48 THE CHICAGO ACADEMY OF SCIENCES. 
Several European zoologists have experimented in the same man- 
ner, producing very interesting results. Thus, Semper! bred speci- 
mens of Lymnea stagnalis from the same egg mass and placed them 
in aquaria of different volume varying from 100 to 2000 cubic centi- 
meters. All of the essential conditions of life were kept as uniform as 
possible, especially the food supply. At the end of sixty-five days the 
specimens from the 100 c. c. aquarium measured 6 mill. in length, those 
from the 250 c. c. aquarium measured 9 mill. in length, while those 
from the largest aquarium, 2000 c. c. were 18 mill. long. Semper con- 
cluded from these experiments that the size of the shell varied in 
proportion to the volume of water, and that the effect was the same 
whether one or several individuals were confined in the same aquarium. 
The factor of temperature was accidentally shown in Semper’s experi- 
ments; when the temperature fell to 55°. The aquaria were placed 
near a window through which the sun shone, raising the temperature 
of the smaller aquaria but having no effect upon the larger aquarium. 
The consequence was that the individuals in the 2000 c. c. vessel, 
which should have been 10 mill. in length when 25 days old, were but 
little longer than those individuals which were contained in the smaller 
aquaria, the water of which had been warmed by the rays of the sun. 
De Varigny? experimented in a similar manner by breeding snails 
in aquaria containing equal volumes of water, but with varying surface 
areas. The result was that the largest individuals were produced in 
those vessels having the greatest surface areas. De Varigny was led 
to the conclusion from the result of these experiments, that the chief 
factor in growth was an extensive surface area which permitted ex- 
tended locomotion. 
Willem,’ having in mind the experiments of Semper and DeVar- 
igny, carried on two distinct sets of experiments. The first experi- 
ment consisted of two similar aquaria, one filled with stagnant water 
and the other with water into which a constant air stream was intro- 
duced. The result after three or four months was that the snails in 
the aeriated jars were very much larger than those in the aquarium 
containing stagnant water. In the second experiment two vessels of 
different sizes were used, the surface areas of which bore the ratio 
of 8 to 1, the volumes of water being respectively 3110 and 310 ¢. ¢. 
An equal amount of air was kept passing through either jar, and at 
the end of three months the snails in the larger vessel were of the 
1Arbeiten Zool.-Zoot. Inst., I, p. 137, 1874. 
2Journ. Anat. Physiol., XXV, pp.147-188, 1894. 
sBull. Acad. Roy. Sci. Brussels, XXXII, p. 566. 
