101 



ORDER II. PNEUMONOBRANCHIATA. 



THE respiratory organs consisting of a number of 

 pulmonary vessels spread over an open or closed bag- 

 like cavity on the back of the neck, they breathe free 

 air, and either live constantly on the land or in the 

 water, in which latter case they come periodically to 

 the surface to respire. 



The shell is rarely wanting. 



This order is divided into families in the following 



manner : 



Sect. I. Inoperculated. Edge of the mantle adherent 

 to the back of the neck, forming a closed pulmo- 

 nary chamber, leaving a hole for the entrance and 

 exit of the air ; operculum none ; hermaphrodite. 



A. Terrestrial. Tentacles cylindrical, retractile; 

 upper pair having the eyes at their tip. 



Fain. 1. Arionidce. Head and tentacles re- 

 tractile ; end of the tail truncated, bearing a 

 mucous gland. 



Fam. 2. Helicidce. Head and tentacles retractile ; 

 end of tail simple, conical. 



B. Aquatic. Tentacles contractile, with the eyes 

 at or near their base. 



Fam. 3. Auriculida. Head elongated into a 



F3 



