CHAPTER XXV. 



GASTEROPODA {Goyitimied). 



Heteropoda and Pulmoxifeea. 



Oeder III. Heteropoda, or Nucleobeanchiata. — The Gas- 

 teropods of this order differ from the typical members of 

 the class in being organised to lead an existence in tlie open 

 ocean, locomotion leing effected hy a Jin-lile tail, or by a fan- 

 shaped xertiecdly-fiattened ventral fin. They are found swim- 

 ming at or near the surface of the ocean ; and the body may 

 be completely protected by a shell, within which the animal 

 can retire, and which can be closed by an operculum. In 

 other cases, as in Carinaria (iig. 427), the body is large, and 



Fig. 427. — Heteropoda. Carinaria cynibium. p. Proboscis ; f, Tentacles ; ?j, Brancliiie ; 

 s, Sliell ; /, Foot ; d, Disc. (Mter Woodward.) 



there is only a small shell protecting the gills and heart. In 

 other cases, again, the shell is completely wanting. The 

 order is divided into the two families of the Firolidcc and 



