PARASITISM AND DEGENERATION 



301 



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

 spiders, livo parasitically. Most of them live as external para- 

 sites, sucking the blood of their host, but some live under- 

 neath the skin like the itch 

 mites (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 the 

 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, bones, 

 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 by lacerating the flesh with its rasp- 

 like teeth, remaining attached by the round sucking disk of 

 its mouth. 



Certain birds, as the cowbird and the European cuckoo, 

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



FIG. 219. The large ichneumon fly, 

 Thalessa, with long ovipositor. 



