342 MOLLUSCS. 



The colour and ornamentation of the surface are also as varied as 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 Gastro- 

 pods entirely or partly close the aperture of their shells with what is termed an 

 operculum. This is sometimes horny, like that of the common periwinkle, or it may 

 be solid and calcareous as in the Turbinidce. The different forms assumed by this 

 structure have afforded characters 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 aperture, and thus 

 protects itself. In many species the operculum is very small, and would not serve 

 as a defensive weapon, whilst 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 Auriculidce. The latter forms a considerable group, the members 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 pro- 

 vided 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. This 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 

 individual. The eggs of terrestrial molluscs are far less numerous than those of 

 the marine forms, and are deposited separately. They are mostly round or egg- 



