1 8 comparative anatomy 



Sub-Phylum Urochorda (Tunicata) 



The urochordates, the tunicates or sea-squirts, are so named because 

 the notochord, absent in the sessile adult, is always limited to the tail 

 region. Another character common to the group is the presence of a 

 tunicin mantle which is secreted by the skin. Tunicin is a chemical 

 substance that resembles cellulose. A coelom is sometimes present, but is 

 limited to the region of the ventral heart. Nephridia or coelomoducts are 

 wanting. The body is unsegmented, and the alimentary canal is bent on 

 itself so that the anus lies near the mouth. The pharynx is perforated 



TENTACLES 

 CIUATED GROOVE 



NERVE CORD 

 ESOPHAGEAL 



— STOMACH 



ASCIDIA - A OROCHORDATE 



Fig. 12. — Ascidia, a urochordate. The animal is viewed as if cut in median longitudinal 

 section and as seen from the right side. (Redrawn from Sewertzoff, after Boas.) 



by gill-slits, the number of which varies greatly in the different species. 

 The nervous system consists of a nerve ganglion dorsal to the pharynx, 

 from which nerves extend to the various organs. In some forms both 

 sexual and asexual methods of reproduction occur. Individuals however, 

 are usually hermaphroditic. Development generally involves metamor- 

 phosis. The sexually-produced tailed larva bears certain striking resem- 

 blances to the larva of amphioxus. 



Some systematists recognize 1400 species. 



ClONA is a sessile tunicate, three or four inches in length, which is 

 attached by tunicin stolons to its substratum. A tunicin test or tunic, 

 which is secreted by the skin, encloses the entire animal as a sac. Beneath 

 the test and loosely connected with it, except in the region of the two 

 apertures of the body, lies the body-wall or mantle. This consists of an 



