i68 NATURAL SCIENCE. Sept.. 



of late directed their attention, will be interested in a paper that Mr. 

 H. de Varigny has just published in the Journal de V Anatomic et de la 

 Physiologie (1894, PP- 147-188). 



In 1874 Professor Semper published the results of some experi- 

 ments made with specimens of Limmva stagnalis, in order to ascertain 

 the effects of the environment on the growth of the animal, and more 

 especially the causes of dwarfing. By isolating young individuals 

 obtained from the same mass of ova, and placing them in vessels 

 each containing a different amount of water, he found that reduction 

 in the bulk of water, below a certain quantity favourable to full 

 development, was attended by a corresponding reduction in the size 

 of the animals. Since the young snails were fully supplied with food, 

 and placed under similar conditions of temperature and light, and 

 since the state of aeration of the water would be without effect on 

 these air-breathers, the cause of the dwarfing was not very apparent ; 

 and Dr. Semper was finally driven to conclude that the reason must 

 be sought in the presence in the water of some substance, not as yet 

 ascertained, which was necessary to the growth of the animal, and 

 the amount of which in the smaller volumes of water was insufficient. 



To test these observations more fully, and more especially the 

 Professor's conclusion, Mr. de Varigny undertook a series of experi- 

 ments. Some of the results arrived at he has embodied in the paper 

 before us. He selected as subjects Limnaa stagnalis and a species 

 which he terms L. auricularis, but which from his figures is un- 

 doubtedly an inflated form of L. pereger. Individuals hatched from 

 the same mass of ova were isolated in vessels containing different 

 amounts of water and offering equal or different areas of aeration in 

 the several experiments. The vessels were placed under equal and 

 favourable conditions of temperature and light, and the animals fur- 

 nished with abundant food. The results went to show, first, as 

 Semper's trials did, that, within limits, the size of the shell increased 

 with the increase of volume of the water the animal was placed in ; 

 secondly, that the dimensions of the Limnaa increased in proportion 

 to the increase of surface exposed to the air, and that increase was 

 almost more pronounced than in the case of increase of volume of the 

 water alone, and even persisted when the superfices was increased 

 while the volume was diminished ; thirdly, other conditions being 

 equal, the size of the shells varied in inverse proportion to the 

 number of individuals inhaling a given volume of water. Semper's 

 conclusion, however, as to dwarfing being due to some unknown 

 substance in the water, was neatly disposed of by experiments in 

 which individual Limnaa were penned off from their fellows, and 

 compelled to occupy a small space in the same vessel, while a 

 thorough circulation of the water was effected. The results were just 

 the same as if they had been confined in separate small receptacles. 



Seeking, therefore, to explain the cause of this dwarfing in some 

 other way, Mr. de Varigny suggests that it is due to lack of room in 



