102 PNEUMONOBKANCHIATA. 



rugose muzzle ; tentacles subcylindrical, eyes 

 near the inner side of their base. 



Fam. 4. Limnceidce. Head bifid ; tentacles com- 

 pressed, with the eyes on the outer side of 

 their base. 



Sect. II. Operculated. Mantle edge separate from 

 the back of the neck, leaving the pulmonic cavity 

 open ; tentacles contractile ; dioecious ; operculum 

 distinct. 



Fam. 5. Cyclostomidce. Muzzle ringed ; tentacles 

 two ; operculum spiral. 



Sect. I. INOPERCULATED. (Inoperculata.} 



The edge of the mantle adherent to the back of 

 the neck of the animal, forming a closed pulmonary 

 chamber, leaving only a hole for the entrance and exit 

 of the air, which is closed by an external valve on 

 the side of the cavity. 



They are all destitute of any operculum, but close 

 the shell, during the torpidity of the animal, with a 

 lid or epiphragm formed of its inspissated humours, 

 and sometimes hardened with a little calcareous 

 matter. 



They are all hermaphrodite, but require mutual 

 impregnation, and feed on vegetables ; but some few 

 have carnivorous propensities, and others, when they 

 live near man, acquire bad habits, and eat paper and 

 dead animal matter. 



They may be divided into two groups, by the form 

 of their tentacles, which conform to their more or less 

 aquatic habits. 



