NO. 7 A NEW COPEPOD HABITAT ^WILSON 9 



for crawling about in sand and mud. There is also sometimes a re- 

 duction in the number and size of the endopod segments until in a 

 few species the entire endopod is reduced to a mere knob, of no use 

 except to show that the leg is still biramose. 



Another modification is concerned with the external ovisacs, which 

 in the free-swimming copepods hang loosely from the genital segment 

 and often diverge considerably from the body. The eggs themselves 

 are of moderate size and fairly numerous, and may be carried in one 

 or two ovisacs, or even extruded singly into the water without being 

 carried at all. In the parasitic copepods a large number of eggs seems 

 to be the primal requisite. When the eggs are arranged in a single 



Fig. 6. — f to /, first, second, third and fourth leg of Emertonia 

 gracilis, showing substitution of spines fcr plumose setae. 



row, as in the Caligidae, the increase in number is obtained by length- 

 ening the ovisacs, which sometimes become several times as long as 

 the entire body. When the eggs are multiseriate, the diameter of the 

 ovisac is increased and the size of the egg is at the same time dimin- 

 ished. As a result, the number of eggs in some copepods parasitic upon 

 deep-sea fish may reach 10,000 or more in each ovisac. In the com- 

 mensal copepods there are often no ovisacs, the eggs being gathered 

 into a brood sack situated in the dorsal portion of the thorax. 



In contrast with these three groups, the ovisacs of the terraqueous 

 copepods are nearly always flattened and closely appressed to the 

 surface of the genital segment and abdomen. Sometimes the fifth 

 legs are enlarged and modified to cover the anterior ends of the ovisacs 

 and thus partially protect them. The number of eggs is reduced, and 



