PECTINIBRANCHIATA. 51 



Class II. 

 GASTEROPODA*, or UNIVALVES. 



Body of a coRical shape : mantle forming a single lobe, which 

 only covers the front: head usually distinct, and furnished 

 with tentacles, of which the upper pair (in those kinds which 

 have four), or the single pair, have in most cases two eyes, 

 placed either at their tips or base, or on separate stalks : foot 

 a muscular disk, by means of which the animal generally 

 crawls, or sometimes floats in an inverted position on the 

 under surface of the water : repy^oductive system various ; some 

 kinds being hermaphrodite, and ha\'ing both sexes united in 

 the same individual, but requiring impregnation by another 

 individual ; while in other kinds the sexes are separate, each 

 individual being either male or female : respiratory system con- 

 sisting of gills, or lung-like organs ; the former, and some of 

 the latter, being possessed by aquatic kinds ; while the terres- 

 trial kinds are only furnished with the lung-like organ : those 

 kinds which are aquatic, and have this last organ, eliminate 

 oxygen from the water and also respire atmospheric air ; but 

 the terrestrial kinds, or Snails, breathe only the pure air, like 

 vertebrated land animals. 



Shell usually present, conical or spiral, and covering the 

 whole, or most important parts, of the body. 



The only two Orders which we have to deal wdtli in 

 this division of the subject are as follows : — 



I. Pectinibranchiata. 



II. Pulmonobranchiata. 



Order I. PECTINIBRANCHIATA t 



Body spiral : respiratory apparatus consisting of a single 

 comb-like gill, which is placed within the mantle, on the upper 

 side of the head. 



Shell external and spiral. 



* Foot forming the belly. f Having comb-like gills. 



D 2 



