GASTEROPODA. 



51 



fresh water, and have mostly a spiral shell. [This order 

 embraces more than Cuvier's Pulmonata.] 



7. Nudibranchiata. — The animals breathe through 

 lungs of various forms, which are naked, and situated on 

 the back, and never form a shell. They live, without 

 exception, in the sea. (Cuvier's Inferobranchiata.) 



8. Heteropoda. — The foot of the animal is changed 

 into a vertical fin, Avhich serves for swimming ; they live 

 only in the sea. Shell, when present, generally thin. 



Order I. 



PECTINIBRANCHIATA. 



Organs of respiration in the shape of a comb, and for 

 the most part single ; the animal always forming a shell. 



This order of the class Gasteropoda contains the greatest 

 number of genera ; but as long as the animals of all the 

 genera and species are not perfectly known, the classifi- 

 cation must remain in some degree uncertain. The shells 

 of many of them are amongst the handsomest and most 

 striking known. The genera Conus, Cyprgea, Murex, 

 Strombus, Mitra, and Oliva, are favourites with all col- 

 lectors of shells — and deservedly so — the varied forms, 

 characters, colouring, and markings rendering them objects 

 of great beauty and interest. 



Section I. 



The animals have a distinct breathing tube, and the 

 shell is channelled, with the exception of that of the 

 genus Coriocella, in which it is internal. 



