THE SPECIFIC GRAVITY OF SOME FRESH- 
WATER ANIMALS IN RELATION TO 
THEIR HABITS, DEVELOPMENT, 
AND COMPOSITION.! 
STEPHEN R. WILLIAMS. 
THE questions I have examined are: What is the specific 
gravity of animals living in water, and what bearing does their 
specific gravity have on their life histories ? 
These questions suggest the underlying problem of the spe- 
cific gravity of protoplasms. I say protoplasms because two 
eggs, though very similar in appearance, existing under exactly 
the same conditions, may develop into two widely different 
animals. 
The first part of my paper deals with the specific gravities 
of adult animals. The second part is a description of a number 
of series of growing tadpoles, in which I have ascertained, 
besides their specific gravity, their dry weight, and thus the 
percentage of water they contain. The third section of the 
paper has reference to the location of the water which, as 
shown by Davenport (97-99, p. 285), is imbibed by the tadpole 
in its period of rapid growth. 
I. 
Protoplasm with its structure and functions very little spe- 
cialized will be best found in simple, free-swimming water 
animals. An Amceba is apparently a body of simple proto- 
plasm, and if a method delicate enough to find its specific 
gravity were devised, we should know very exactly how amoebic 
protoplasm compares in weight with its own volume of pure 
water, and how much effort would be necessary on the part of 
l Contributions from the Zovlogical Laboratory of the e of Comparative 
Zoölogy at Harvard College. E. L. Mark, Director. No. 1 
95 
