428 TUNIC ATA. ASCIDLE. 



respiratory sac ; and it would seem probable that substances 

 unfit to enter it are kept out by the closure of the muscular 

 ring, excited through the nervous apparatus by irritation of the 

 tentacula ( ANIM. PHYSTOL. 436) ; beyond these, no organs of 

 sense can be detected. The solitary Ascidiae are found in all 

 climates ; they generally frequent the shallow waters of the 

 shore, so as to be occasionally left uncovered by the tide. In 

 some localities they are so abundant, as to afford an important 

 source of nutriment to Fishes ; and some species are employed 

 as food even by Man. The power of ejecting the contents of the 

 branchial sac is their principal means of defence. Some of the 

 larger species are able to shoot the fluid to a height of three 

 feet ; and thus the person who places his hand upon one of them, 

 is very likely to receive a deluge of water in his face. We are 

 not justified, however, in attributing this action (as some have 

 done) to any intelligence on the part of the animal. It is the 

 only movement which these simple beings can perform ; and is 

 evidently a reflex action, which any bodily irritation, whether 

 internal or external, will excite. Some of the Ascidias have 

 the power of changing their hue; the Cynthia Momus, for 

 instance, is sometimes white, sometimes orange, and sometimes 

 of flesh colour. 



978. The Ascidia are 

 not all, however, solitary 

 in their habits. The ac- 

 companying figure repre- 

 sents one of the compound 

 forms ; in which different 

 individuals are united toge- 

 ther by a common stem, in 

 the same manner as the 



PolvDeS Of the Sprtllliril Fia> 602.-POROPHOR4, a compound Asci- 



ro vP e ^ -uiana dian; b> branchial orifice , or mouth; 



(Fig. 615). Each has its a, anal orifice; e, stomach ; i, intestinal 



i . canal ; t, common stem. 



own heart, respiratory ap- 

 paratus, and digestive system; but each is fixed on a foot- 

 stalk that branches from a common creeping stem, through which 

 a circulation takes place that connects them all. The integu- 



