Origin of the Land Fauna
Gasteropoda.—(1) Operculate. On a priori grounds, one might predict a double origin for land operculates. Marine species might be imagined to accustom themselves to a terrestrial existence, after a period, more or less prolonged, of littoral probation. Or again, fresh-water species, themselves ultimately derived from the sea, might submit to a similar transformation, after a preliminary or intermediate stage of life on mudbanks, wet swamps, branches overhanging the water, etc. Two great families in this group, and two only, seem to have undergone these transformations, the Littorinidae and the Neritidae. The derivation of almost all existing land operculates may be referred to one or other of these groups.
Fig. 16.—Two rows of the radula of Littorina littorea L., × 72.
The power of the Littorinidae to live for days or even weeks without being moistened by the sea may be verified by the most casual observer. In the tropics this power seems even greater than on our own shores. I have seen, in various parts of Jamaica, Littorina muricata living at the top of low cliffs among grass and herbage. At Panama I have taken three large species of Littorina (varia, fasciata, pulchra), on trees at and above high-water mark. Cases have been recorded in which a number of L. muricata, collected and put aside, have lived for three months, and L. irrorata for four months.[29] These facts are significant, when we know that the land operculates almost certainly originated in a tropical climate.
The Cyclophoridae, Cyclostomatidae, and Aciculidae, which, as contrasted with the other land operculates, form one group, have very close relations, particularly in the length and formation of the radula, or lingual ribbon, with the Littorinidae.
Fig. 17.—Two rows of the radula of Cyclophorus sp., India, × 40.
On the other hand, the Helicinidae, Hydrocenidae, and Proserpinidae are equally closely related to Neritina. The Proserpinidae (restricted to the Greater Antilles, Central America and Venezuela) may perhaps be regarded as the ultimate term of the series. They have lost the characteristic operculum, which in their case is replaced by a number of folds or lamellae in the interior of the shell. It has already been noticed how one group of Neritina (Neritodryas) occurs normally out of the water. This group furnishes a link between the fresh-water and land forms. It is interesting to notice that here we have the most perfect sequence of derivatives; Nerita in the main a purely marine form, with certain species occurring also in brackish water; Neritina in the main fresh-water, but some species occurring on the muddy shore, others on dry land; Helicina the developed land form; and finally Proserpina, an aberrant derivative which has lost the operculum.[30]
