THE ENTRANCE OF PARASITES 341 



their appearance differs considerably from that which 

 they presented in the first. And often they exist in one 

 kind of animal without doing any harm, and are only 

 poisonous when introduced by insects into the blood of 

 other kinds of animals ! 



There is, further, another set of disease-causing proto- 

 zoan parasites which are similar to the amoeba or proteus- 

 animalcule, and a third, which belong to the group 

 of " ciliated infusoria." They are not so minute as the 

 preceding set, and are not usually referred to as "microbes." 

 They inhabit the intestine of man and animals, and cause, 

 in some instances, dysentery. These two later kinds of 

 protozoan parasites I will at the moment leave out of 

 consideration, as well as the " coccidia," which multiply in 

 the tissue-cells of animals for instance, rabbits and mice 

 and cause an unhealthy growth and excessive multiplica- 

 tion of the cells of the tissues, which in some respects 

 resembles that seen in the terrible disease known as cancer. 

 Indeed, it is held by many investigators that some such 

 parasite though not yet discovered is the cause of 

 cancer. 



A very important question is : How do these poison- 

 producing parasites (for it is by the poison which they 

 manufacture that they upset the healthy life of their 

 hosts) make their way into the human body? The 

 surface of the body of animals, like man, is pro- 

 tected by a delicate, horny covering the epidermis 

 through which none of these parasites can make their 

 way. They can only get through it, and so into the soft, 

 juicy tissues and the fine blood-vessels which it covers, 

 when it is cracked, broken, pierced, or cut. But they 

 also have a way open to them through the softer moist 

 surfaces of the inner passages, such as the digestive canal 

 and the lungs. They enter (some kinds only and not a 

 few) with food and drink into the digestive canal, and 



