PARASITISM 303 



their appropriate hosts, sometimes to satisfy their appe- 

 tites, sometimes to remain. Should death overtake the 

 particular host, they desert him for another, but should 

 none be available, they too must die, having mouth 

 parts solely adapted for sucking blood. 



The final step is reached with the lice. One species 

 inhabits the clothing and visits the skin to feed; other 

 species attach themselves to the hairs and are permanent 

 as well as obligatory parasites. 



All the parasites of this class, whether they visit the 

 surface of the body occasionally, attach themselves to it 

 permanently, or even burrow into it, like the Sarcoptes . 

 scabei, or itch mite, are ectoparasites. A still more close ^ 

 relationship conjunctive symbiosis is seen in those 

 cases in which the parasite actually enters the body of 

 the host and inhabits its blood, tissues, or alimentary 

 canal. Such are known as endoparasites. 



Parasitic symbiosis not only takes place by design, 

 but also by accident, and indeed since it may be con- 

 jectured that accident first brought about and fostered 

 the relationship, it is difficult in some cases to say 

 whether we are dealing with true parasitism or not. 

 Among the infectious diseases of man and animals, and 

 indeed of plants, we are not infrequently puzzled to 

 know whether certain bacteria are parasites in the true 

 sense or not. Thus, the bacillus of tetanus or lock-jaw 

 is a rather common tenant of the alimentary apparatus 

 of herbivorous animals with whose dejecta it finds its 

 way to the soil in which it is sometimes found in large 

 numbers. When this bacillus is accidentally admitted 

 to the tissues of certain animals, it proceeds to live and 

 multiply, and eventually, in many cases, to destroy the 

 animal through the virulence of its toxic metabolic 

 products. Here the accidental circumstance of a wound 

 leads to an unexpected symbiosis that is fatal to the host 

 and later to the parasite as well. 



When this circumstance is carefully analyzed we find 

 that one of the fundamental conditions of true parasit- 



