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, Pliysa 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- 
