EFFECT OF CDKRENTS. 209 



from the egg have so little clinging power in the foot, that the 

 feeblest current suffices to sweep them off the leaves on which 

 they seek for food, although the clinging surface is just as 

 large in proportion to the length of the shell as in the fully 

 grown snails, but the older ones can move about freely in a 

 current of moderate strength. Dr. Kobelt, in Frankfort, took 

 occasion, when discussing my experiments on Lymnsea, to show 

 that this fact tends to explain the remarkably wide distri- 

 bution of this mollusc. It usually lives in ponds and lakes ; 

 sometimes, however, it is found in brooks or rivers where the 

 current is feeble ; but in these, only fully grown specimens are 

 found, and they are by no means so numerous as in stagnant 

 waters. Hence we may infer that they were carried by floods 

 into the stream at these spots, when ah'eady half grown, while 

 the young ones transported at the same time perished for lack 

 of food, in consequence of their inability to cling to leaves in the 

 current. Multiplication in a strong current is equally impos- 

 sible to the LymnEea, and for the same reason ; since, even if the 

 young escaped from the egg, they must shortly perish in conse- 

 quence of the weakness of the foot. This explains why LymnoPM 

 stagnalis seeks situations where the water is stagnant, for it is 

 only in such spots that the conditions of life are suitable to 

 every stage of its growth. It would be interesting to institute 

 similar experiments with other species of Molluscs, and I have 

 no doubt that the results that might thus be obtained would 

 contribute to explain many facts hitherto inexplicable as to the 

 distribution of fresh-water snails. 



In some cases an increase of clinging power is accompanied 

 by peculiar modifications of structure in the creature itself, which 

 may perhaps be even regarded as its direct result. Among the 

 fresh-water Univalves of the tropics of the eastern hemisphere 

 there is a genus, Navicella, which is identical with Neritina in 

 all the essential characters of its anatomical structure.^' Both 

 belong to the same family of univalves, and bear a calcareous 

 plate on the hinder part of the foot, known as the operculum 

 (see fig. 61), which is commonly of the same shape as the month! 

 of the shell, so as to close it perfectly when the animal draws in 

 the foot. But, in order to do this, it must necessarily leave 



