Sub-Phylum Urochordata || characteristics || B. Sc. & M. Sc. || Zoology

Sub-Phylum Urochordata characteristics
Urochordates are characterized by :
1. Habitat. These are exclusively marine and are widely distributed from the polar oceans to tropical seas. Most of these are sedentary (eg, ascidians) and may inhabit upto the depth of 4.80 km while some urochordates are pelagic (eg. Poliolum, Salpa). These are more numerous in shallow waters.
2. Size. It varies widely and ranges from 0.25 mm to over 250 mm.
3. Colour. It also varies widely. Many urochordates are grey or green but these may colours as well.
4. Body form. It also varies widely and may be solitary (eg. Herdmania) or colonial (eg. Pyrosoma).
5. Body covering. Body of urochordates is generally covered by a thick leathery and protective covering called test. It is chemically formed of tunicine [(CHO) - a carbohydrate similar to celluloseleg. Herdmania, so called Tunicata. Some are covered by a gelatinous house.e.g. Oikopleura.
6. Body appendages. Adults generally lack paired or unpaired appendages except Oikopleura whose larva has a tail fin
7. Body wall or mantle. It is formed of:
(a) Outer single layered epidermis of flat and hexagonal cells. It secretes test.
(b) Middle mesenchyme of connective tissue fibres, nerve fibres, smooth muscle fibres and blood sinuses
(c) Inner atrial epithelium of flat and ectodermal hexagonal cells.
8. Notochord. It is a rod of vacuolated cells found only in tail of their larval forms, so called Urochordates. Notochard is generally lost during retrogressive metamorphosis, so adults generally lack both tail and notochord except appendicularians eg. Oikopleura.
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