TUNICATA I — SOCIAL ASCIDIANS. 



589 



selves or to the creature which encloses them. In general, how- 

 ever, particles which are unsuited for reception into the stomach 

 are ejected by the sudden contraction of the Mantle (or muscular 

 tunic), the vent 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 characteristic of the 

 Class generally (§ 452), may be particularly well studied in Pero- 

 phora. The creeping-stalk (Fig. 304) that connects the individuals 



Fig. 304. 



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; g, oral orifice; g', oral tentacula ; 

 h, downward stream of food ; h', oesophagus ; i, stomach ; k, vent ; 

 I, ovary (?); n, vessels connecting the circulation in the body 

 with that in the stalk. 



of any 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 vessels (or rather 



