428 SUBDIVISIONS OP TUNICATA. 



tendency to aggregation exhibits itself among the Tunicata in 

 various ways. Sometimes we find a number of individuals 

 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. 



1060. 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 Ascidite; 

 and the other, the Salpce. The prominent differences in these 

 two Orders are these. In the Ascidice the two orifices usually 

 approach one another more or less closely (Figs. 701, 702). The 

 body is either fixed immediately to some solid mass, or attached 

 to it by a peduncle. And the branchial apparatus consists of a 

 sac or bag, bearing a branchial network of small square meshes, 

 occupying the greater part of the cavity of the mantle, and hav- 

 ing the entrance to the oesophagus at its lower part. In the 

 Salpce (Fig. 703), on the other hand, the two orifices are placed 

 at the opposite extremities of the body, which is generally more 

 elongated than in the Ascidire. 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 bran- 

 chial 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. 



