210 MOLLUSCA. 
tion of the branchiae, the circulation was distinctly visible 
in the mantle near the openings, and the particles in the 
blood were only of about the same size as above. 
b. Body sessile. 
The branchial orifice with eight or nine rays, and the 
anal with six. The angles of the branchial meshes with 
papilla. No liver. A cylindrical rib extending from the 
pylorus to the anus. 
(A.) Tunic and branchial cavity straight. 
Genus Prrena.—The branchial sac as extended as the 
tunic. Stomach not resting on the intestine. P. phusca of 
Forskal is the type, to which Savigny has added three other 
species. The Ascidia prunum of Miiller, a native species, 
may be referred to this genus. 
Genus Crona.—Branchial sac shorter than the tunic, and 
exceeded by the viscera. Ascidia intestinalis, Lin. is a 
native example of this genus. 
(B.) Tunic turned up at the base. 
Genus Puaruiusta.—Branchial sac extending beyond 
the viscera into the pouch of the sac. Stomach resting on 
the mass of viscera. The Ascidia mentula of Miller, a na- 
tive species, is the type. 
There are two genera supposed to be nearly related to 
the preceding, which are involved in great obscurity. The 
genus Bipapilaria of Lamark appears to be pedunculated, 
with two apertures, each furnished with three setaceous 
tentacula. The Mammaria of Miiller has only one termi- 
nal aperture. One species inhabits the British seas. 
B. Compound. 
The animals included under this division were formerly 
inserted in the genus Alcyonium of Linnzeus, and placed 
