I/O ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA 



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. Some Pteropoda are prosobranchiate, others intermediate, others 

 opisthobranchiate. 



3. The foot consists of four parts : — three, the propodium, meso- 

 podium, and metapodium, such as are found in the Heteropoda ; and a 

 fourth, the epipodium, not found in the Heteropoda. All of these 

 parts (propodium ?) ma)' be distinguished in Pneuviodentwn and 

 Euribia, while all but the epipodium and metapodium have disap- 

 peared 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 ha\'e just 

 endea\'Oured in a very general way to sketch and illustrate, may 

 be regarded, in some respects, as opposite poles of the development 

 of the archet)'pe of the Cephalous MoUusca. 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 solu- 

 tion of these questions is connected the meaning and justification of 

 cei tain new terms of which I have made use. 



The most proper way of proceeding in this matter would of course 

 be, to trace the development of the Heteropoda and Pteropoda. Un- 

 fortunately, however, 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.^ 



But in any natural group of animals the grand laws of development 

 and growth are so uniform (the uniformity in fact constituting the true 

 bond of union of its members), that this want may be supplied by the 

 ver)- full information we possess with regard to other Mollusca. If 

 from these data certain general propositions can be established, it will, 



^ This, it will Ije 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 Firoloides 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. 



"- Since the above paragraph was written, this hiatus has been filled, so far as the Pteropoda 

 are concerned, by Vogt (Bilder aus dem Thierleben, p. 289) and by Johannes Midler (Ueber 

 die Entwickelungs-formen einiger niedern Thiere. Monatsbericht d. k. Akad. zu Berlin, 

 Octolier, 1852.) 



