THE TULMOGASTEROPODA AND PTEROPODA. 39 



the snails and slugs, which agree with the Branchiogastero- 

 pocla in the general characters of their body, mantle, nervous 

 and respiratory systems, and in possessing an odontophorc ; 

 but differ from them, not only in breathing air by means of 

 the thin lining of the pallial chamber, but, as I believe, by 

 the direction of the flexure of their intestine. A careful dis- 

 section of a common snail, for example (Fig. 15), will prove 

 that, though the anus is situated in the same way as in the 

 Brancldogasteropoda, on the dorsal or haemal side of the body, 

 the primary bend of the intestine is not to the htemal, but to 

 the neural, side, the eventual termination of the intestine on 

 the haamal side being the result of a second change in its 

 direction. 



How far this neural flexure of the intestine really prevails 

 among the Pulmo-gasteropods is a question which must be 

 decided by more extensive investigations than I have as yet 

 been enabled to carrv out. 



The members of the class Pteropoda are small, or even 

 minute, molluscs ; all marine in habit, and for the most part 

 pelagic, or swimmers at the surface of deep seas. Like the two 

 preceding groups, they possess three principal pairs of ganglia, 

 an odontophore, a mantle, which is not divided into two lobes, 

 and which secretes a univalve shell, if any. But the propodium, 

 mesopodium, and metapodium are usually rudimentary, and 

 locomotion is almost wholly effected by the epipodia, which are 

 enormously developed, and, in most of the genera, perform the 

 office of aquatic wings still more efficiently than those of the 

 Aplysize. Furthermore, the intestine is flexed towards the 

 neural side of the body ; and the head, with the organs of sight, 

 are usually quite rudimentary. I include in this group not 

 only Criseis, Cleodora, Hyal&a, Pneumodermon, &c, but also 

 the aberrant genus Dentalium* 



The last class of this series is that of the Cephalopoda — 

 comprising the Poulpes, the Cuttle-fishes, the Squids, and the 

 pearly Nautilus ; a group definable by most marked and distinct 

 characters from all the preceding, though it resembles them in 



* Dentalium resembles the Pteropoda in its rudimentary head, the neural flexure 

 of its intestine, its epipodial lobes, and the character of its larva. 



