xni PHYLUM CHORDATA 23 



Systematic Position of the Example. 



The genus Ascidia, of which there are very many species, is a 

 member of the family Ascidiidcs of the Ascidics simplices. The 

 Ascidiidae differ from the other families of simple Ascidians by the 

 union of the following characters : The body is usually sessile, 

 rarely elevated on a peduncle. The oral aperture is usually 

 8-lobed and the atrial 6-lobed. The test is always of gelatinous or 

 cartilaginous consistency. The wall of the pharynx is not folded ; 

 the tentacles are simple and filiform. The gonads are placed close 

 to the intestine. 



The genus Ascidia is characterised by having the oral and atrial 

 apertures not close together, by the dorsal lamina being a continu- 

 ous undivided fold, and by the ganglion and neural gland being 

 situated at a little distance from the dorsal tubercle. 



3. GENERAL ORGANISATION. 



General Features. The Larvacea are minute transparent 

 animals, in shape not unlike tadpoles, with a rounded body and a 

 long tail-like appen- 

 dage attached to the 

 ventral side. At the 

 extremity of the body 

 most remote from the 

 tail is the aperture of 

 the mouth. This leads 

 into a tolerably wide 

 pharynx (Fig. 732, 

 ph.), in the ventral 



co-all of w"hir>>i foYpprf FIG. 731. Oikopleura in " house." The arrows show 



cn ^except the courge o{ jg current> (From Herdmani after FoL) 

 in Kowalevskia) is 



an endostyle similar to that of the simple Ascidian, but com- 

 paratively short. Round the pharynx there run obliquely two 

 bands covered with strong cilia the peripharyngeal bands, which 

 join a median dorsal ciliated band. On the ventral side of the 

 pharynx there are two ciliated openings the stigmata (stig.), which 

 communicate with the exterior by short passages the atrial 

 canals, situated on either side behind the anus. The axis of the 

 tail is occupied by a cylindrical rod the notochord or urochord 

 (noto.). 



A remarkable peculiarity of the Larvacea is the power which 

 they possess of secreting from the surface, by the agency of certain 

 specially modified epidermal cells, a transparent envelope which is 

 frequently discarded and quickly renewed. The chief object of 

 this structure seems to be the capture of the very minute plankton- 

 organisms on which the Larvacea feed. In Oikopleura (Fig. 731) 

 the " house" is a comparatively large structure within which the 



