100 THE AMERICAN NATURALIST. [VoL. XXXIV. 
the gum above the 1 cc. mark and not to get any on the sides 
of the test-tube. Then I lifted the animals with as little water 
as possible into the solution. Where I knew that the specific 
gravity was less than 1.111, I took 1 cc. of water with them. 
Animals must be placed underneath the surface of the liquid, 
not dropped upon it, as some are buoyed up by the surface film, 
and so do not sink in solutions lighter than themselves.  (Pla- 
naria and snails are thus supported in common pond water.) 
As it is very difficult to force out the gum adhering to the sides 
of the pipette, I made an allowance for the diminution of bore 
due to the adhering gum when adding the first cubic centimeter 
of pure water. By using this pipette to add the water, the 
gum mixture will soon be washed completely out into the test- 
tube, where it belongs. 
= The very heavy bodies — such as the Sigs and the very 
light ones — such as the older tadpoles (in general, the ends of 
the series) —afford the least accurate determinations of their 
specific gravity. To have a uniform standard for all, I assumed 
that in each case, after the addition of more water, an animal 
floated unless in sinking it touched the bottom or curving end 
of the test-tube. 
I give my results in the form of a table (in which the animals 
are arranged according to their systematic relations) showing 
the number of specimens used, the average specific gravity, and 
the probable error. In the case of starred animals the solutions 
given by Table II were employed. This involved less rough 
treatment than by using the method of Table I. 
There are five animals at the end of the table which were 
not tested in sufficient numbers to warrant comparison with the 
others. 
All the free-swimming poses tested (excepting old tadpoles) 
tend, when quiescent, to be at the bottom and to rise must 
swim. That the lighter animals need less effort to swim is 
well illustrated by the two entomostracans, Cypridopsis and 
Simocephalus. Simocephalus moves without much effort and 
often very slowly in the water. Cypridopsis, on the other hand, 
must move rapidly and continuously or it sinks. It requires 
a distinct effort for a mosquito larva to get to the surface. 
