MOLLUSCA. 7 



of it ; a muscular foot attached to the body in its whole length, 

 situated under the belly, and serving the purpose of locomotion. 



Order III. TIIACHELIPODA. Body in a great measure spi- 

 rally contorted, separated from the foot, and always enveloped 

 by a spiral shell ; a free flat foot attached to the base of the 

 neck beneath, for the purpose of crawling. 



Order IV. CEPHALOPODA. Lower part of the body contain- 

 ed in a bag-shaped mantle ; head protruding from the sac, 

 crowned by inarticulated arms, furnished with cups, and sur- 

 rounding a mouth having two horny mandibles. 



Order V. HETEROPODA. No arms arranged round the head; 

 no foot under the belly or throat for crawling ; one or more fins 

 without regular order, and not disposed in pairs. 



This arrangement is adopted in the following pages, but with 

 the order reversed ; and as a detailed account is afterwards given 

 of the distribution, it is unnecessary to present it here. 



In a very elaborate work by M. Blainville, entitled Manuel 

 de Malacologie et Conchyliologie, published in 1 825, there is 

 an exposition of the organization, physiology, and natural history 

 in general of the animals of this and the following classes, which 

 he places together under the name of Malacozoa. After treat- 

 ing of the shells of these animals, considered as a distinct de- 

 partment of science under the name of Conchyliology, he offers 

 an arrangement founded on the characters presented by the or- 

 ganization, of which the following are the leading divisions, in 

 so far as regards the animals of the present class : 



CLASS I. CEPHALOPHORA. 

 Order I. CRYPTODIBRANCHIATA, Order III. POT.YTHALAMACEA. 



II. CELLULACEA, 



CLASS II. PARACEPHALOPHORA. 



SUB-CLASS I. PARACEPHALOPHORA DIOICA. 



Order I. SIPHONOBRANCHIATA, Order II. ASIPHONOBRANCHIATA. 



SUB-CLASS II. PARACEPHALOPHORA MONOICA. 

 Order I. PULMONOBRANCHIATA, Order V. POLYBRANCHIATA, 



II. CHISMOBRANCHIATA, VI. CYCLOBRANCHIATA, 



III. MONOPLEUROBRANCHIATA, VII. INFEROBRANCHIATA, 



IV. APOROBRANCHIATA, VIII. NUCLEOBRANCHIATA, 

 SUB-CLASS III. PARACEPHALOPHORA HERMAPHRODITA. 



Order I. CIRRHOBRANCHIATA, Order III. SCUTIBRANCHIATA. 



II. CERVICOBRANCHIATA, 



