234 ASCIDIZ. 
Bohatch* and Plancus‘, in their works, have described and 
represented them with exactitude under the name Thethyrium. 
Bastert has also well distinguished this genus, and names 
it Ascidia, which he derives from aoxds, a pouch or budget. 
He has with great exactness shown its affinity with the Ostrea, 
oyster, in its anatomical structure. 
Pallas § proposes the reunion of Thethyrium with Ascidium, 
from which Linnzeus, in his twelfth edition, has given the name 
Ascidia : it was in this edition that he has referred all the 
inhabitants of the class Ginglymoconche, Bivalves, to the genus 
which he names Thethys. 
Baster and Pallas have both of them observed the affinity 
of the Ostrea (oyster) with Aseidia. 
The bodies of the species are more or less cylindric, liege 
two cylindric tubes situated at the upper part of the sac: both 
of them can be withdrawn at the will of the animal. One of 
them is always placed on the summit of the sac: this takes in 
the water, and conveys it to the branchial cavity ; and by the 
same tube the animal rejects the water and renews it, when 
it requires food, or water for respiration : when irritated, it 
throws up the water several inches. 
The second tube is generally placed a little lower than the 
other; through this it also rejects the water received by the 
other tube: this tube contains the rectum and the organs of 
generation ; it has no communication with the branchial cavity. 
The integument is generally cartilaginous; and is always 
transparent, varying in thickness: it is well-organized, and re- 
ceives from the body, properly so called, the branches of the 
arteries and the veins, forming a most elegant network, which 
can be traced by the naked eye. 
Besides this outer integument, there is an inner one, formed 
by the reflection of the outer one through the tubes, which 
thus forms a second integument, which is very thin and serous. 
The body, properly so called, is suspended in the cavity of 
the sac, and is always less voluminous than the body itself 
(which Cuvier thinks may probably contain, when the animal 
* Animalia marina, pl. 10. + De Conchis minus notis, pl. 5 & 7. 
t Opuscula subseciva. § Miscellanea Zoologica, p. 74, 
