PNEUMODERMA. — Another genus of the Pteropod Mollusca, anatomized by Cuvier, ernbraces the Pneunzoderma, which presents many peculiarities of structure, more es pecially as relates to the position of the re spiratory organs and the tentacula placed at the sides of the mouth and other anatomical details, so that it will require to be described at length. In this genus, the body is without a shell, having two fins situated on the sides of the neck, but is distinguished by having two bunches of tentacula in the vicinity of the mouth, and by carrying its branchial organs at the surface of its body near its posterior ex tremity.
The body of this mollusk is of an oval shape (fig. 115. 1, a), the head (b) is round, and the neck constricted. The mouth opens upon the summit of the head, and is guarded in front by two longitudinal prominent lips (d, d), beneath which is a pointed, fleshy ap pendage (e).
The fins ( f, f), attached to the sides of the neck, are fleshy and much smaller than in either of the preceding genera.
The branchim (g,g) are situated at the op posite extremity of the animal, and form two prominent lines somewhat in the shape of two capital Cs placed back to hack and united by a transverse band. These lines give off from each side small prominent laminm, ar ranged much in the same way as the leaflets of a pinnate leaf.
On the right side of the body, and a little above the branchial apparatus, is seen a simple prominent line (h), which, on opening the animal, is discovered to be the branchial vein opening into the auricle of the heart (0, which, enclosed in its pericardium, is situated upon this side (k).
On opening the integument, which is com paratively soft, the mass of the viscera is found to be enclosed in a muscular envelope, the fibres of which are almost all longitudinal (fig. 115. 4, /, /). The pericardium is not con tained within this fleshy envelope, which is only adherent to the skin in the vicinity of the branchim ; for in this place are situated the arterial trunks, which convey the blood of the body into the pulmonary organ.
On dividing. the muscular layer (fig. 115. 5 and 6), it is seen that almost the whole space within is nearly equally shared between the liver (m), the testicle (n), and the ovary (o), the latter being slightly the largest viscus of the three. The ovary occupies the bottom of the visceral sac, the testicle is on the left, and the liver on the right side.
The stomach is very capacious, and sur rounded on all sides by the liver, which pours the bile directly into its interior through nu merous orifices, exactly as in Conchiferous Mollusks. The walls of the stomach are thin and internally present numerous little cavities, into which the biliary pores open (fig. 115. 7, p). The rectuni is short, and opens beneath the right fin (fig. 115.7,g).
The mouth is a fleshy mass of considerable size, from which two fleshy appendages are prolonged backwards (fig. 115. 7, 8, r r). The tongue is covered with short reverted spines, the use of which is evidently to assist in de glutition (fig. 115. 8, x). The posterior part of the mouth, in which the tongue is situated, is separated from the anterior (fig. 115. 7,8,u), which is membranous, by a fleshy construction (fig. 115. 8, ttt), upon which are perceived three small tubercles.
The opening of the mouth is guarded by two bunches of tentacula (fig. 115. 1 to 8, c), which the animal can, at will, either protrude or retract within the oral orifice. Each of these tentacles consists of a delicate filament, terminated by a minute tubercle excavated in the centre. These organs forcibly remind us of the complicated oral apparatus of the Clio already described, and most probably are in struments of prehension analogous to those of the Cephalopoda.
The salivary glands (fig. 115. 7, 8, v v) are long and ample, and their excretory duct, as it passes above the brain, is obviously dilated (x x).
The brain (fig. 115. 9, y) is a narrow trans verse band, and among the nerves which it furnishes, two may be observed on each side, which are connected beneath the oesophagus with a group of six ganglia, four of which are mesial and of considerable size, while the other two placed at the bides are of smaller dimen sions.
There is nothing peculiar in the structure of the generative apparatus, which nearly resembles that of Hyalea and Clio. The penis is small, and situated beneath the mouth. It is protruded between the two little lips situated upon the anterior surface of the head (fig. 115. I, 5, d d). The common generative orifice is found immediately in front of the anus, and is prolonged externally into a kind of furrow, which is directed forwards.
(T. Rymer Jones.)