SUBDIVISIONS OF TUNlCATA. ASCIDLE. 427 



simply adhering externally, but forming a mass of a certain 

 regular aspect. In other cases, we observe several individuals 

 included within a common envelope, their own external coats 

 being absent. And in some instances, there is a continuous cir- 

 culation of blood among .several individuals, through vessels 

 passing along a stem, with which they are all connected by short 

 peduncles or foot-stalks. 



976. The Tunicata may be better subdivided according to 

 the anatomical characters, and the mode of existence, of the 

 respective species, than by arranging them according to their 

 solitary or united condition. On this principle two Orders will 

 be formed ; one including the isolated and aggregated Ascidice ; 

 and the other, the isolated and aggregated Salpce. The promi- 

 nent differences in these two Orders are these. In the Asddice 

 the two orifices approach one another more or less closely (Figs. 

 601, 602). The body is either fixed immediately to some solid 

 mass, or attached to it by a peduncle. And the branchial ap- 

 paratus consists of a simple sac or bag, occupying the greater 

 part of the cavity of the mantle, and having the entrance to the 

 oesophagus at its lower part. In the Salpce (Fig. 603), on the 

 other hand, the two orifices are placed at the opposite extremi- 

 ties of the body, which is generally more elongated than in the 

 Ascidiae. They seldom attach themselves to any fixed basis, 

 but rather to floating bodies ; and many of them seek no support 

 from other masses, but trust themselves to the waters of the 

 ocean, in which they seem to have some power of spontaneous 

 motion. Instead of a capacious but simple branchial sac, we 

 find a long narrow tube, in which a riband-like fold of vascular 

 membrane stretches from end to end, and serves as the special 

 apparatus for the aeration of the blood. 



977. It is among the solitary Ascidice , that the highest 

 organisation presents itself, which occurs in this family. The 

 two orifices are both evident on the upper part of the body ; but 

 the branchial aperture is the most prominent. "Within this may 

 be observed a fringe of tentacula, which are sometimes of con- 

 siderable length and minutely divided, sometimes short and 

 simple. Their office appears to be, to guard the entrance to the 



