430 
Birge—The Crustacea of the Plankton. 
strokes of the antennae resuming their former position. In this 
way they pass up and down through the water utilizing the 
material available for food. After a time the animal may swim 
off to a new place, but soon begins to repeat these alternate 
movements. The movements of Diaptomus are far less regular, 
yet it, too, keeps at about the same level, unless some at¬ 
traction causes it to move up or down. Cyclops , which hunts 
for food of all sorts, and is decidedly a more predacious ani¬ 
mal than either of the first two named, is far less regular in 
its movements, and Leptodora , as a true carnivore, swims ac¬ 
tively in all directions. 
The amount of energy required of the Crustacea in order to 
maintain their position in the water is not inconsiderable, and 
is doubtless the main muscular labor demanded of them. They 
are all of them heavier than water, and sink at a rather rapid 
rate, which very quickly becomes uniform. The full-grown 
Daphnia , 3 to 4 millimeters long, sinks at the rate of 20-30 
centimeters per minute even with expanded antennae. Small, 
newly-hatched individuals, one millimeter or less in length, have 
a rate less than one-third as great, from 5 to 10 centimeters 
per minute. The specimens experimented upon almost always 
fell edgewise through the water, with the head down, if the an¬ 
tennae were folded, and with the head up, if the antennae were 
expanded. Diaptomus sinks at about the rate of about 12 cm. 
per minute, and medium-sized adult Cyclops without eggs at a 
rate of 9.5 cm. per minute. 
Live Daphnias sink at the same rate as those freshly poisoned, 
as far as the eye can determine. This is easily determined in 
the case of half-grown and adult individuals, but young speci¬ 
mens are so active that it is hard to be accurate. At the rate 
given, an adult Daphnia would sink through as many as 250-400 
meters in a day, and must, therefore, maintain itself against the 
force which would cause it to fall through this distance. Of course 
the weight to be lifted is very small, being the excess of the weight 
of the animal over that of an equal bulk of water. It seems im¬ 
possible that the animal should ever sleep. As the creatures be¬ 
come older and larger the exertion becomes greater than in the 
case of young individuals, and the older and, especially, the 
