COMPOUND ASCIDIANS. 429 



ment is so transparent, that the whole interior and its living 

 actions may be observed without difficulty. As in other Ascidiae, 

 the branchial sac occupies a large part of the cavity of the mantle ; 

 at the sides and point, however, a vacant space exists between 

 these two membranes, which terminates in the funnel. The sac 

 is perforated with four rows of narrow oval openings, the edges 

 of which are thickened, and fringed with closely-set cilia. 

 Through these openings, a part of the water which flows into 

 the sac by the branchial orifice, escapes into the space between 

 the sac and the mantle, and is thus discharged immediately by 

 the funnel. Whatever little substances, alive or inanimate, the 

 current of water brings, flow into the sac, unless stopped by the 

 tentacula 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 (by some unseen 

 agency), until they arrive at the part of it, down which the cur- 

 rent proceeds into the entrance to 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, with- 

 out any apparent injury to themselves, or to the animal which 

 incloses them, for some days. In general, however, particles 

 which are unsuited for being received into the stomach, are 

 ejected by the sudden contraction of the mantle, the funnel 

 being at the same time closed, so that they are forced out by a 

 powerful current through the branchial orifice. 



979. It is in the Circulating apparatus, that the chief pecu- 

 liarities of these compound Ascidiae present themselves. The 

 creeping tube which unites the individuals of a 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, of which some ramify over the 

 branchial sac, branching off at each of the passages between the 

 oval apertures ; whilst others are first distributed to the stomach 

 and intestines, and the soft surface of the mantle. All these 

 reunite, and form a trunk which enters the peduncle, and con- 

 stitutes the returning branch. The circulation through these 



