40 INTRODUCTION TO CLASSIFICATION. 



among the Pulinogasteropods is a question which must be 

 decided by more extensive investigations than I have as yet 

 been enabled to carry out. 



XIV. THE PTEROPODA. 



The members of this class are small, or even minute, mol- 

 luscs; 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 odonto- 

 phore ; a mantle, which is not divided into two lobes, and which 

 secretes a univalve shell, if any. But the propodium, mesopo- 

 dium, 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 Aplysise. 

 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, Hyalsea, Pneumodermon, &c., but also the aberrant 

 genus Dentalium* 



XV. THE CEPHALOPODA. 



This class comprises the Poulpes, the Cuttle-fishes, the 

 Squids, and the pearly Nautilus; and it is definable by most 

 marked and distinct characters from all the preceding, though 

 it resembles them in fundamental characters. Thus, the mantle 

 is related to the body, as in Pteropoda and Gasteropoda ; when 

 an external shell exists it is composed of a single piece ; and the 

 Cephalopods have an odontophore constructed upon just the 

 same principle as that of the other classes. The nervous 

 system, the foot, and the epipodia exhibit the same primary 

 relations as in these groups, and there is a distinct head, with 

 ordinarily well-developed optic and olfactory organs. That 

 which essentially characterises the Cephalopoda, in fact, is simply 

 the manner in which, in the course of development, the margins 



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

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



