The classification of the /Gasteropoda/ is primarily a division into three great orders: the /Opisthobranchiata/, the /Prosobranchiata/, and the /Pulmonata/. The /Pulmonata/, which include the land and fresh-water gasteropod shells, are characterized by the presence of a breathing-sac, or lung, instead of gills. They are, therefore, essentially air-breathers, and would perish if kept too long under water. The /Opisthobranchiata/ and /Prosobranchiata/ are marine, and, like all animals which breathe by means of gills, soon die when removed from the water. We have already noted, however, how some genera of marine gasteropods that find their station about high-tide mark are able to live for considerable periods out of the water, notwithstanding the fact that they are fully equipped with gills and not possessed of lungs. Indeed, there is one large family of prosobranchs (the /Cyclostomatidæ/) that has become entirely terrestrial in habit, its members having lost their gills and acquired lungs, but their organization otherwise is so essentially that of the prosobranchs that they have never been considered as pulmonates. The main difference between these two orders of marine gasteropods is that in the prosobranchs the breathing-organs (the gills) are placed in a position forward of the heart, and the degree of torsion (page 330) characteristic of this molluscan class has been continued until the auricle of the heart is in front of the ventricle. There is always a shell, usually spiral, and, with few exceptions, an operculum. In the opisthobranchs, on the other hand, the relative position of the heart and gills is reversed, and they further differ from the prosobranchs in that the sexes are always united in each individual. The opisthobranchs are not always provided with a shell. One division of the order, known as the "nudibranchs," are entirely naked. These are commonly called the "sea-slugs," and are to be found crawling about the marine vegetation in shallow water, in tide-pools, and on the piling of old docks. Curiously enough, the sea-slugs (see page 354) have no gills at all, but, having lost these apparently essential organs, are [pg350] enabled to "breathe water" through their skin. They are wonderfully beautiful creatures, and the collector must not fail to secure some specimens, put them into a jar of sea-water, and watch them expand. There are other more superficial differences between the prosobranchs and the opisthobranchs, which will enable even a beginner to distinguish them at a glance. When the latter are possessed of an external shell, it is bulbous, generally glassy, and with a simple lip, the aperture extending the entire length of the shell. Again, the mantle or the propodium of the foot is greatly extended and usually covers the shell almost wholly.