Scalariform specimens of Planorbis are said to occur most commonly in waters which are choked by vegetation, and it has been shown that this form of shell is able to make its way through masses of dense weed much more readily than specimens of normal shape.
Continental authorities have long considered Limnaea peregra and L. ovata as two distinct species. Hazay, however, has succeeded in rearing specimens of so-called peregra from the ova of ovata, and so-called ovata from the ova of peregra, simply by placing one species in running water, and the other in still water.
According to Mr. J. S. Gibbons[200] certain species of Littorina, in tropical and sub-tropical regions, are confined to water more or less brackish, being incapable of living in pure salt water. “I have met,” says Mr. Gibbons, “with three of these species, and in each case they have been distinguished from the truly marine species by the extreme (comparative) thinness of their shells, and by their colouring being richer and more varied; they are also usually more elaborately marked. They are to be met with under three different conditions—(1) in harbours and bays where the water is salt with but a slight admixture of fresh water; (2) in mangrove swamps where salt and fresh water mix in pretty equal volume; (3) on dry land, but near a marsh or the dry bed of one.
“L. intermedia Reeve, a widely diffused E. African shell, attaches itself by a thin pellicle of dried mucus to grass growing by the margin of slightly brackish marshes near the coast, resembling in its mode of suspension the Old World Cyclostoma. I have found it in vast numbers in situations where, during the greater part of the year, it is exposed to the full glare of an almost vertical sun, its only source of moisture being a slight dew at night-time. The W. Indian L. angulifera Lam., and a beautifully coloured E. African species (? L. carinifera), are found in mangrove swamps; they are, however, less independent of salt water than the last.”
Mr. Gibbons goes on to note that brackish water species (although not so solid as truly marine species) tend to become more solid as the water they inhabit becomes less salt. This is a curious fact, and the reverse of what one would expect. Specimens of L. intermedia on stakes at the mouth of the Lorenço Marques River, Delagoa Bay, are much smaller, darker, and more fragile, than those living on grass a few hundred yards away. L. angulifera is unusually solid and heavy at Puerto Plata (S. Domingo) among mangroves, where the water is in a great measure fresh; at Havana and at Colon, where it lives on stakes in water but slightly brackish, it is thinner and smaller and also darker coloured.
(c) Changes in the Volume of Water.—It has long been known that the largest specimens, e.g. of Limnaea stagnalis and Anodonta anatina, only occurred in pieces of water of considerable size. Recent observation, however, has shown conclusively that the volume of water in which certain species live has a very close relation to the actual size of their shells, besides producing other effects. Lymnaea megasoma, when kept in an aquarium of limited size, deposited eggs which hatched out; this process was continued in the same aquarium for four generations in all, the form of the shell of the last generation having become such that an experienced conchologist gave it as his opinion that the first and last terms of the series could have no possible specific relation to one another. The size of the shell became greatly diminished, and in particular the spire became very slender.[201]
The same species being again kept in an aquarium under similar conditions, it was found that the third generation had a shell only four-sevenths the length of their great grandparents. It was noticed also that the sexual capacities of the animals changed as well. The liver was greatly reduced, and the male organs were entirely lost.[202]
K. Semper conducted some well-known experiments bearing on this point. He separated[203] specimens of Limnaea stagnalis from the same mass of eggs as soon as they were hatched, and placed them simultaneously in bodies of water varying in volume from 100 to 2000 cubic centimetres. All the other conditions of life, and especially the food supply, were kept at the known optimum. He found, in the result, that the size of the shell varied directly in proportion to the volume of the water in which it lived, and that this was the case, whether an individual specimen was kept alone in a given quantity of water, or shared it with several others. At the close of 65 days the specimens raised in 100 cubic cm. of water were only 6 mm. long, those in 250 cubic cm. were 9 mm. long, those in 600 cubic cm. were 12 mm. long, while those kept in 2000 cubic cm. attained a length of 18 mm. (Fig. [37]).
An interesting effect of a sudden fall of temperature was noticed by Semper in connection with the above experiments. Vessels of unequal size, containing specimens of the Limnaea, happened to stand before a window at a time when the temperature suddenly fell to about 55° F. The sun, which shone through the window, warmed the water in the smaller vessels, but had no effect upon the temperature of the larger. The result was, that the Limnaea in 2000 cubic cm., which ought to have been 10 mm. long when 25 days old, were scarcely longer, at the end of that period, than those which had lived in the smaller vessels, but whose water had been sufficiently warm.
