This taxon constitutes a unique
group of animals exhibiting diversityin form and habit. In Urochordata the
notochord is confined to the tail re- gion of the larva. The adults are mostly
degenerate, sessile forms. The body is enveloped by a tunic or test. The free
end of the body bears two openings, the mouth and the atriopore. The proximal
part of the alimentary canal is greatly enlarged to form a spacious pharynx.
They are hermaphroditic ani- mals. The development occurs through free swimming
tadpole like larva.
Example : Ascidia, Doliolum, Salpa
.
Sub phylum : Vertebrata (Craniata)
This group is characterized by the
presence of brain caseor cranium and a vertebral column which forms the chief
skeletal axis of the body. The notochord is an embryonic structure. It is
replaced in the adult stage by a cartilaginous or bony vertebral column . The
body is covered with an integument having
an outer epidermis and an inner dermis .
The skin has many modifications such as glands, scales, feathers, claws horns
and hairs.
The digestive system is ventral to
the vertebral column. It is provided with a large liver and pancreas. The circulatory
system consists of the ven- tral, chamberd heart. The circulatory system is of
a closed type with arteries,veins and capillaries. The blood plasma contains
red and white blood cor- puscles. Gill slits are limited in number (usually 5
pairs). There are two pairs of appendages. The anterior part of the nerve cord
becomes differentiated into brain and spinal cord. The special organs of sense
like the nose, eyes and ears are closely connected with the brain. \ Urinary
and genital systems are closely connected to form an urinogenital system.
No comments:
Post a Comment