THE president's ADDRESS. 



some cases the host is another species of Protozoon, into the 

 body of which the intruder penetrates, living either in its 

 cytoplasm or its nucleus. Amoebae are very subject to the 

 attacks of intra -nuclear 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, intra-cellular or intra-nuclear in habitat ; or it may 

 penetrate into the tissues, living amongst and between the con- 

 stituent cells ; or, finally, it may be found in one of the internal 

 cavities of the body, such as the digestive tract, body-cavity, 

 blood or lymph-spaces, urinary organs, etc., either living free 

 in the cavity or attached to the lining epithelium. We may 

 distinguish, in general, two ways in which these parasites may 

 tax the resources of the host. Some of those living in the diges- 

 tive tract may simply absorb the soluble products of digestion 

 occurring there, thus diminishing the nutriment of the host by 

 intercepting its food-supply. Other parasites, in the digestive 

 tract or elsewhere, ravage the very substance of the body of the 

 host, either by devouring cells and tissues, as in the case of 

 the dysenteric amoebae already mentioned ; or, and more usually, 

 by absorbing the vital fluids and juices of the cells, tissues, or 

 organs into which they penetrate, sapping in many cases the life- 

 springs of the organism that harbours them. 



As diverse as the modes of parasitism amongst Protozoa are 

 the effects they produce upon their hosts. Some forms of these 

 parasites cause no perceptible disturbance in the well-being of 

 the host ; even when they destroy cells and portions of the tissues, 

 the damage may be slight, and is quickly made good without 

 appreciable harm being done to the host. From this condition 

 of more or less perfect harralessness there is a continuous grada- 

 tion in the ascending scale of capacity for harmful ness possessed 

 by Protozoan parasites, culminating in species which bring about 

 the death of their hosts with greater or less rapidity. Hence, 

 parasitic Protozoa are commonly characterised as pathogenic, 

 that is to say disease-producing, and non- pathogenic ; these two 

 terms imply, however, a distinction which is purely relative, 

 since a " disease " may be anything from a slight and transitory 



