308 THE EFFECT OF FOOD 



Those of the last generation had altered so much that a 

 conchologist of experience was of the opinion that they 

 could bear no possible specific relation to those of the 

 first. Thus in addition to the diminution in size, the 

 spire had become very slender. In a second experi- 

 ment of a similar kind, the shells of the third genera- 

 tion were only 4-7ths as long as those of the parent 

 stock, and, still more remarkable, the male organs had 

 disappeared, whilst the liver had become considerably 

 reduced in size. 



These extraordinary effects were probably due to the 

 cumulative action of the increasing quantities of meta- 

 bolic products in the water in which the molluscs were 

 living. 



Still another series of observations on the effect of a 

 confined volume of water was described by Warren * 

 only a year or two ago. These were made upon 

 Daphnia magna (Water-flea). Four adult individuals 

 were placed separately in covered glass vessels contain- 

 ing 200 cc. of water, together with some Conferva and 

 some mud containing algSB, etc. Four others were 

 placed in similar, but uncovered vessels, and four more 

 in still other vessels, of which the water was changed 

 about once a day. The water in the former vessels was 

 never changed, but the loss due to evaporation in the 

 uncovered vessels was compensated for by the occasional 

 addition of rain water. The Daphnias produced broods 

 of four or five offspring each after about 15 days, and 

 these offspring were allowed to grow in the vessels, and 

 after a time produced offspring in their turn. It was 

 interesting to note, however, that in the vessels in 

 *Q. J. Microsc. Sci., vol. 43, p. 212, 1900. 



