1>ARASITISM AND DEGENERATION 



3G1 



Almost all of the mites and ticks, animals allied to the 

 spiders, hve parasitieally. Most of them live as external jxira- 

 sites, siiekinoj the blood of their liost, l)ut some live under- 

 neath the skin like the itch 

 mite§ (Fig. 222), which cause, 

 in man, the disease known 

 as the itch. 



Among the vertebrate ani- 

 mals there are not many ex- 

 amples of true parasitism. 

 The hagfishes or borers 

 (Myxine, etc.) have been al- 

 ready mentioned. These are 

 long and cylindrical, eellike 

 creatures, very slimy and 

 very low in structure. The 

 mouth is without jaws, but 

 forms a sucking disk, by 

 which the hagfish attaches 

 itself to the body of some 

 other fish. By means of tlic 

 rasping teeth on its tongue, 

 it makes a round hole 

 through the skin, usually at 

 the throat. It then devours 

 all the muscular "substance 

 of the fish, leaving the vis- 

 cera untouched. When the 

 fish finally dies it is a mere 

 hulk of skin, scales, l)ones, 

 and viscera, nearly all the 

 muscle being gone. Then 

 the hagfish slips out and at- 

 tacks another individual. 



The lamprey, another low 

 fish, in similar fashion feeds leechlike on the blood of other 

 fishes, which it obtains l)y hicerating the flesh with its ras|>- 

 hke teeth, remaining attached by tlie round sucking disk of 

 its mouth. 



Certain birds, as the cowbii-d and the Kurojiean cuckoo, 

 have a parasitic habit, laying their eggs in the nests of other 



Fig. 219. — The larjre ichneumon fly, 

 Thalessd, with long ovipositor. 



