94 EFFECT OF VOLUME OF WATER chap. 



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 

 Lorenqo Marques Eiver, 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 tliat 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. Limnaea 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.^ 



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 weU. The liver was greatly reduced, and the male 

 organs were entirely lost.^ 



K. Semper conducted some well-known experiments bearing 

 on tliis point. He separated^ 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 



1 W'hitfield, Bull. Amer. Mas. N. H. i. p. 29. 

 2 Amcr. Nat. xiv. p. 51. ^ Animal Li/c, Ed. 1, p. 160 f. 



