MU. T. H. HUXLEY ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA. 39 
The Eyes {i, fig'. 6) resemble those of Firoldides, except that their pigment is black. 
The Auditory Vesicles are spherical and about slolh of an inch in diameter. Each 
contains a single strongly refracting globular otolithe of about g^dth of an inch in 
diameter. In some cases this had a slow movement of rotation upon its axis, 
%• 6.J- 
Now in regarding Firoloides and Atlanta, whose structure has just been described, 
as illustrations of a typical form, the following circumstances appear to me to be of 
importance : — 
1. The intestine is bent dorsad, or towards the side on which the heart is placed. 
The visceral mass is situated below and behind the posterior portion of the alimentary 
canal; it may be called a post-abdomen. 
2. Atlanta is prosobranchiate ; Firoldides is neither opisthobranchiate nor proso- 
branchiate. 
3. The foot consists of three parts, the propodium, mesopodium and metapodium, 
in Atlanta-, but of these the mesopodium disappears in Firoloides, and the raetapodium 
becomes very rudimentary. 
4. The auditory organs appear to be connected with the cephalic ganglia. 
5. The animals are unisexual. 
Ill, Anatomy of Pteropoda. (Plate IV.) 
The variation of form undergone by the members of this group is perhaps greater 
than that which takes place in any other, except it may be the Nudibranchiata, and 
hence their structure is particularly instructive. 
Three very distinct modifications of the type present themselves at first sight. The 
first is characterized by the non-development of the mantle and the full development 
of the foot, ex. Pneumodermon, Clio, fig. 1. 
The second, by the great development of the mantle, by its cavity opening upon 
the ventral surface, and by the minuteness or absence of the mesial portion of the 
foot, ex. Cleodora, fig. 4. 
The third resembles the second, but the mantle-cavity is placed upon, or at any 
rate opens upon the dorsal surface, ex. Spinalis, Limacina. 
1. It is very remarkable that Cuvier should not have recognized in the '‘espece de 
menton,” and the “deux petits levres*” of Pneumodermon, nor in the “ deux tenta- 
cules triangulaires ” of CUo-\-, the homologues of the foot of the Gasteropoda. In 
fact it was on the strength of their having no such appendage that he founded his 
new order of Pteropoda:{;, and yet the resemblance of the inter-alar appendages in 
these twogenera§ to the foot of Gasteropods is so striking as at once to point to their 
real nature. 
* Mem. sur le Pneumoderme, p. 7. Mem. sur le Clio, p. 6. 
t Mem. sur le Clio, p. 9 ; sur I’Hyale et le Pneumoderme, p. 10. 
§ This is fuUy recognised by Leuckaet, “ Ueber die Morphologic,” &c. p. 149. 
G 2 
