 12 pairs.

(11) Senses well developed. Eyes protected by lids, the upper of which is movable. External ear opening protected by a large fleshy and cartilaginous flap called pinna. Middle ear cavity with 3 ear ossicles–malleus, incus and stapes. Cochlea of internal ear spirally coiled.

(12) Sexes separate.

(13) Fertilization internal preceded by copulation.

(14) Except egg-laying monotremes, mammals are viviparous, giving birth to living young ones.

(15) Development uterine.

Classification of Mammalia :  Mammals have been thoroughly described and adequately classified. The main characters forming the basis of their classification into orders include : 

(1) Mode of caring of their young,

(2) Nature of dentition

(3) Foot posture,

(4) Nails, claws and hoofs,

(5) Complexity of nervous system and

(6) Systematics.  

(a) Subclass I prototheria : (Gk. protos = first therios = beast). Primitive, reptile-like, oviparous or egg-laying mammals.

Order 1. Monotremata : (Gk. monos = single trema = opening), Cloacal opening present Confined to Australian region.

Examples : Monotremes. Platypus or duckbill (ornith orhynchus) spiny anteater (Tachyglossus = Echidna).

(b) Sub class II. Theria : (Gk. ther, = animal)

Subclass Theria is divided into two infraclasses : Metatheria and Eutheria.

Infraclass 1. Metatheria : (Gk. meta = between or after)

(1) Metatherians are pouched mammals young born in very immature state.

(2) Corpus collosum absent.

(3) Epipublic bone present.

Order 2. Marsupialia : (Gk. marsypion = pouch).

(1) ਋orn in a very immature state, and complete their development attached to teats or nipples in the abdominal pouch or marsupium. (2) Usually 3 premolars and 4 molars in each jaw on either side. (3) Vagina double

Examples : Opossum (Didelphis). Kangaroo (Macropus),

koala (Phascolarctos)

$\bullet$ Kangaroo is the native of Australia.

Infraclass 2. Eutheria : (Gk. eu = true therios beast)

(1) Higher viviparous placental mammals without marsupium.

(2) Young born in a relatively advanced stage.

(3) Dentition never exceeds $\frac{3.14.3}{3.14.3}=44$ . On the basis of characteristics like skull, teeth and limbs, eutheria is arranged into 16 orders.

Order 1. Insectivora : (L. insectum = insect vorare = to eat)

(1) Small mammals with long pointed snout.

(2) Feet plantigrade, usually 5–toed, with claws.

(3) Molars with pointed, peg-like cusps for insect feeding.

Examples : Talpa, Sorex, Solenodon, Erinaceus, Paraechinus

Order 2. Dermaptera : (Gk. derm = skin pteron = wing).

(1) Nocturnal in trees.

(2) A gliding mammal called flying lemur, resembling a flying squirrel.

Examples : One living genus Cynocephalos (= Galaeoithecus) with 2 species from South eastern Asia.

Order 3. Chiroptera : (Gk. Cheiros = hand pteron = wing)

(1) Flying mammals or bats in which forelimbs are modified into wings.

(2) Hind legs short and included in wing membrane.

Examples : Pteropus, Rhinolophus, Desmodus

Order 4. Edentata : (L. edentatus = toothless) Teeth absent or reduced to molars. Without enamel

Examples : Mryrmecopha, Dasypus, Bradypus.

Order 5. Pholidota : (Gk. pholis = a scale)

(1) Body covered with large overlapping scales with sparse hair in between. No teeth.

(2) Tongue long and protrusible, used to capture insects.

Examples : Single genus of scaly anteaters pangolins (Manis)

Order 6. Rodentia : (L. rodo = gnaw).

(1) Largest order including usually small gnawing mammals.

(2) Each jaw with one pair of long, rootless, chisel-like incisors growing throughout life.

(3) Canine absent.

Examples : Rattus, Mus, Funambulus.

Order 7. Lagomorpha : (Gk. logos = hare morphe = form)

With a second pair of small upper incisors behind first pair of large chisel like incisors. No canines.

Examples : Oryctolagus, Lepus, Ochotona.

Order 8. Carnivora : (L. caro = flesh vorare = to eat) Small to large predatory, flesh-eating mammals.

Examples : Canis familiaris, C. lupus, C. aureus, Odobenus, Phoca, Panthera tigeris.

Order 9. Cetacea : (Gk. ketos or L. cetus =a whale)

(1) Large marine fish-like mammals well adapted for aquatic life pectoral limbs modified into broad paddle-like flippers.

(2) Tail divided in two broad horizontal fleshy flukes with a notch, used in propulsion.

(3) No claws, no hind limbs and no external ears.

(4) Mostly gregarious and carnivorous. The living Cetacea are divided into two suborders Odontoceti (toothed whales) and Mysticeti or Mystacoceti (whalebone whales).

Examples : Phocaena, Orcinus, Delphinus, Platanista Physeter, Balenoptera.

Order 10. Sirenia : (Gk. siren = sea nymph).

(1) Large, clumsy herbivorous, aquatic mammals with paddle-like forelimbs, no hindlimbs and a flattened tail with horizontal lateral fleshy flukes with or without a notch.

(2) No external ears.

(3) Muzzle blunt. Hairs few.

(4) Stomach complex.

(5) Inhabit estuaries and coastal sea.

Examples : Trichechus, Dugong (Halicore), recently extinct Steller&aposs sea-cow (Rhytina).

Order 11. Tubulidentata : (L. tubulus Tube like dens = tooth) With tubular mouth tongue protrusible, no incisor or conines, limbs clawed and adapted for digging ant and termites nests. Examples : Single genus of pig-like aardvark or Cape anteater (Orycteropus) of South Africa.

Order 12. Proboscidea : (Gk. pro = in front boskein = to eat)

(1) Largest living land animals having large heads, massive ears, thick practically hairless skins (pachyderm).

(2) Bulky straight legs and 3 to 5 toes with small, nail like hoofs.

(3) Conspicuous feature is the nose and upper lip modified as an elongated flexible proboscis or trunk. 2 upper incisors elongated as ivory tusks.

(4) Cheek teeth lophodont.

Examples : Elephas maximus, Loxodonta africana, Elephas cyclotis.

Order 13. Hyracoidea : (Gk. hyrax = shrew eidos = form)

Small, guinea-pig like mammals distantly related to elephants. No canines. Cheek teeth lophodont.

Example : Hyrax (Procavia) from S. Africa, Syria and Arabia.

Order 14. Perissodactyla : (Gk. perissos = odd dactylos = toes)

The odd-toed hoofed mammals or ungulates have an odd number of toes (1 or 3) incisors present in both jaws.

Examples : Equus cabalus, Equus asinus, Equus zebra

Order 15. Artiodactyla : (Gk. artios = even dactylos = digit)

(1) The even-toed hoofed mammals having an even number of toes (2 or 4)

(2) Incisors and canines in upper jaw usually lacking.

(3) Stomach 4 – chambered.

(4) Many with antlers or horns.

Examples : Sus, Hippopotamus amphibius, Camelus, Cervus, Moschus, Ovis.

Order 16. Primates : (L. primus = of the first rank)

(1) Generalized or primitive mammals except for the great development of brain. 

(2) Mostly arboreal.

(3) First digit usually opposable, an adaptation for grasping.

(4) Eyes typically large and turned farward.

Example – Gibbon, Mandrillus, Chimpanzee, Ateles, etc.