624 



INVERTEBRATE MORPHOLOGY. 



deferens (vd\ opening into the atrial cavity near the opening 

 of the oviduct. 



On account of the larval characters being more important 



than the adult in indicating the 

 affinities of the Tunicates and in 

 justifying the term Urochorda applied 

 to the class, it seems convenient to 

 depart from the usual arrangement 

 and consider the development of the 

 simple Ascidians here, before passing 

 on to a description of the various 

 orders. 



Development of the Simple Asci- 

 dians. For an account of the early 

 development reference must be made 

 to embryological text-books, the 

 present description being confined to 

 the larva and the changes it under- 

 goes in transforming to the adult. 

 Suffice it to say that the early stages 

 resemble very closely those of Am- 

 phioxus, and they result in the forma- 

 tion of a remarkable structure usually 

 known as the Ascidian tadpole, a 

 term which indicates its general ap- 

 pearance. This larva is a free-swim- 

 ming organism and consists (Fig. 

 284) of an anterior somewhat globular 

 portion, the body, and a posterior 

 flattened region, the tail. The entire 

 body is enclosed w r ithin a continuous 

 case, the test (c), which, in the tail 

 region, is elevated into a dorsal and 

 ventral ridge, serving as fins. Upon 

 the anterior end of the body are 

 papillae (ap) which serve for fixation 

 when the larval life is completed, while in the interior of 

 the body region indications of the various adult organs 

 may be seen. Certain interesting modifications of these 



FIG. 284. DIAGRAM OF THE 

 TADPOLE LARVA OF A 

 TUNICATE. 



a anus. 

 ao = atrial orifice. 

 ap = adhesive papilla. 

 at = atrial cavity. 



c = cellulose test. 



ce = brain. 

 en = eudostyle. 



h = heart. 

 m = mouth. 



n = nerve 

 nc = notochord. 

 ph = pharynx. 

 sg = subueural gland. 



