io8 ANIMAL AGGREGATIONS 



In 1854 Jabez Hogg with some right apologized to the London 

 Microscopical Society for taking their time with observations on the 

 subject of the pond snail Limnaeus stagnalis, which had already been 

 well studied; but in the midst of his tedious record he states that a 

 snail kept in a "small narrow cell will grow only to such a size as will 

 enable it to move freely." This is the first recorded observation that 

 has come to my attention of the limiting effect of volume on growth. 



Semper (1874, 1881) took up the problem twenty years after 

 Hogg's observations, using the fresh-water isopod Asellus and the 

 pond snail Lymnaea stagnalis. With the former he found that when 

 animals living in a balanced aquarium were sealed into glass dishes, 

 they might be left for nearly 2 years, with an adequate food supply 

 and, he beheved, an adequate oxygen supply for the three or four 

 generations that would be produced; but under these conditions the 

 last generation was abnormally small. With the snails Semper divid- 

 ed the same mass of eggs into different lots, which were placed in 

 variously sized containers ranging from 100 to 5,000 cc. Food was 

 kept at an optimum, but the snails placed in the containers of smaller 

 volume grew more slowly than did their fellows placed in the larger 

 vessels. Similar results were obtained regardless of whether the 

 snails were isolated into a given volume or were put in groups, so 

 long as the volume per snail was the same in both cases. Semper 

 found that optimum growth lay between 400 and 500 cc. per snail 

 and that increases beyond this point gave no further increase in 

 growth. The effect of increase in volume was much more marked in 

 the smaller volumes. Later workers are agreed that relatively large 

 volumes of water per snail are necessary for optimum growth. 



Semper recognized the complex nature of the problem and at- 

 tempted, by chemical analyses made by a trained chemist, to find a 

 chemical cause. Failing in this attempt, he advanced the hypothesis 

 that some substance unknown to him was present in the water, prob- 

 ably in a very minute quantity, "which, by its relations to the water 

 which holds it in solution, and by its osmotic affinity to the skin of 

 the animal, can be absorbed only in a determined and extremely 



small quantity Since, according to this hypothesis, the 



amount of the substance absorbable in a given time depends on the 



