648 ENTOMOSTRACA. 



In this he discerns a decided fitness, as tending to prevent the 

 hurtful effects of putrid air that might attend the decomposition 

 of the amazing number of these animals abounding in ponds and 

 ditches. These Crustaceans, however, in their turn, become the 

 prey of other animals. They form a considerable part of the 

 food of fishes; and it is thought that the quality of some of the 

 fresh-water fishes, of which a species of trout may be particularly 

 mentioned, may, in some degree, depend upon the abundance of 

 this portion of their food. Among the genera belonging to this 

 order, we refer first to the Cyclops, (see Chart,) (Gr. circular or 

 rounded eye,) a fresh-water genus, in which the body is pear- 

 shaped, and the upper, or larger pair of antennae, are employed 

 as aids to locomotion. Species of these may be seen jerking 

 themselves along in springs and stagnant waters. When they 

 lose part of an antenna, it reappears as, in the case of some others 

 of the class, at the time of the next moult. They are carnivo- 

 rous, and when without other food, even eat up their own young. 

 Some of the kindred marine species appear to be phosphorescent. 

 These minute Crustaceans are very prolific. They are tena- 

 cious of life, reviving after having been frozen, though they soon 

 die when removed from water and dried. Many of them furnish 

 food to the water larvae of insects. 



(2) Daphnia, (Gr. a laurel-berry,) the ARBORESCENT WATER- 

 FLEA. In this genus the body is enclosed in a bivalve shell, 

 though the head is exposed, having a compound and somewhat 

 movable eye. These Crustaceans are found in stagnant waters 

 in company with the Cyclops, which they resemble in their move- 

 ments. They are sometimes so numerous in water as to give it 

 "a muddy hue, like the red dust of iron, or as if blood had been 

 mixed with it." On the back of the shell is seen, at certain sea- 

 sons, a black saddle-shaped appendage, containing two eggs, from 

 which, in the spring, the species are reproduced. 



(3) Cypris, (see Chart.) This is likewise enclosed in a 

 bivalve shell, with a dorsal hinge. The antennas are four, the 

 second pair large, and fitted to aid in swimming. Many species 

 may be seen in summer-time swimming about in stagnant pools, 

 and they often show beautiful variations of color. 



(4) Limulus or Polyphemus, this is sometimes referred to the 

 order XIPHOSURA, (Gr. sword-tail,) a name referring to the long, 

 hard, and sharp tail-spine of this creature, which, in some places, 

 is used for pointing spears. The body is covered with a large 

 carapace shield, (Plate XVI. fig. 1 ;) is rounded in front, having 

 the hind part smallest, with spines on the sides, and deep notches 

 behind ; the gill-feet are appended to the abdomen. Six feet, 



