96 THE AMERICAN NATURALIST. [Vor. XXXIV. 
the Amoeba to maintain itself at a given level. I have, how- 
ever, not been able to get the specific gravity of Amæbæ. 
But the larger soft-bodied animals, or animals with their shells 
removed, those with a small amount of foreign matter only, 
and especially embryos, offer favorable fields for the study of 
the specific gravity of protoplasms, and consequently I have 
made use of them. 
Any information concerning the specific gravity of organisms 
wil be of use in explaining motions, positions, and possible 
foods of the different animals. Tadpoles, found to be nega-: 
tively geotactic by Miss Platt (99), are forced to swim or to 
adhere by their suckers if they wish to remain above the 
bottom. The attitude of young tadpoles in an aquarium, as 
they hang tail downwards while clinging to the glass sides by 
their suckers, is due to the pull of gravity. Outdoors very few 
young tadpoles are seen at the surface of ponds where one 
knows they abound, though older stages are common at the 
surface. Stir up the bottom, and the surface for an instant 
will be black with tadpoles clinging to the bottom débris. An 
individual of a later stage whose intestine is filled with gas 
floats, can no longer feed on the bottom alga, and is out- 
stripped in growth by his normal fellows. 
In determining specific gravity I have used the method sug- 
gested by Davenport (97-99) and used by Miss Platt (99), 
that of placing the animal in a solution of its own density. 
The density of this solution can be varied at will by the addition 
of known amounts of the dissolved substance or of water. 
I found it necessary to stupefy all the free-swimming ani- 
mals experimented upon. For this purpose I generally used 
chloroform as the most convenient reagent. It is, however, 
unsatisfactory for Planaria, as the irritation causes a slime to 
be secreted, which buoys up the animal greatly. It causes 
Protozoa to burst. For Planarians a solution of CO2 gas in 
water is excellent. For Stentors and Bursaria, being unable 
to stupefy without killing, I tried four per cent formol, absolute 
alcohol, CO2, and chloroform, each without success. One-half 
per cent osmic acid and hot Perenyi's solution were fairly good, 
though most of the Stentors contracted to balls under the 
