506 



GASTEROPODA OPISTHOBRANCHIATA. 



A LARGE section of marine gasteropodous Mollusca is 

 unprovided with shells except in the larva state. The 

 majority of this group are hermaphrodite. When a shell 

 is present it is convoluted or reduced to a simple corneous 

 or calcareous branchial lid. The branchife are not lodged 

 in a supra-cervical cavity, and the heart, in the great 

 majority of instances, is placed in advance of the gills. 

 The auricle of the heart is usually behind the ventricle. 



The orders Tectibranchia and NudihrancJda of Cuvier 

 form two very natural sections of this division of Gas- 

 teropods. 



BULLIDiE. 



This tribe may be considered intermediate between the 

 two great sections of Gasteropoda, The shells of its 

 mollusks are always convolute, and more or less enveloped 

 by the animal, sometimes entirely invested, more rarely 

 absent. Except in the case of Tornatella there is no oper- 

 culum. The head of the animal is in the form of a simple 

 or lobed disk, and its lateral lobes are often greatly deve- 



