THE GASTROPODA 



183 



of the oviduct and penis may be approximated, a condition found 

 in the majority of the Stylommatophora (Fig. 104). 



The Pulmonates never have a free larval form ; if a veliger is 

 developed it is always contained in the egg membranes. The 

 majority of Stylommatophora do not pass through a veliger stage, 

 and in other forms the velum is almost always ill developed (Fig. 



Flo. 172. 



A series of Stylomniatophorous Pulmonata, showing the reduction of the shell. A, Helix 

 pomatia ; , Daudebardia brevlpes ; C, Testacella haliotidea ; D, Arion ater. a, external shell in 

 A, B, C ; shell-sac (closed) in D ; b, orifice of the pallial or pulmonary cavity. (From Lankester, 

 after Ferussac, Pfeiffer, and Reeve.) 



119, A, re). The Pulmonates are for the most part aerial, but 

 some live in fresh water, and others, but they are exceptional cases, 

 are marine. The Pulmonates are distributed over the whole world, 

 and include some seven thousand species, of which more than half 

 belong to the genus Helix. Most of them enter into a resting stage 

 during some part of the year ; in the summer in hot climates, in 

 the winter in cold climates. In our country the hibernation lasts 

 for rather more than a third of the year. 



