ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA 169 



rudimentary eyes and tentacles. The anterior pair of the lower mass 

 {the. pedal ganglia) give off branches to the epipodia and expansion of 

 the foot generally ; posteriorly they carry two small auditory vesicles, 

 with many otolithes. 



The posterior (j>arieto-splanchnid) 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 

 Spinalis and Liniacina, 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 for- 

 tunate enough to obtain specimens of these genera, and as the atten- 

 tion of those anatomists who have examined them does not appear to 

 have been specially directed to this 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 continua- 

 tion of the process by which the anus is thrown to the left side in 

 Cleodora and to the right in Pneumodennon, it (with the branchial 

 cavity) is thrown to the dorsal side in Liniacina and Spinalis. Such 

 a change would be completely paralleled 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 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 Cymbnlia, 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 Spinalis (E. and S., plate 11, p. 13, &c.), greatly 

 strengthens this view of the ca32. 



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 corresponding to those in which the organiza- 

 tion of the Heteropoda was summed up : — 



' See below, p. 181. 



