THE LUNG-BREATHING GROUP 3311 



the shape. Nearly all spiral shells are dextral, but a few genera (Physa and 

 Lanistes} are normally sinistral; while in other groups (Achatinella, Amphidromus> 

 etc. ) some of the species are indifferently dextral or sinistral. A large proportion 

 of the Gastropods entirely or partly close the aperture of their shells with what is 

 termed an operculum. This is sometimes horny, like that of the common periwin- 

 kle, or it may be solid and calcareous as in the Turbinidce, The different forms 

 assumed by this structure have afforded characteristics for separating many groups 

 generically. The operculum is generally attached to the hinder part of the foot, so 

 that when the animal withdraws within the shell it more or less closes the aper- 

 ture, and thus protects itself. In many species the operculum is very small, and 

 would not serve as a defensive weapon, while in others it is altogether wanting. 

 Gastropods may be classified in three principal divisions or orders, namely, the 

 Pulmonata, Opisthobranchia, and Prosobranchia. 



THE LUNG-BREATHING GROUP Order PULMONATA 



The order Pulmonata comprises all the true land snails, excepting such as 

 are provided with an operculum, the inoperculated forms of fresh- water snails, 

 and the family of the Auriculida : The latter forms a considerable group, the mem- 

 bers of which are chiefly met with in salt or brackish marshes, although there are 

 three or four genera which are strictly littoral in their habits. All the Pulmonata 

 are provided with a breathing cavity, which is not freely open as in the terrestrial 

 and fresh-water Prosobranchs, but has only a small opening which is contractile so 

 as to exclude the water in the aquatic species, and the hot air during the heat of 

 summer in the terrestrial forms. The lung opening is seen on the right side of 

 most snails and slugs a little behind the head, and at once closes up if the animal be 

 molested. With a few exceptions, the fresh-water Pulmonates rise to the surface 

 to breathe, and are suffocated if prevented from obtaining the requisite supply of 

 air. They can be drowned just as easily as a slug or snail beneath the water, but 

 naturally the process would be longer in these animals, in which the breathing is 

 habitually much slower. In the case of Ancylus, however, this could not be 

 effected; and a fresh-water pulmonate, Physa lamellata, from Madagascar, possesses 

 a well-formed gill. 



The Pulmonata may be separated into two main divisions, according to the 

 position of the eyes. In the Stylommatophora, represented by slugs and snails, 

 these are situated at the tip of retractile tentacles, whereas in the Basommatophora 

 they are placed at the base of the tentacles, which are only contractile and not 

 retractile within themselves as in the former section. The Stylommatophora are 

 mostly more or less slimy, and leave a mucous track behind when crawling. 

 Although they are chiefly vegetarians, some are not only carnivorous, but even 

 cannibals at times. They are found in all parts of the globe, from the Arctic 

 regions to the Tropics, but are most abundant in those countries where there is 

 plenty of lime and moisture. The sexes are not distinct but united in each indi- 

 vidual. The eggs of terrestrial mollusks are far less numerous than those of the 



