MOLLUSC A. 167 



Class V. HETEROPODA. 



NUCLEOBRANCHIATA. 



Free-swimming pelagic mollusks, mostly with a shell, and pro- 

 vided with a fin- like tail or with a ventral fin. With or without 

 branchige. Sexes distinct. 



The locomotive organs are modifications of the foot ; these are 

 directed upwards, the animal swimming on its back. In Atlanta 

 with a spiral shell, the foot is provided with an operculum. 

 Firola has no shell ; Carinaria has a small conical transparent 

 shell which hangs below, protecting the heart, branchiae, &c. The 

 remainder of the animal is a gelatinous transparent sac with two 

 eyes and a pair of tentacles. Bellerophon is one of the genera 

 of shells which goes to form the limestone beds of the Silurian 

 and Carboniferous epochs. 



This class is generally regarded as a simple order of Gastro- 

 poda ; but it is a subclass for Glaus. Its peculiarities are only 

 developed in the later stages of the embryo. 



Only two families are included in the Heteropoda. 



Firolida. Atlantidce. 



Firola = Pterotrachea. Atlanta. 



Carinaria. Oxygyrus. 



Cardiopoda. *Beilerophon. 



