THE MODES OF LIFE OF THE PROTOZOA 19 



or urinary organs, etc., either living free in the cavity it inhabits, or 

 attached to the lining epithelium. 



As diverse as the modes of parasitism among Protozoa are the 

 effects they produce on their hosts. Some parasites produce no 

 perceptible disturbance in the well-being of their host ; even when 

 they destroy cells and portions of tissues, the damage may be slight, 

 and is quickly made good without appreciable permanent injury 

 being done. From this condition of more or less perfect harmless- 

 ness there is a continuous gradation in the ascending capacity for 

 harmfulness, culminating in species which bring about the death of 

 their hosts with greater or less rapidity. Hence parasitic Protozoa 

 are commonly distinguished as pathogenic or non-pathogenic ; but 

 since there is no precise limit to the degree of sickness or indis- 

 position which justifies the application of the term " pathogenic," 

 it is perhaps more convenient to distinguish them as lethal or non- 

 lethal. It is not possible, however, to lay down hard-and-fast 

 distinctions in these matters,* since a parasite which is not lethal 

 under some circumstances may become so under others ; for instance, 

 an animal living a free and natural life may be quite well able to 

 resist the attacks of parasites to which it succumbs in captivity. 

 Moreover, it must be borne in mind that such terms as " lethal " or 

 " pathogenic " can only be applied to a parasite in its relation to a 

 particular host, since, as will be shown below, a parasite which is 

 harmful to one host may be harmless to another. 



It is far from clear in what way the pathogenic effects of parasitic 

 Protozoa are produced. If the action and reaction of host and 

 parasite were relations dependent simply on the number or relative 

 total bulk of parasites present in a given host, the problems of 

 parasitism would be comparatively simple ; but in many cases this 

 is obviously very far from being the case. The effect produced by 

 a given species of parasite upon a given species of host is a specific 

 reaction, which differs markedly when one of the two dramatis 

 personce is changed. It is not uncommon to find insects vrith their 

 digestive tract or body-cavity crammed with parasitic Gregarines 

 of relatively large size, but apparently none the worse for it. On 

 the other hand, large mammals may succumb to the effects of 

 minute parasites in relatively scanty numbers in the sense, that 

 is, that the aggregate bulk of the parasites may be infinitesimal 

 compared to the bulk of the host. A better comparison is furnished 

 by considering closely-allied species of parasites and hosts respec- 

 tively. A rat may have its blood swarming with Trypanosoma 

 leivisi, without apparently being any the worse for it. On the other 

 hand, in a man dying of sleeping sickness, caused by T. gambiense, 

 or in a ruminant dying of nagana (tsetse-fly disease), caused by 

 T. brucii, the trypanosomes may be so scanty as to be exceedingly 



