SOCIAL ASCIDIANS. 



503 



stolons, instead of being included in a common investment (Fig. 

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

 as that of the zooids of Laguncula ( 322), the chief difference 



FIG. 250. 



A, Group of Perophora (enlarged), growing from a common stalk: B, single Perophora; a, test; 

 b, inner sac ; c, branchial sac, attached to the inner sac along the line c' c'; e, e, finger-like processes 

 projecting inwards; /, cavity between test and internal coat ;/", anal orifice or funnel ;#, oral orifice; 

 g f , onil tentacula ; h, downward stream of food ; h', oesophagus ; i. stomach ; fc, vent ; I, ovary (?) ; w, 

 vessels connecting the circulation in the body with that in the stalk. 



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 branchial ori- 

 fice (g) escapes into the space between the sac and the mantle, 

 and is thus discharged immediately by the funnel (/'). What- 

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

 brings, flow into the sac, unleae stopped by the tentacula (g'} 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 until they arrive at that part of 

 it down which the current proceeds to the entrance of the sto- 



