REPORT OH THE PTEROPODA. 
9 
are of a rectangular outline, elongated in the direction of the axis of the digestive tube, 
the dorsal pair being a little shorter than the ventral. Their free surface bears a rather 
sharp prominent crest. The fifth plate is situated behind the other four, between the two 
dorsal ones ; its form is almost triangular, one of the angles being directed forwards. 
These masticatory plates have a structure resembling that of the “shell” of the Cymbuliidse, 
but are rather more dense ; striae of growth may be observed in them, and they are 
covered by the gastric epithelium. 
Behind the muscular band the stomach gradually narrows to pass into the intestine, 
and into this hinder portion of the organ on the left side opens the bile-duct. 
Huxley 1 asks with respect to the Limacinidse “whether the first flexure of the 
intestine is also dorsal” (like the pallial cavity) “ or whether, as in all other Pteropods, it is 
ventral.” In Limacina from its origin the intestine bends towards the right and dorsally, 
and eventually opens at the right side of the pallial cavity. Its flexure is thus the same 
as that called “dorsal” or “ haemal ” among the Gastropods (PI. I. fig. 3). 
The Circulatory and Excretory Organs. — The heart is situated at the posterior end 
of the pallial cavity, and is even visible, owing to the transparency of the mantle, behind 
the shield. It presents an auricle directed towards the left and a ventricle lying 
posteriorly, both contained in a pericardium, which is quite excluded from the circulation. 
In front of the heart is the elongated, thin-walled kidney, arising from the pericardium, 
with the cavity of which it communicates. It enlarges anteriorly in such a way as to 
form an elongated triangle whose base is forwards, and opens into the pallial cavity by a 
small narrow aperture. 
The Generative Organs. — The gonad, which in this case is a hermaphrodite gland, 
occupies all the initial portion of the visceral mass. The efferent duct arises anteriorly 
at the ventral aspect of the gland, and then passes forwards across the intestine and to the 
right of the oesophagus. This duct (PI. I. fig. 3, j) is very thin at its origin but expands 
about its middle, where its walls become glandular ; it then contracts again and reaches the 
accessory glands (albuminiparous and muciparous glands) and the receptaculum seminis. 
The genital aperture is situated at the right side of the cephalic region, and is protected 
by a kind of little operculum. At this opening commences the spermatic groove (PI. I. 
fig. 3 ,T), formed by a fold of skin, which is directed towards the dorsal surface of the head, 
passes to the right side of the right tentacle, and reaches the anterior surface of the fins, 
where it terminates at the opening of the copulatory organ. This is the same disposition 
as that already known to exist in the Cavoliniiclse. When protruded the copulatory organ 
divides into two branches, for instance in Limacina lesueuri (PI. I. fig. 2, c). 
Nervous System . — The nervous centres are united around the oesophagus behind the 
buccal mass. The cerebral ganglia (PI. I. fig. 7, c) are situated at the sides of the 
oesophagus, and connected by a long supra-cesophageal or dorsal commissure. 
1 On the Morphology of the Cephalous Mollusca, loc. cit., p. 43. 
(ZOOL. CHALL. EXP. PART LXVI. — 1888.) 
Uuu 2 
