166 GENERAL ZOOLOGY 



two-branched appendages, also used in swimming. One pair 

 of antennae and the legs are not shown in the figure. Two 

 long appendages extend from the end of the abdomen. The 

 body consists of fifteen somites, five in each of the three 

 regions, head, thorax, and abdomen. The female, in the 

 summer season, carries about with her two large brood sacs 

 of eggs, which extend diagonally out behind. 



Parasitic Crustacea. A large number of different forms re- 

 sembling Cyclops in many respects have adopted a parasitic 

 life. They occur on various hosts, but the greater number 

 of them are found on fishes. They live on every part of the 

 body. Some live in the alimentary canal, or in the gill 

 region, feeding only on the food of the host ; some tempo- 

 rarily seek their host for the body fluids, while others are 

 permanent parasites external or internal. In general these 

 forms are spoken of as "fish lice." To the extent that they 

 are dependent on a host, the normal, external structure tends 

 to be modified out of all resemblance to the type represented 

 by Cyclops. The somites lose their distinctness, and the 

 form of the body is altered by protuberances of various 

 kinds. The mouth parts become adapted as holding and 

 sucking organs. As the external organs degenerate, the tend- 

 ency of certain internal organs is also to become rudimen- 

 tary. Some of these parasitic organisms are so degenerate 

 in form and structure that they have lost all appearance of 

 being animals at all. In many cases it is the female only 

 which is parasitic, the males leading a free life and showing 

 the normal structure of their race. 



Barnacles. Despite the great apparent difference between 

 the fixed, shell-bearing barnacles and the free-swimming 

 crabs and other forms discussed in this chapter, naturalists 

 have shown that they are in reality closely allied both in 

 development and in structure. In some places barnacles 

 literally incrust the coast-rock between tide lines with hard, 



