TUNICATA 



915 



pides of Laguncula, the chief difference being that a regular cir- 

 culation 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 Perophora, first discovered by Mr. Lister, 

 which occurs not unfrequently on the south coast of England and in 

 the Irish Sea, living attached to seaweeds, and looking like an assem- 

 blage 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 distinctly 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 tha,t enters its oral orifice escapes 



FIG. (J91.Botryllus violaceus \ A, cluster on the surface of a Fucus ; 

 B, portion of the same enlarged. 



into the space between the sac and the mantle, and is thus dis- 

 charged immediately by the atrial funnel. Whatever little particles, 

 animate or inanimate, the current of water brings flow into the 

 sac unless stopped at its entrance by the tentacles, 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 ap- 

 parent injury either to themselves or to the creature which incloses 

 them. In general, however, particles which are unsuited for reception 

 into the stomach are rejected by the sudden contraction of the mantle 

 (or muscular tunic), the atriopore being at the same time closed, so 

 that they ^ are forced out by a powerful current through the oral 

 orifice. The curious alternation of the circulation that is character- 

 istic of the class generally may be particularly well studied in 

 Perophora. The creeping stalk that connects the individuals of 



3 N 2 



