COPEPODA 747 



There is a marked difference in form and structure between the 

 Gopepoda living in the open water and those that are limnetic in 

 their habit. Those that live among the weeds alongshore, or in 

 pools, are relatively short and stout, and frequently deeply colored. 

 A good example of this is Cyclops ater, which has received its name 

 because of its dark color. It is to be presumed that Cyclops viridis 

 also received its name from its color, for many of these shore forms 

 show a distinctly green coloration. These colors, doubtless, are pro- 

 tective, for, because of them, the animals are almost invisible when 

 stationary upon a background of bottom mud or of the stems of 

 aquatic plants. 



The limnetic species have long and slender bodies, Limnoca- 

 lanus macrurus being an especially good type. Some species of 

 Cyclops live either as limnetic or as littoral inhabitants; in these 

 cases one finds the same differences in form, the littoral variety 

 being short and stout, and the limnetic long and slender. This is 

 especially well shown in the varieties of Cyclops viridis and Cyclops 

 serrulatus. The figures in the systematic discussion of these species 

 show these differences which are especially well marked in the furcal 

 rami (Figs. 1214, 1215). The littoral species have short and stout 

 furcal rami, whereas in the limnetic species these structures are 

 long and slender. The limnetic species are ordinarily colorless, 

 their transparent bodies making beautiful objects for examination 

 under low magnifying powers; for much of the internal anatomy 

 of the animal can be observed, while the animal is still alive: the 

 movements of the alimentary canal can be followed, and the beat- 

 ings of the heart observed. This lack of color is doubtless an 

 adaptation to the environment, for in open waters colorless animals 

 are much less conspicuous. 



Occasionally the Copepoda are of a marked red color. This is 

 sometimes due to oil globules, and is especially marked in some of 

 the species appearing in the early spring, or living in the cold 

 waters of lakes at great altitudes. In other cases, and this is 

 markedly true of some of the Diaptomi, the integument may be 

 deeply colored in reds, blues, and purples. Diaptomus shoshone, a 

 large species found in the mountain regions of the West, is an espe- 

 cially good example of a highly colored copepod. 



