430 COMPOUND ASCIDIANS. 



different individuals are united together by a common stem, in 

 the same manner as the Polypes of the Sertularia (Fig. 718). Each 

 has its own heart, respira- & a 



tory apparatus, and digestive 

 system ; but each is "fixed 

 on a footstalk that branches 

 from a common creeping 

 stem, through which a cir- 

 culation takes place that con- 

 nects them all. The integu- 

 ment is so transparent, that ^ 



the whole interior and its FIG. 702. PEROPHQRA, a compound Ascidian: 6, 

 ... . , , branchial orifice, or mouth; a, anal orifice; e, 



living actions may be Ob- stomach ; i, intestinal canal ; t, common stem. 



served without difficulty. 



Whatever little substances, alive or inanimate, the current of 

 water brings, flow into the sac, unless stopped by the tentacula 

 at its entrance, which do not appear fastidious. The particles 

 which are admitted usually lodge somewhere on the sides of the 

 sac, and then travel horizontally (by some unseen agency), until 

 they arrive at the part of it, down which the current proceeds 

 into the entrance to 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, without any apparent 

 injury to themselves, or to the animal which incloses them, for 

 some days. In general, however, particles which are unsuited 

 for being received into the stomach, are ejected by the sudden 

 contraction of the mantle, the funnel being at the same time 

 closed, so that they are forced out by a powerful current through 

 the branchial orifice. 



1063. It is in the Circulating apparatus, that the chief pecu- 

 liarities of these compound Ascidise present themselves. The 

 creeping tube which unites the individuals of a group, contains 

 two distinct canals, which send off branches into each peduncle. 

 One of these branches terminates in the heart, which is nothing 

 more than a contractile dilatation of the principal trunk. This 

 trunk subdivides into branches, of which some ramify over the 

 branchial sac, whilst others are first distributed to the stomach 



