190 MR. A. HANCOCK ON THE ANATOMY OF DORIDOPSIS. 
Doridopsis nigra has also been dissected, and will be used in this paper as the type of 
the smooth division to which it belongs. It differs from D. gemmacea chiefly by the 
smoothness of the mantle. The branchial plumes, too, are tripinnate, and are eight in 
number. The specimens dissected were about one inch and a half in length. 
Besides these two species, reference will likewise be made to D. £uberculosa, D. cla. 
vulata, D. miniata, and one or two others. 
On laying open the dorsal skin of .D. gemmacea (Pl. XV. fig. 1), the whole of the viscera 
are found to be packed together beneath a thin delicate membrane, similar to that 
which in Doris has been named the peritoneum *. This membrane is attached along 
the sides to the inner surface of the skin, and, behind, to the anterior margin of the peri- - 
cardium, and seems to furnish a pretty complete lining to the visceral cavity. It is per- 
forated with numerous circular orifices (c), no doubt for the passage of the blood from 
the visceral cavity to the skin on its way to the heart. 
The anterior extremity of the alimentary canalis seen in front as a constricted tube, 
passing through the nervous collar. On the right, in front, is the great bulk of the 
reproductive organs; on the left, and further back, the liver-mass, with the anterior 
stomach or crop lying between it and the genitalia. At the posterior extremity of the 
visceral cavity, above the liver, is the heart enclosed within the pericardium, which is 
apparently enveloped in a fold of the peritoneum. 
Digestive System.—The mouth (Pl. XVIII. fig .1) opens downwards and forwards, in a 
depression or cleft in the thickened anterior margin of the foot, and is a small cireular 
orifice; there are no fleshy lips as in Doris, but it leads at once into a rather wide 
chamber, with strong muscular walls (Pl. XVIII. fig.15). From the inner surface of the 
posterior wall of this chamber or sac, a nipple-formed proboscis (d) projects exactly in à 
line with the oral or outer orifice (c). The proboseis is perforated in front, and is formed 
by the invagination of the posterior wall of the sac ; and on laying it open (Pl. XVIL fig. 2) 
it is found to enclose the anterior extremity of the alimentary tube, the wall of which, 
in front, is fused with that of the proboscis around the perforation; or, in other words, 
the extremity of the alimentary tube forms the inner wall of the proboseis, the outer 
wall being nothing more than the posterior wall of the sac reflected over it. 
Two or three stout muscular bands (Pl. XVI. fig. 3 %) pass from either side of the sac or 
sheath of the proboscis, and, stretching backwards, become attached to the skin at the 
sides of the body. These are the retractors of the proboscis or sheath, and are the ho- 
mologues of the retractors of the oral channel in Doris, the sac or sheath being the true 
representative of the channel of the mouth in that animal. A few minute threads 
(Pl. XVIII. fig. 14), apparently muscular, pass backwards from the wall of the sac to 
the outer surface of the alimentary tube, which probably regulate the action of the 
parts on the exsertion of the proboscis. 
For some distance behind the sac or sheath, the alimentary tube continues constricted 
(Pl. XVI. fig. 3 7), and must, in fact, form the inner wall of the proboscis when that orga? 
's exserted. The tube then rather suddenly dilates (m), and is shortly again constricted. 
Behind, at each side of the dilated portion, is placed a glandular nodule (0); which opens 
* “On the Anatomy of Doris,” by Albany Hancock and Dennis Embleton, M.D., Phil. Trans. 1852, p- 208. 
