CRUSTACEA 



267 



The amphipods are a third group of Malacostraca distinguished from 

 the two preceding groups by being laterally compressed and by the 

 absence of a carapace. The first thoracic segment may be fused to the 

 head and there is usually no sharp distinction between thorax and 

 abdomen. The first five pairs of thoracic appendages are used in feeding, 

 and the last three pairs aid in crawling. Of the six pairs of abdominal 

 appendages, the first three serve in swimming and directing water toward 

 the gills; the last three pairs are directed posteriorly and are the jumping 



Fig. 170. Fig. 171. 



Fig. 170. — An isopod, Oniscus asellus Linnaeus. (From Paulmier, in Bull. 91, N. Y. 

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



Fig. 171. — An amphipod, Hyalella dentata (Say). (From Paulmier, in Bull. 91, N. Y. 

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



appendages. They are found in all waters, a common fresh-water form, 

 Hyalella (Fig. 171), being one of the most generally distributed of all 

 North American animals. Amphipods are also found on the beach 

 between tide marks, where, because of their power of jumping, they are 

 termed beach fleas. 



302. Entomostraca. — Entomostraca (en to mos' tra ka; G., entomos, 

 cut in pieces, and ostrakon, 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. 172) 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. 



