84 IIETEROPODA. 



Messina, and would seem to be one of the most simply 

 organized among the Mollusca ; unfortunately there is no 

 figure given. 



Sub-class HETEROPODA. 



Body formed for floating, the anterior portion usually 

 greatly developed, the abdomen rudimentary, and exposed, 

 or spiral, and protected by a shell. Tongue armed with 

 recurved hooks. Head proboscidiform, usually provided 

 with eyes and tentacles. Heart prosobranchiate. Gills 

 tufted, situated in advance of the heart. Foot laterally 

 compressed and fin-like, or flat, and furnished with a 

 vesicular appendage or float. 



Shell spiral, sub-spiral, or altogether wanting. 



Animal pelagian, predatory, bisexual, with reciprocal im- 

 pregnation. Larva shell-bearing, with ciliated head-lobes. 



In the Branchiferous Gasteropods which we have already 

 considered, and also in the Pulmoniferous tribes of Mol- 

 lusca, the fleshy part of the foot is dilated and flattened, 

 and the visceral mass is situated above it ; but in the Hete- 

 ropocls, which inhabit the high seas, and which are organized 

 for swimming, the locomotive organ, or foot, is laterally 

 compressed into the form of a vertical and rounded fin, or 

 it is rudimentary and provided with a vesicular appendage, 

 or float, to enable the animal to sustain itself near the sur- 

 face of the water. Not being formed for crawling on the 

 earth, or creeping on the bed of the sea, these animals, 

 moreover, have the front portion of the body sometimes 

 enormously developed, so as to qualify them better for their 

 pelagian life. In Ianthinidie and Atlantida, where the 

 shell is well developed, the animals are retractile within 



