ORDER HETEROPODA; FIROLID^. 391 



part of the body, which is also regarded as analagous to one of 

 the divisions of the foot (seen very distinctly in the lobe which 

 bears the operculum in the Strombidae) is enlarged to form a 

 powerful caudal fin, by the agency of which the creatures swim 

 rapidly through the water. At the edge of the true foot, in 

 many species, there is a sort of sucker, by which the animals can 

 attach themselves to floating sea- weeds and other objects. They 

 swim with the back downwards. The branchiae are often want- 

 ing ; and it is curious that this is sometimes the case in particu- 

 lar species of a genus, or even in certain individuals of a species 

 in which branchiaB usually occur. When present, they are 

 generally collected together, with the viscera, into a mass on the 

 back of the animal, which is often protected by a shell ; and in a 

 few species, which are furnished with an ample shell, the branchiae 

 are placed in a regular dorsal cavity. The body is gelatinous in 



FIG. 682. CARINARIA . a, mouth ; b, tentacula ; c, eyes ; d, stomach ; , gills ; /, anus ; g, shell ; 

 h, liver; t. foot; *, sucker. 



its consistence, and so transparent as to permit much of its in- 

 ternal organisation to be seen. The mouth is furnished with 

 a long muscular proboscis, which can be protruded or drawn 

 in at will ; it contains a rasp-like tongue, armed with recurved 

 spines. They appear to feed on small marine animals. These 

 singular Mollusks are divided into two families. In the FIRO- 

 LIDJE, which present the characters of the Order in their typical 



