586 HISTORY OF SYSTEMS. 



funnel-shaped expansion ; the gills are internal, and the 

 creatures use their fins as oars to their boat -like shell when 

 they swim on the calm ocean. 



This order embraces four families : 1, Cleodorid^;, which 

 have an elongate, or subglobose conical glassy shell ; and the 

 fins are simple without any intermediate foot-like lobe. 

 2, Limacinid^e, distinguished from the first by its spiral 

 discoidal shell. 3, Cuvierid^:, with a glassy conical cylin- 

 drical shell, which becomes truncate in its adult state : and 

 4, Cymbuliad.e, in which the shell is only of a firm gela- 

 tinous substance, variable in figure, but generally somewhat 

 resembling a slipper. 



The second order, or Gymnosomata, are destitute of any 

 shell ; the head is distinct, and there are two or four distinct 

 fins on the neck, and a central foot-like appendage between 

 their bases. The gills are external. Three families enter 

 into the composition of the order. The Pneumodermid^e 

 have a fusiform body, and the head is furnished with two 

 contractile arms, armed with peduncled suckers ; they have 

 two wings, and the gills are posterior. The Cymodoceid^e 

 have two wings on each side, placed in the space that sepa- 

 rates the body into two parts ; and the Clionid^e have two 

 wings only, which are said to be covered with a vascular net- 

 work, and serve the purpose of gills. They and the Li- 

 macinas are the chief food of the whales.* 



3. Gasteropoda. 



The few alterations made by M. Rang in the arrangement 

 of the Gasteropods, are in harmony with Cuvier's principles. 

 His Nucleobranches, corresponding pretty exactly to the He- 

 teropods of Cuvier, are placed at the head to make the alli- 

 ance of the class with the Pteropods less abrupt ; but it 

 would have been better to have removed the Pteropods from 

 the position they occupy to a lower rank ; for we now know 

 that their structure is of lower organization, and we know, 

 moreover, that the pectinibranchial Gasteropods leave no 

 space between them and the Tetrabranchiate Cephalopods to 

 be occupied by any intervening class. f The orders Pul- 

 mones-opercules and Cirrhobranches were rejected by Cuvier, 

 and with good reason : the former rested on characters incon- 

 gruous with the system, and apparently of not more than sub- 

 ordinal value ; and the structure of the Dentalium, the only 

 cirrhobranchial Mollusk, seems to prove it to be a mere family 



* Syn. Brit. Mus. 1842, p. 86. 



t Blainville and Souleyct maintain that the Pteropods are a tribe of Gas- 

 teropods allied to Bulla and Aplysia. 



