What are subphylum urochordata
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The subphylum Urochordata constitutes a unique group of animals under the, Phylum Chordata. The members of this subphylum exhibit a high degree of diversity in form, habit and habitat.
1. AII urochordates are marine, sessile (Ascidiacea) or permanently pelagic (Larvacea or thaliacea) forms.
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2. The adults are mostly degenerated and un-segmented.
3. The notochord is confined to the tail region in the larval stage. For this characteristic feature, the name of the group is given Urochordata. Other features — a dorsal hollow nerve cord, pharyngeal slits and a post-anal tail are found in the larval stage. They retain only pharyngeal (branchial) slits in the adult.
4. The body is enveloped by flexible tunic or test. Hence commonly known as tunicates. The material of the tunic is unique because it contains cellulose, a plant materials.
5. The free end of the body bears two openings — the mouth and the atriopore.
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6. The proximal part of the alimentary canal is greatly enlarged to form a spacious pharynx, adapted for filter feeding.
7. The coelom is not recognisable.
8. Respiration performs by gaseous exchange across the tunic or the pharyngeal slits.
9. Blood vascular system is open. Heart is simple, tubular and ventral, enclosed in a pericardial cavity. The heart is unusual because it periodically reverses the direction in which it pumps blood. Blood contains cells but not erythrocytes.
10. Excretion is performed by neural gland, pyloric gland and nephrocytes.
11. The urochordates are mostly hermaphrodite, but in some species of Distaplia, Holozoa and Sycozoa (Ascidiacea) and in Oikopleura dioica (Larvacea), the uni-sexuality is noticed. The eggs and sperm attain maturity at different times in most species.
12. Fertilization is cross and external and takes place in sea water.
13. Asexual reproduction accomplishes by the formation of buds.
14. Development occurs through a free- swimming tadpole larva.
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15. The urochordates exhibit certain exceptions to the characters of the chordates, like retrogressive metamorphosis, alternation of generations and the formation of colonies.
Fossil history of Subphylum Urochordata:
Jaekelocarpus oklahomensis recorded from the upper Carboniferous, Gene Autry Shale Formation near Ardmore of Oklahoma, U.S.A. is considered as fossil tunicate, belonged to the stem group of Urochordata, because it possessed calcite skeleton, ciliated pharyngeal slits and a downward flexing tail.
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