MILNE-EDWARDS. 591 



the hinder part of the body ; the cervical region is always 

 naked ; the animals are hermaphrodite ; and the shell, well 

 developed on the larva, becomes rudimentary, or entirely dis- 

 appears in the adult. 



This order is constituted of Gasteropods, distributed in 

 three of Cuvier's orders, viz. his Nudibranches, Infero- 

 branches, and Tectibranches. In the classification of La- 

 marck we find them united together in the first section of 

 Gasteropods ; but they are mixed up with the Patellae and 

 the Chitons, whose structure is very alien. M. de Blainville 

 again scattered them throughout four orders, which have no 

 nigh affinity to each other ; and some are widely sundered 

 from their relations by the introduction of the Pteropods 

 between the Aplysiae and the Eolides. The Opistobranches 

 form, however, a very natural group ; and the characters 

 which combine them together, as well as those which sepa- 

 rate them from other Gasteropods, seem to justify their 

 elevation to an ordinal rank in their class. 



In the second division of the branchiferous Gasteropods, 

 the abdominal portion of the body does not become rudi- 

 mentary as in the Opistobranches, but is developed, on the 

 contrary, in due proportion to the cephalic and pedal por- 

 tions ; and it is, throughout life, protected by a shell suffi- 

 ciently large to allow the body to be drawn within its cavity. 

 The cloak is always directed forwards, and forms, above the 

 cervical region, a vaulted chamber of variable capacity where 

 the excretory vents open externally, and in which the 

 branchiae are lodged. These respiratory organs are com- 

 posed of simple lamellae, laid parallel to each other, inserted 

 along a vascular support, and affecting a pectinated or comb- 

 like figure. In general they are situated in front of the 

 heart, and even when they are prolonged to the posterior part 

 of the body, the branchio-cardiac vessels trend from before 

 backwards, so that the blood comes to the heart in a current 

 the reverse of that in the Opistobranches. Lastly, every 

 species has its male and female individuals. 



In the classification of Cuvier the Gasteropods that pos- 

 sess this assemblage of anatomical and physiological charac- 

 ters are disseminated in the orders Pectinibranches, Tubuli- 

 branches, Scutibranches, and Cyclobranches. Lamarck has 

 arranged a part of them amongst his Gasteropods, and an- 

 other part amongst his Trachelipods, where they mix but do 

 not combine with the Pulmonata. M. de Blainville forms 

 with them the first and the third of his subclasses of Parace- 

 phalophores, and intercalates between these two groups all 

 the other Gasteropods. Milne-Edwards justly prefers to re- 



