262 



METAZOAN PHYLA 



known as sow bugs (Fig. 160) and pill bugs, live under stones, boards 

 and other objects upon the ground and are also found in damp cellars 

 and in greenhouses, where the air is moist. 



The amphipods are a third group of Malacostraca distinguished from 

 the two preceding groups by being laterally compressed and by having 

 an elongated abdomen. They have three pairs of anterior thoracic legs 

 and on the abdomen three pairs of posteriorly directed jumping append- 

 ages. They also lack a carapace. They are found in all waters, a 

 common fresh-water form, Hijalella (Fig. 161), being one of the most 

 generally distributed of all North American animals. Amphipods are 



Fig. 160. Fig. 161. 



Fig. 160. — An isopod, Oniscus asellus Linnaeus. (From Paulmier, in Bull. 91, A''. Y. 

 State Mus., by the courtesy of the New York State Museum.) X 3. 



Fig. 161. — An amphipod, Hyalella dentata (Say). {From Paulmier, in Bull. 91, N. Y. 

 State Mus., after Stnith, by the courtesy of the New York State Museum,.) X 6. 



also found on the beach between tide marks, where, because of their 

 power of jumping, they are termed beach fleas. 



300. Entomostraca. — Entomostraca (en to m6s' tra ka; G., entomos, 

 cut in pieces, and ostrako7i, a hard shell) are, generally speaking, of small 

 size but they occur in numbers that can hardly be realized. It has been 

 estimated that on the average each cubic meter of water in the small 

 Wisconsin lakes contains about forty thousand individuals. Cladocerans 

 have been observed in a small alkaline lake in Cherry County, Nebraska, 

 in such numbers that the whole lake, when seen from a distance, was of 

 a red color. Entirely around the shore was a windrow of these animals, 

 cast up by the water, a foot wide and from an inch to two inches in depth. 

 A wide-mouthed bottle filled by one dipping from the water of the lake 

 at the shore was about half filled with the organisms after preservation of 

 the material and on settling. The group (Fig. 162) includes Cladocera 

 (kla dos' er a; G., klados, sprout, and keras, horn), also known as water 

 fleas; and Copepoda (ko pep' o da; G., kope, oar, and podos, foot), some 

 of which are parasitic on fish, being called fish lice. A third order is 

 Ostracoda (6s tra ko' da; G., ostrakodes, having a shell), which are 

 inclosed in bivalve shells and look like miniature mollusks. 



