172 ReconJs of the Indian Mu.^ewm. [Vol. XIV, 



toads as Rana afghana,^ Bufo penangensis^ and Megalophrys montana.^ 

 On the other hand, in countries in which large lakes exist many of the 

 most highly specialized genera are found only in them, e.g., the peculiar 

 Mollusca of Lake Tanganyika in Central Africa, of Lake Tali Fu in 

 Western China and of the deeper lakes of Celebes, or the peculiar prawns 

 and crabs of Tanganyika, or even the peculiar fish of the Lile Lake. We 

 may assume therefore that in hill streams and large lakes there are 

 certain factors that encourage extreme plasticity ; but they cannot be 

 the same in both instances. 



In most of the highly modified genera and species that inhabit hill 

 streams, the modifications in structure have a definite function in 

 enabling the animal to cling tightly to rocks or other solid bodies in 

 rapid-running water. But this is not the case with most modified 

 organisms from large lakes. 



It is possible* that in Limnaea, a pulmonate mollusc that has no 

 gills or other special organ by means of which oxygen can be absorbed 

 direct from the water, it may be advantageous in deep water or in water 

 highly charged with vegetable matter to have a relatively large aperture 

 to the shell ; for this enables a large surface of skin to be extruded and 

 it is probable that oxygen, in conditions in which the animal cannot rise 

 to the surface, must be absorbed through the skin. If, however, this 

 is the case, it is probable that the benefit is to a large extent fortuitous, 

 for we have no evidence whatsoever that the modification has been, 

 produced through any kind of selection of individuals ; it rather seems 

 to be the result of the direct influence of physical or chemical forces 

 working suddenly or gradually on a plastic organism, as is suggested 

 by the series of shells figured on plate X, figs. 5-8, as well as by the 

 experiments cited above. Further, the deep-water forms of the genus 

 resemble the young of shallow-water forms in the shape as well as the 

 size of the shell. They are essentially forms which retain when adult 

 the external characters of immaturity. 



As I have already pointed out it is not uncommon in Ampullaria for 

 ridges to be produced on the surface of the body-whorl owing to 

 the resumption of growth after a period of rest. There is every 

 reason to think that the varix of H. nassa has a similar origin, 

 but whereas in Ampullaria the lip of the shell is thin and therefore the 

 ridge representing it is low, in H. nassa the lip and the ridge representing 

 it are thick. Our visit to the Lile Lake was made in early spring, at a 

 time when many hibernating animals were just beginning to awake 

 from their winter sleep, t noticed that large numbers of young in- 

 dividuals of H. nassa of from 2 to 3 mm. long had at this season a very 

 thick lip, but that in others only slightly longer or even a little shorter, 

 the thickening had disappeared and the lip was thin and sharp. It 

 seems probable, therefore, that the marginal thickening can be absorbed 

 in the course of growth. The retention of the varix in adult shells would 

 seem to indicate merely that at a certain period of growth the animal 



1 Annandale, Eec. Ind. Mus., VIII, p. 9 (1912). 



2 Flowor, Proc. Zool. Soc. London, (1899), p. 908. 



^ Annandale, Mem. As. Soc. Bengal, VI, p. 155 (1917), 

 * Pelseneer, Arch, de Biol. XIV, p. 379 ( 1895). 



