Fig. 13.—Illustrating the development of the fresh-water genus Navicella, through the brackish-water Neritina, from the marine Nerita, with corresponding changes in the operculum. 1. Nerita; 2, 3. Neritina; 4. Neritina, intermediate form; 5, 6. Navicella.
The great family of the Melaniidae, which occurs in the rivers of warm countries all over the world, and that of the Pleuroceridae, which is confined to North America, are, in all probability, derived from some form or forms of Cerithium. The origin of the Paludinidae, Valvatidae, and Ampullariidae is more doubtful. Their migration from the sea was probably of an early date, since the first traces of all three appear in the lower Cretaceous, while Melaniidae are not known until Tertiary times. Ampullaria, however, shows distinct signs of relationship to Natica, while the affinities of Paludina and Valvata cannot as yet be approximately affirmed.
(2) Pulmonata.—Intermediate between the essentially fresh-water and the essentially marine species come the group sometimes known as Gehydrophila, consisting of the two families Auriculidae and Otinidae. These may be regarded as Mollusca which, though definitely removed from all marine species by the development of a true lung or lung cavity in the place of a gill, have yet never become, in respect of habitat, genuine fresh-water species. Like Potamides, they haunt salt marshes, mangrove swamps, and the region about high-water mark. In some cases (Otina, Melampus, Pedipes) they live on rocks which are moistened, or even bathed by the spray, in others (Cassidula, Auricula) they are immersed in some depth of brackish water at high tide, in others again (Scarabus) they are more definitely terrestrial, and live under dead leaves in woods at some little distance from water. Indeed one genus of diminutive size (Carychium) has completely abandoned the neighbourhood of the sea, and inhabits swampy ground almost all over the world.
Fig. 14.—Examples of the Auriculidae: A, Auricula Judae Lam., Borneo; B, Scarabus Lessoni Blainv., E. Indies; C, Cassidula mustelina Desh., N. Zealand; D, Melampus castaneus Mühlf., S. Pacific; E, Pedipes quadridens Pfr., Jamaica.
Fig. 15.—An example of Amphibola (avellana Chem.), the only true Pulmonate which possesses an operculum.
To this same section Gehydrophila have been assigned two remarkable forms of air-breathing “limpet,” Siphonaria and Gadinia (see page [151]), and the aberrant Amphibola, a unique instance of a true operculated pulmonate. Siphonaria possesses a pulmonary cavity as well as a gill, while Gadinia and Amphibola are exclusively air-breathing. Siphonaria lives on rocks at or above high-water mark, Gadinia between tide marks, Amphibola (Fig. [15]) in brackish water at the estuaries of rivers, half buried in the sand. There can be little doubt that all these are marine forms which are gradually becoming accustomed to a terrestrial existence. In Gadinia and Amphibola the process is so far complete that they have exchanged gills for a pulmonary cavity, while in Siphonaria we have an intermediate stage in which both organs exist together. A curious parallel to this is found in the case of Ampullaria, which is furnished with two gills and a pulmonary chamber, and breathes indifferently air and water. It is a little remarkable that Siphonaria, which lives at a higher tide level than Gadinia, should retain the gill, while Gadinia has lost it.
The ultimate affinities of the essentially fresh-water groups, Limnaea, Physa, Chilina, cannot be precisely affirmed. The form of shell in Latia, Gundlachia, and perhaps Ancylus, may suggest to some a connexion with the Otinidae, and in Chilina, a similar connexion with the Auriculidae. But, in a question of derivation, similarities of shell alone are of little value. It is not a little remarkable, for instance, that we should find a simple patelliform shell in genera so completely distinct from one another in all anatomical essentials as Ancylus, Patella, Siphonaria, Propilidium, Hipponyx, Cocculina, and Umbrella.
Some recent authors, on grounds of general organisation, regard the Limnaeidae and their allies as Opisthobranchs adapted to an aerial life. It is held[24] that the Nudibranchiate Opisthobranchs have given birth to the Pulmonata Stylommatophora or land snails, and the Tectibranchiate Opisthobranchs to the Pulmonata Basommatophora or fresh-water snails. Such a view seems at first sight open to some objection from other views than those which deal simply with anatomy. The Opisthobranchiata are not, to any marked extent, littoral genera, nor do they specially haunt the mouths of rivers. On the contrary, they inhabit, as a rule, only the very lowest part of the littoral zone, and are seldom found, except where the water is purely salt. In other cases, when the derivation of land or fresh-water genera is fairly well established, intermediate forms persist, which indicate, with more or less clearness, the lines along which modification has proceeded. It has, however, recently been shown that Siphonaria[25] and Gadinia,[26] which have, as has been already mentioned, hitherto been classified as Pulmonata, are in reality modified forms of Opisthobranchiata, which are in process of adaptation to a life partly marine, partly on land. They may therefore be regarded as supplying the link, hitherto missing, between the land Pulmonata and the marine groups from one or other of which the latter must have been derived. The general consensus of recent opinion inclines towards accepting these views, some writers[27] being content to regard the Pulmonata, as a whole, as derived from the Tectibranchiate Opisthobranchs, while others[28] go further and regard the Stylommatophora as derived directly from the Basommatophora.