No. 398. ] SPECIFIC GRAVITY OF ANIMALS. 107 
Fic. 3. — Cross-section R. silvatica, 20 mm. long, magnified 3o diam. 
tissue is greater in the 20 mm. tadpole is due to the fact that 
the tadpole has been taking in solid food for ten days. 
RESULTS. 
The specific gravity of certain fresh-water animals was deter- 
mined and found to vary from 1.0095 (Hydra viridis) to a 
maximum of 1.0460 (Cypridopsis). 
The movements of an animal are closely related to its density, 
and there is also a correlation between density and food habits. 
Of the animals tested, Stentor may represent a typical case 
of specific gravity in little modified protoplasm. The heavier 
animals certainly have some specialized tissues which are 
