CLASSIFICATION OF THE GASTEROPODA. 



407 



brauchiata) or gasteropods with the gills on the back and sides 

 and towards the hind part of the body ; as Bulla, Aplysia, 

 Doris, Eolis, etc. The name is from the Greek opisthe, be- 

 hind, and bragchia, gills. 



4. rlETEROPODA or Nucleobranchiata, or marine gasteropods 



with a fin-like tail, or fan-shaped ventral fin provided with 

 a sucker for attachment. They swim rapidly at the surface 

 of the sea. Oarinaria is an example. The name Heteropoda 

 conies from the Greek heteros, different from others, and 

 pous,.foot. 



5. PTEROPODA or Wing-footed gasteropods of the open sea ; as 



Hyalea, etc. The name is from the Greek pieron, a wing, 

 and pous, foot. 



1. The PROSOBRANCHIAT!, as already stated, have pec- 

 tinated or comb-teeth-like gills situated towards the 



FIG. 551. 



Scorpion-shell, Pteroceras lambis, L. Reduced. Chinese Seaa.* 



forward part of the body. They have a mantle forming 

 a chamber over the back of the head ; and in this chamber 

 are the excretory orifices, and, in most cases, the gills 

 also. A shell is always present, and into this all the soft 

 parts can be withdrawn. 



* Foreign shells are quite as likely to fall in the hands of American 

 students as those of our own coast. 



