Biology Sikshya

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    Gr.,cephale=head; L.,chorda=cord

• Exclusively marine animals and are cosmopolitan, present in shallow water.

• Triploblastic animals i.e., body is derived from 3 embryonic germ layer (ectoderm, mesoderm and endoderm).

• Body is bilaterally symmetrical. Body structure is fish like.

• Body lacks head and is divisible into trunk and tail. Exoskeleton is absent and epidermis is single layered.

• Metamerism is well marked.

• Coelom is enterocoelous, reduced in the pharyngeal region by development of atrial cavity.

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• Median fins are present but the paired appendages are absent.

• Notochord is persistent through out the life, extending from rostrum to tail.

• All are filter feeders.

• Respiration through general body surface i.e., no specialized respiratory structure is found.

• Complete alimentary canal. Pharynx large, perforated by numerous persistent gill-slits opening to atrium.

• Excretion by protonephridia with solenocytes.

• Circulatory system is closed type. But, heart and respiratory pigments are absent. Hepatic portal system is developed.

• Nerve cord is dorsal and hollow, without ganglion and brain. Dorsal and ventral roots are separate.

• Sexual reproduction occurs. External fertilization takes place.

• Development is direct, but an intermediate juvenile form exist.

• All dioecious animals.

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