64 THE ZOOLOGIST. 



But when we reach the tranquil waters of our ponds, we find 

 the characteristic genera,* Limncea and Physa, have monochro- 

 matic horn-coloured shells, of a comparatively thin suhstance. 

 Their apertures too are, as a rule, large in comparison with the 

 whole shell, and they are in the habit of protruding a considerable 

 portion of their bodies as they move about. All this points to a 

 safer life than marine mollusca lead. In the sea there are almost 

 innumerable enemies devouring any mollusca they can find. Gwyn 

 Jeffreysf enumerates a long list of destructive animals, from the 

 cod to star-fishes ; and mentions that the number of Turtonia 

 minuta in the stomach of a small mullet from Lough Larne was 

 estimated by Hyndman at 35,000. In fresh water their enemies 

 are far fewer; cod and the rest of the large fish are replaced 

 chiefly by the trout which occurs in few such ponds as Limncea 

 likes. Gwyn Jeffreys % records a case where a specimen of L. 

 peregra burnetii was found in the stomach of a gillaroo trout 

 (Salmo stomachiens) from Ireland ;§ and I have heard of several 

 shells being found in trout from the Lugg. The lobster, which 

 readily breaks open even a whelk, || is replaced by Astacus, at once 

 less numerous and less powerful ; though it will, however, eat 

 molluscs, shells and all.H Among other enemies are sticklebacks, 

 birds (especially ducks), frogs, &c, but the number of mollusca 

 destroyed annually by all these must be far less, both absolutely 

 and relatively, than in the sea. 



I have for some time been inclined to think that the shell of 

 Limncea is degenerate, and that this degeneracy has arisen from 

 its quieter organic surroundings.** ,Of course, the physical condi- 

 tions of animal life are far more severe in fresh water than in the 

 sea. Organisms are subjected to the drying up of ponds, con- 



* These may, however, occur in the sea ; c/., e.g. K. Semper, ' Animal 

 Life,' 1890, p. 145. 



f ' British Conchology,' vol. i. p. lix ; cf. A. H. Cooke, op. cit. chap. iii. 



I E. Kimmer, ' Land and Freshwater Shells,' 1880, p. 59. 



§ Prof. Seeley considers that the muscular thickenings of the walls of the 

 stomach characteristic of this species (" local race "), have arisen from its 

 feeding on Limnaea, Aucylus, &c. ' Freshwater Fishes of Europe,' 1886, 

 p. 280. 



II Gwyn Jeffreys, op. cit., loc. cit. 

 IT Huxley, ' Crayfish,' 1884, p. 9. 



** Darwin (' Descent,' ed. vi. p. 83) makes a similar remark, but draws 

 a somewhat different conclusion. 



