MR. T. H. HUXLEY ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA. 43 
the nervous ring, and eventually dividing into two branches, one for each epi- 
podium. 
There is a more distinct rudiment of a venous system in this mollusk than in 
Firolmdes or Atlanta. A wide canal traverses the mantle towards its upper part ; it 
is crossed by various muscular bands. Another distinct canal can be traced from 
the auricle towards the right side, skirting the lower border of the branchial chamber. 
Whether it becomes continuons with the right extremity of the previous canal or not, 
I could not certainly determine. The blood flows from both ends of the first-men- 
tioned canal towards the auricle, but on the left side there does not appear to be so 
distinct a venous canal as on the right. 
In all species of Cleodora there is an elongated sac, in its structure, contractions, 
and position relatively to the heart, exactly resembling that of the Heteropoda. It 
communicates by a small aperture with the cavity of the mantle. 
The nervous system in all these species consists of three ganglia on each side of the 
oesophagus. Four of these form a mass, placed entirely below the oesophagus, and 
the other two, placed in contact with and immediately above them at the side of the 
oesophagus, are united above by a broad flattened supraoesophageal commissure. 
The upper {cephalic) ganglia give off two principal branches to the rudimentary 
eyes and tentacles. The anterior pair of the lower mass (the pedal ganglia) give otf 
branches to the epipodia and expansion of the foot generally ; posteriorly they carry 
two small auditory vesicles, with many otolithes. 
The posterior {parieto-splanchnic*) ganglia give off their principal branches to the 
mantle. 
The aorta passes between the pedal and parieto-splanchnic ganglia. 
3. I have mentioned that a third subtype appears to be formed by Spirialis and 
Limacina, which are the only Pteropods with spiral shells. Spirialis, again, is the 
only Pteropod with an operculum. But a more important difference for my present 
purpose consists in the fact, that in these genera the mantle-cavity (and with it the 
anus) opens on the dorsal side of the animal. I have not myself been fortunate 
enough to obtain specimens of these genera, and as the attention of those anatomists 
who have examined them does not appear to have been specially directed to tiiis point, 
it is impossible to make out with certainty whether the first flexure of the intestine 
is also dorsal, or whether, as in all other Pteropods, it is ventral. I cannot think 
that any real variation will be found to occur among closely allied forms, in a matter 
so fundamentally connected with their whole structure and mode of development ; 
and I would suggest that here also the bend of the intestine is truly ventral, but that 
by a continuation of the process by which the anus is thrown to the left side in Cleodora 
and to the right in Pneumodermon, it (with the branchial cavity) is thrown to the 
dorsal side in Limacina and Spirialis. Such a change would be completely paral- 
leled by the arrangement of the parts in the Ascidians, where the first bend of the 
intestine is dorsal ; but the cloaca, which corresponds to the mantle cavity, opens on 
* See below, p. 54. 
