416 PROCEEDINGS OF THE ACADEMY OF [July, 



There are eight variable factors that have been considered to affect 

 the growth processes of aquatic animals. Each factor, however, is not 

 of equal weight, but should be held in mind and controlled, if possible, 

 in an experimental study. Using the topics named above as a basis for 

 study, the general plan of the experiments that follow in the subsequent 

 section will be to keep every other factor constant and vary one alone. 



Precautions. — ^The experiments were carried out principally in 

 battery jars. The size used in the majority of the experiments was 

 4 inches in diameter by 5 inches high. Other vessels used were 5x6 

 battery jars, 8 x 10 battery jars and 12-inch dishes. 



In order to save repetition we will consider here the methods em- 

 ployed in every case. Where this order has been deviated from, it 

 will be mentioned in its place. 



(1) Before each experiment the jars were washed out and wiped 

 clean. In the later experiments the jars were washed with oxidizing 

 solution (potassium bichromate in concentrated sulphuric acid) as an 

 extra precaution. 



(2) The water used in the experiments was taken from a large 

 aquarium in the University Vivarium which contained fish. This was 

 done to introduce algae, etc., without the danger of adding young snails 

 with it. 



(3) In any one experiment the jars used were similar, the water 

 was taken from the same source, and the same amount of water was 

 used unless stated otherwise. These conditions being fulfilled, the 

 composition of the water, the algae for food, and the temperature must 

 vary in the same way. 



(4) In a given experiment the jars were placed near together and 

 care was taken so that each received an equal amount of light. 



(5) Over each was placed a glass plate to prevent evaporation and 

 the escape of the snails. This latter apparently suicidal behavior, as 

 described by Walter, was found often occurring. In many of the 

 cases that have come under observation, this was caused by the vapor 

 condensing on the glass sides of the jar above the water. Up this wet 

 glass the snail crawls, until temperature changes occur that dry the 

 glass. The snail is then dried and killed. Other cases are not so easy 

 to explain. 



(6) Just before an egg case was ready to hatch, with a section lifter it 

 was carefully removed from its substratum and isolated in a jar of 

 water. If the eggs are freshly laid it is usually fatal to the embryos 

 to remove them. In some of the earlier experiments, after the young 

 snail has broken through the egg membrane, the jelly mass was divided 



