224 PARASITES. 



Epizoa infest the skin of their hosts, and in the case of man and 

 other mammals suck his blood — such, for instance, as mosquitoes 

 or gnats, fleas, lice (three or four species), with small suctorial 

 mouths^ and a bug. In the case of the biting, or bird hce, they 

 have bitifig frtouths furnished with mandibles and hooked maxillae 

 to suit their mode of life, as they do not suck the juices of their 

 hosts, but feed upon the tegumentary appendages. Sometimes 

 these biting or bird lice are found on mammals. One of the most 

 familiar is the dog-louse, Trichodectes latus. Some mites may also 

 be included with the external parasites. 



In many parasites the male is free all his life ; but the female, 

 while free when young, seeks a host when she becomes sexually 

 mature. Such are chiefly Epizoans, as the Lernaeas and fish-lice. 

 Many parasites, especially tape and thread worms, have two hosts, 

 the intermediate one being generally an herbivorous animal, which 

 swallows the parasite either in the egg or whilst young ; and the 

 final host some carnivorous animal, which feeds upon the inter- 

 mediate host, and which swallows with it the contained parasite. 

 The parasite, on reaching its final host, generally undergoes a 

 change of form and becomes sexually mature, inhabiting chiefly 

 the alimentary canal and its outgrowths, so as to apply for a ready 

 exit for the eggs. Most parasites are introduced into the system 

 through the medium of meat and drink. The most terrible of 

 man's parasites are the Trichina spiralis (which causes the disease 

 known as trichinosis) and the tape worms, in meat insufficiently 

 cooked. 



It is believed that mosquitoes imbibe embryo Filarice with the 

 blood of man, and that many of these reach full development 

 within the mosquitoes, acquiring their freedom when the latter 

 resorts to water, where it dies after depositing its eggs. Mosquitoes 

 are thus the means of introducing the Filarice into the human 

 body through the medium of water. 



Certain parasitic flies lay their eggs within reach of the tongues 

 of horses, and by which means they are conveyed into the 

 stomach. Other insects, as the Ichneumon flies, pierce the 

 bodies of caterpillars by means of their ovipositors, and insert 

 their eggs, which hatch into fat larvae, and devour their hosts' 



