NEET Biology Animal Kingdom Notes - Phylum Chordata

Chordata

 Chordata.

(i) General Characters of Phylum Chordata

            (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.

Outline Classification of Phylum Chordata

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

Group A. Acrania (Protochordata) : (Gr. a, absent; kranion, head, or, Gr. 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 : (Gr. 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 : (Gr. enteron, gut; pneustos, breathed). Body large and worm-like. Gill slits numerous. Intstine straight. Acorn or togue worms. 70 species. Balanoglossus, Saccoglossus.

Class 2. Pterobranchia : (Gr. pteron, feather; branchion, gill). Body small and compact. Gill-slits one pair or none. Intstine U?shaped. Pterobranchs. 20 species. Cephalodiscus, Rhabdopleura.

Subphylum II. Urochordata or Tunicata : (Gr. 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. Ascidacea : Sessile tunicates with scattered muscles in tunic. Solitary, colonial or compound. Gill-clefts numerous. Ascidians or sea squirts. 1,200 species. Herdmania, ciona, Molgula. 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 : (Gr. kephale, head; 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. Branchiostoma (= Amphioxus), Asymmetron.

Group B. Craniata (Euchordata) : Aquatic or terrestrial, usually large-sized, higher chordates or vertebrates with distinct head, a 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 : (Gr. a, not; gnathos, jaw). Jaw less primitive fish-like vertebrates without true jaws and paired limbs.

Class 1. Ostracodermi. (Gr. ostrakon, shell; derma, skin). Several extinct orders of ancient primitive heavily armoured, Palaeozoic, world's first vertebrates, collectively called the ostracoderms. Caphalaspis, Drepanaspis.

Class 2. Cyclostomata. (Gr. cyklos, circular; stoma, mouth). Body eel-shaped, without scales, jaws and lateral fins. Mouth rounded and suctorial. Gills 5?16 pairs. Parasites and scavengers. 45 species. Lampreys (Petromyzon) and hagfishes (Myxine).

Division II. Gnathostomata: (Gr. 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: