TUNICATA. ASCIDI^:. 429 



ORDER I. ASCIDI^E. 



1061. It is perhaps among the solitary Ascidia, forming the 

 the family ASCIDIID^:, that the highest organisation presents 

 itself, which occurs in this class. The two orifices are both 

 evident on the upper part of the body ; but the branchial aper- 

 ture is the most prominent. Within this may be observed a 

 fringe of tentacula, which are sometimes of considerable length 

 and minutely divided, sometimes short and simple. Their office 

 appears to be, to guard the entrance to the 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. PHYSIOL. 

 436) ; beyond these, no organs of sense can be detected. The 

 solitary Ascidiae are found in all climates ; they generally fre- 

 quent the shallow waters of the shore, so as to be occasionally 

 left uncovered by the tide. In some localities they are so abund- 

 ant, 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 prin- 

 cipal 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 Ascidi<TD have the power of changing their 

 hue; the Cynthia Momus, for instance, is sometimes white, 

 sometimes orange, and sometimes of a flesh colour. 



1062. The Ascidice are not all, however, solitary in their 

 habits. The accompanying figure represents one of the com- 

 pound forms, constituting the family CLAVELLINID^E ; in which 



VOL. II. H H 



