38 INTEODUCTION TO CLASSIFICATION. 



consists essentially of a cartilaginous cushion, supporting, as on 

 a pulley, an elastic strap, which bears a long series of trans- 

 versely disposed teeth. The ends of the strap are connected 

 with muscles attached to the upper and lower surface of the 

 hinder extremities of the cartilaginous cushions ; and these 

 muscles, by their alternate contractions, cause the toothed strap 

 to work, backwards and forwards, over the end of the pulley 

 formed by its anterior end. The strap consequently acts, after 

 the fashion of a chain-saw, upon any substance to which it is 

 applied, and the resulting wear and tear of its anterior teeth 

 are made good by the incessant development of new teeth in 

 the secreting sac in which the hinder end of the strap is lodged. 

 Besides the chain-saw-like motion of the strap, the odontophore 

 may be capable of a licking or scraping action as a whole. 



The other peculiarity of the alimentary canal of the Bran- 

 chiogasteropoda is that it is always bent upon itself, at first, not 

 to the neural, but to the haemal, or heart side of the body the 

 rectum very commonly opening into the mantle cavity, above 

 the cephalic portion of the body. 



In most Branehiogasteropoda the foot is a broad, flat, mus- 

 cular body, without any distinct division of parts ; but in some 

 forms, such as the Heteropoda of Cuvier, it is divided into three 

 very well-marked portions an anterior, a middle, and a pos- 

 terior, which are termed respectively the propodium, mesopo- 

 dium, and metapodium ; while the Aplysise, in which the foot 

 proper has the ordinary composition, exhibit processes from the 

 lateral and upper surfaces of that organ, having the form of 

 great muscular lobes, which serve as a sort of aquatic wings to 

 some species, and are termed epipodia. 



The Brancliiogasleropoda are such of the Gasteropoda of 

 Cuvier as breathe water either by means of the thin wall of the 

 mantle cavity (Atlanta, e.g.}, or by special pallial branchise 

 (Pectinibranchiata, Tectibranchiata, &c.), or by certain parts of 

 the integument of the body (Nudibranchiata) more or less 

 specially modified. 



