128 CLASS ACEPHALA. 
Some have but a single one at this same extremity. (Holo- 
thuria Zonaria, Gm., &c.) 
The greater number is simply oval or cylindrical. (Salpa 
octofera, Cuv., &c.) In the 
AscipiA, Lin., THEYTON of the ancients, 
The mantle and its cartilaginous envelope, which is fre- 
quently very thick, resemble sacs every where closed, except 
at two orifices which correspond to the two tubes of several 
bivalves, one serving to admit water, and the other to give a 
passage to the feces. The gills form a large sac, at the 
bottom of which are the mouth and the visceral mass. The 
envelope is much larger than the mantle, which is fibrous and 
vascular, and on which, between the two tubes, is one of the 
ganglions. These animals attach themselves to rocks and 
other bodies, and are deprived of all power of locomotion. 
The chief sign of vitality which they exhibit consists in the 
absorption and evacuation of water, through one of their 
orifices: when alarmed they eject it to a considerable dis- 
tance. They abound in every sea, and some of them are 
eaten. The whole genus ASCIDIA, Gm. with some addi- 
tional species. 
Some species are remarkable for the long pedicle which 
supports them. (Ascidia pedunculata, Edw., &c.) 
The second family of naked acephala, 
AGGREGATA, 
Consists of animals more or less analogous to the ascidiz, but 
united in a common mass, so that they seem to communicate 
organically with each other, and in this respect to connect 
the mollusca with the zoophytes ; but, independent of their 
peculiar organization, these animals, according to the obser- 
vations of Messrs. Audouin and Milne Edwards, at first live 
and swim separately, only becoming united at a certain sub- 
