No. 398. ] SPECIFIC GRAVITY OF ANIMALS. IOI 
TasBLE III. 
j APAE NuMBEROF| AVERAGE | PROBABLE 
SPECIMENS. | Sp. GR. ERROR. 
*Stentor cceruleus — : : ; ‘ ; 72 1.016+ | .oo16 
*Bursaria (sf.?) . : i ; ; ; 66 1.0142 00067 
Hydra viridis L. v : : , : : 18 1.0095 .00I13 
fusca L. 
Hydra 1 ae glia d. i ; : ; 6 1.0115 .0009 
Cordylophora lacustris Allm. . : ; . | 5 heads 1.016 
lanaria maculata Leidy oF 1.030 0026 
*Rotifer vulgaris Ehrbg. . I5 1,021 — 
Dero intermedia Cragin . : aera : 36 1.022 002 
Cyclops albidus Jurine  . : : : : 16 1.0222 .0029 
Simocephalus vetula Müller . : > : r 1.0136 .0012 
Cypridopsis vidua Müller : : : : 40 1.046 -— 
Culex (larva) . : : s : . II 1.0185 .0002 
Agrion larve . i 3 : i : 8 1.046 — 
Aquatic fly larvae (sf. 2) : j : : . | Many 1.0185 p 
Physa (shell hi. ‘ i à ; : 4 1.037 — 
Nephelis (s5.?) . : * : : ; I 1.037 ~— 
Asellus . : äi ; i à : | 3 1.037 m 
When there, the surface film holds up the breathing tube with 
its spreading hairs. Whenever that support is lost the animal 
sinks. A Cyclops, when moving slowly, can be seen to move 
upwards with a jerking motion, then to sink slightly, then to 
jerk again, and so on. To remain motionless it must have its 
antennz on the surface film. 
A sinking animal, like a lifeless body, always falls with the 
heavy end downward. Many animals do not give special evi- 
dence in this direction. Cyclops usually sinks head downward, 
the tail being at an angle of 45? with the horizon. Hydra, 
unless unusually dense near the mouth (for instance, with food 
in the gastrovascular cavity), sinks foot downward. One with 
a bud sank bud downward. 
In the tadpole the center of gravity changes position in the 
course of development. Up toa length of about 6 mm,, it is 
in the head region, for that always sinks first. In later stages 
the center of gravity moves backward nearly to the root of the 
