20 ZOOLOGY SECT. 



with a tail. The test is a permanent structure, usually of con- 

 siderable thickness. The muscular fibres of the mantle (body- 

 wall) are not arranged in annular bands. The pharynx is large, 

 and its walls are perforated by numerous stigmata leading into a 

 surrounding atrium or peri-branchial cavity, which communicates 

 with the exterior by an atrial aperture. Many form colonies by a 

 process of budding ; and most undergo a metamorphosis, the larva 

 being provided with a caudal appendage supported by a notochord 

 similar to that of the Larvacea. 



Sub-Order a. Ascidice. simplices. 



Ascidians in which, when colonies are formed, the zooids are not 

 embedded in a common gelatinous mass, but possess distinct tests 

 of .their own. They are nearly always permanently fixed and 

 never free-swimming. 



Including all the larger Ascidians or Sea-Squirts. 



Sub-Order b. Ascidice compositor. 



Fixed Ascidians which form colonies of zooids embedded in a 

 common gelatinous material without separate tests. 



This order includes Botryllus, Amarcecium, Diazona, and a 

 number of other genera. 



Systematic position of the Example. 



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

 member of the family Ascidiidce of the Ascidias 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 sub-neural gland being 

 situated at a little distance from the dorsal tubercle. 



3. GENERAL ORGANISATION. 



General Features. Appendicularia (Fig. 679), 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 



