44 MR. T. H. HUXLEY ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA. 
the ventral side, carrying the anus with it ; and even in other Pteropoda we find 
changes in the arrangement of the mantle-chamber, which to a great extent modify, 
without however essentially altering, the normal arrangement, e. g. in Hyalcea and 
CymhuUa, where the posterior extremity of the mantle-chamber extends up to the 
dorsal surface. 
Furthermore, the position of the heart, which remains on the ventral side in 
Spirialis (E. and S., plate 11. p. 13, &c.), greatly strengthens this view of the case. 
Leaving this question in abeyance until further light is thrown upon it, we may, I 
think, enunciate the following propositions with regard to the Pteropoda correspond- 
ing to those in which the organization of the Heteropoda was summed up : 
1. The intestine is bent towards the ventral side; the visceral mass is placed 
above, and in front of the anus ; it may be called an abdomen. 
2. So?ne Pteropoda are prosobranchiate, others intermediate, otheis opistho- 
branchiate. 
3. The foot consists of four parts -.—three, the propodium, mesopodium, and 
metapodium, such as are found in the Heteropoda ; and a fourth, the epipodium, 
not found in the Heteropoda. All of these parts (propodium ?) may be distinguished 
in Pneumodermon and Eurihia, while all but the epipodium and metapodium ha\e 
disappeared in Cleodora. 
4. The auditory organs are connected with the pedal ganglia. 
5. The Pteropoda are hermaphrodite*. 
PART II. 
The Heteropoda and Pteropoda, whose anatomy I have just endeavoured in a very 
general way to sketch and illustrate, may be regarded, in some respects, as opposite 
poles of the development of the archetype of the Cephalous Mollusca. We have 
now to consider what that archetype is, and by what process it has become modified 
into the actual forms which have been described ; and with the solution of these 
questions is connected the meaning and justification of certain new terms of which 
I have made use. 
The most proper way of proceeding in this matter would of course be, to tiace the 
develo'p'nicnt of the Heteropoda and Pteropoda. Unfortunately, houevei, I have had 
no opportunity of doing this myself; and so far as I am aware, there is no account 
of the embryogenesis of Mollusks belonging to either of these classes extant 'f-. 
* This, it wiU be observed, is here stated for the first time. In the Heteropoda the nature of the generative 
system has been a matter of controversy, and I therefore gave an account of it at length in Firoldides and 
Atlanta. The hermaphrodism of the Pteropoda, on the other hand, is well-known, and a description of their 
generative organs would only have led to details without any morphological bearing. 
f Since the above paragraph was written, this hiatus has been filled, so far as the Pteropoda are concerned, bj 
Vogt (Bilder aus dem Thierleben, p. 289) and by Johannes Muller (Ueber die Entwickelungs-formen einiger 
niedern Thiere. Monatsbericht d. k. Akad. zu Berlin, October 1852). 
