18 THE PROTOZOA 



type that may be denoted as food-robbers are in general very 

 harmless to their hosts. 



Those parasites, however, that nourish themselves on the sub- 

 stance of the host may produce the most dangerous effects on its 

 health and well-being. As in the case of the food-robbers, parasites 

 of this kind may absorb their food in one of two ways. They may 

 devour solid portions of the host's body in a holozoic manner ; an 

 example of this is seen in Entamceba liistolytica (Fig. 90), the parasite 

 of amoebic dysentery, which devours portions of the host's tissue, 

 such as epithelial cells, or blood-corpuscles. But more usually the 

 parasites absorb their nourishment in a fluid form through the 

 surface of their body, doubtless by the help of enzymes secreted by 

 them. Hence it is typical of true parasites to have lost all trace 

 of special organs for the capture, ingestion, or digestion, of solid 

 food. 



Just as in the epizoic mode of life a harmless or even beneficial 

 commensalism may degenerate by insensible gradations into 

 dangerous parasitism, so the same is true of the entozoic habit. 

 An organism which begins by being a scavenger readily becomes a 

 food-robber. LopJiomonas, for instance, may be seen to contain 

 starch-grains and other substances which probably belong to the 

 food of its host. A further but easy gradation leads to the entozoic 

 organism devouring portions of its host. A good example of this is 

 seen in two of the entozoic amoebae of the human intestine : the 

 common Entamceba coli (Fig. 89) appears to be chiefly a scavenger, 

 harmless to its host, and not deserving the reproach of parasitism ; 

 on the other hand, E, histolytica is a dangerous parasite. So also 

 an entozoic organism, which begins by merely absorbing the pro- 

 ducts of digestion, may end by absorbing the substance of its host. 

 It is highly probable that in many entozoic organisms the mode of 

 feeding may vary according to circumstances, and that an organism 

 which may be a harmless commensal under some conditions may 

 become a more or less dangerous parasite under others. 



The entozoic Protozoa which are truly parasitic may inhabit a 

 variety of situations in the bodies of their hosts. In some cases the 

 host is another species of Protozoon, into the body of which the 

 intruder penetrates, living either in the cytoplasm or the nucleus. 

 Amoebae are ver} 7 subject to the attacks of intranuclear parasites, 

 and the young stages of many Acinetans are parasitic upon other 

 Infusoria. When the host is one of the Metazoa, the invading 

 organism may be in like manner intracellular or intranuclear in 

 habitat ; or it may penetrate into the tissues, living amongst and 

 between the constituent cells ; or it may inhabit, finally, one of the 

 internal cavities of the body, such as the digestive tract, general 

 body-cavity, spaces containing blood or lymph, cavities of the renal 



