MR. T. H. HUXLEY ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA. 37 
and acuminated inferiorly. The elongated lanceolate transparent operculum is fixed 
upon its posterior surface. 
The animal moves by the vigorous flapping of its fin. When it withdraws within 
its shell the head is just retracted, then the fin is folded in, and finally, the tail with 
its operculum covers up the whole. 
The male is distinguished from the female by the presence of a peculiar leaf-like 
penis {p), which is attached upon the right side of the body just above where it 
divides into the three portions of the foot, fig. 1. 
The Alimentary Canal commences above an oval buccal mass, widens gradually into 
the stomach, then narrows again and opens into a quadrangular sac, which commu- 
nicates with the liver. From the anterior and upper part of this sac the rectum is 
continued and runs forward to the upper and dorsal part of the branchial cavity, in 
which it terminates by a tubular anus. 
The mechanism of the tongue exactly resembles that of FiroMdes. Two long 
cylindrical salivary glands (/) open into the anterior part of the oesophagus. They 
are simple caeca lined by a thick epithelium. 
The Liver {I, fig. 3) is a wide conical sac with sacculated and glandular walls ; its 
communication with the quadrangular dilatation of the intestine is so wide that it 
may almost be considered as a diverticulum thereof, and it extends back as far into 
the spire of the shell as any of the viscera. 
The rectum is of a pinkish colour, and is richly ciliated internally. 
Circulatory System (figs. 2, 3, 4). — The heart resembles that oi Firoldides^ and con- 
sists of an auricle {u) and a ventricle {v). It lies parallel to the reetum, with the 
auricle forwards at the base of the mantle-cavity, and the animal is therefore proso- 
branchiate. The aorta proceeds from the apex of the ventricle, and immediately 
after its origin divides into two branches, one of whieh runs backwards to the visceral 
mass, while the other passes forwards close beneath the stomach, until it terminates 
in the buccal mass. After passing over the suboesophageal ganglia, it gives off a 
downvv'ard branch to the fin, but I did not observe the peculiar termination of this 
artery which obtains in Firoloides-, this perhaps may be aecounted for by the greater 
muscularity of tlie fin in Atlanta, rendering it less transparent. The venous blood 
has no distinct channel, but returns to the heart by the cavity of the body. The re- 
turning current of blood-corpuscles is very obvious ; they seem to pass quite freely 
in all directions round the intestine, aorta and nervous centres, the general tendency 
being always backwards towards the heart. 
Respiratojy System. — Very distinct gills are figured by Eydoux and Souleyet in 
most Atlantee, but their presence in this species was decidedly exceptional, the majo- 
rity of specimens presenting no trace of them. Once I noticed a bundle of long 
branchial filaments depending from the wall of the mantle-chamber ; and in another 
case rudimentary and undeveloped short processes of the same kind were to be seen ; 
they contained canals, through which a small portion of the returning venous blood 
was diverted, fig. 4. 
MDCCCLIII. G 
