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 antennas. 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 



