588 COMPOUND AND SOCIAL ASCEDIANS. 



ship to each other is somewhat like that of the several Polypes on 

 the polypidora of a Campanularia (§ 417). 



454. In the family of Didemnians the Post-abdomen is absent, 

 the Heart and Generative apparatus being placed by the side of the 

 Intestine in the Abdominal portion of the body. The Zooids are 

 frequently arranged in star-shaped clusters, their Anal orifices being 

 all directed towards a common vent which occupies the centre. — 

 This shortening is still more remarkable, however, in the family of 

 BotrylUans, whose beautiful stellate gelatinous incrustations are 

 extremely common upon Sea-weeds and submerged rocks (Fig. 303). 

 The anatomy of these animals is very similar to that of the 

 Amaroucium already described ; with this exception, that the 

 body exhibits no distinction of cavities, all the organs being brought 

 together in one, which must be considered as Thoracic. In this 

 respect there is an evident approximation towards the Solitary 

 species. 



455. This approximation is still closer, however, in the i Social ' 

 Ascidians, or Clavellinidce ; in which the general plan of structure 

 is nearly the same, but the Zooids are simply connected by their 

 stolons (Fig. 304) instead of being included in a common invest- 

 ment ; so that their relation to each other is very nearly the same 

 as that of the Polypides of Laguncula (§ 446), the chief difference 

 being that a regular Circulation takes-place through the stolon in 

 the one case, such as has no existence in the other. A better 

 opportunity of studying the living actions of the Ascidians can 

 scarcely be found, than that which is afforded by the genus Pero- 

 phora, first discovered by Mr. Lister, which occurs not unfre- 

 quently on the south coast of England and in the Irish Sea, living 

 attached to Sea-weeds, and looking like an assemblage of minute 

 globules of jelly, dotted with orange and brown, and linked by a 

 silvery winding thread. The isolation of the body of each Zooid 

 from that of its fellows, and the extreme, transparence of its tunics, 

 not only enable the movements of fluid within the body to be dis- 

 tinctly discerned, but also allow the action of the Cilia that border 

 the slits of the Respiratory sac to be clearly made-out. This sac 

 is perforated with four rows of narrow oval openings, through 

 which a portion of the water that enters its Oral orifice (g) 

 escapes into the space between the sac and the mantle, and is 

 thus discharged immediately by the Anal funnel (f). Whatever 

 little particles, animate or inanimate, the current of water brings, 

 flow into the sac, unless stopped at its entrance by the Tenta- 

 cula (g 1 ), which do not appear fastidious. The particles which are 

 admitted usually lodge somewhere on the sides of the sac, and then 

 travel horizontally until they arrive at that part of it down which 

 the current proceeds to the entrance of the Stomach (/), which is 

 situated at the bottom of the sac. Minute animals are often 

 swallowed alive, and have been observed darting about in the 

 cavity for some days, without any apparent injury either to them- 



