24 



ZOOLOGY 



SECT. 



Systematic position of the Example. 



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

 member of the family Ascidiidoi of the Ascidice simph'ces. The 

 AscidiidiB differ from the other families of simple Ascidians by the 

 union of the following characcers : — 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 unciivided fold, and by the ganglion and neural gland being 

 situated at a little distance from the dorsal tubercle. 



3. General Organisatiox, 



General Features. — Appendicularia (Fig. 722), which may 

 be taken as an example of the Larvacea, is a minute transparent 

 animal, in shape not unlike a tadpole, with a rounded body and a 

 long tail-like appendage 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. 

 723, j?7i.), in the ven- 

 tral wall of which is 

 an endostyle similar 

 to that of the sim})le 

 Ascidian, but com- 

 parativ.ely short. 

 Round the pharynx 

 there run two bands 

 covered with strong 

 cilia — the peripharyn- 

 geal lands. 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 {nolo). 



A remarkable peculiarity of Appcndicularia is the power which 

 it possesses of secreting from the surface, by the agency of certain 

 specially modified epidermal cells, a transparent envelope (Fig. 722) 

 in the interior of which the animal can move freely. This 

 structure — the " house',' as it is called — is soon thrown off, and a 

 new one developed in its stead. It represents the test or tunic of 

 the simple Ascidian, though it does not appear to contain cellulose. 



Fig. 722.— Appcndicularia (Oikopleura) in "house' 

 (From Ileiduian, after Ful.) 



