150 



GENERAL ZOOLOGY 



Parasites. A large number of different forms resembling 

 Cyclops in many respects have adopted a parasitic life. 

 One of them is shown in Fig. 76 (Lernceoc'erci). They occur 

 on various hosts, but the greater number 

 of them are found on fishes. They live on 

 every part of the body, and in every degree 

 of commensalism and parasitism. Some live 

 in the alimentary canal, or in the gill-region, 

 feeding only on the food of the host ; some 

 temporarily seek their host for the body- 

 fluids, while others are permanent para- 

 sites, 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 

 FIG. 76. Parasitic their distinctness, and the form of the body 

 Crustacean Much ig altered b protuberances of various kinds. 



enlarged. (After > J 



Von Nordmann) The mouth-parts become adapted as hold- 

 ing and sucking organs. As the external 

 organs degenerate, the tendency of certain internal organs is 

 also to become rudimentary. Some of these parasitic organ- 

 isms 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. As with 

 most parasites, immense numbers of eggs are produced, to 

 overcome the effect of the somewhat isolated positions they 

 occupy. Lernseocera is parasitic on the carp, a fish. The 

 form is considerably modified, but the egg-sacs signify its 

 relation to Cyclops. 



Barnacles. Despite the great apparent difference between the 

 fixed, shell-bearing barnacles (Fig. 77) and the free-swimming 



