THE president's ADDRESS. 



On the other hand, epizoic forms may be parasites, and even 

 dangerous parasites, nourishing themselves at the expense of 

 the animal they infest, and sometimes inflicting much damage 

 upon it. It can easily be understood that an epizoic form which 

 at first lived harmlessly upon some animal, drawing its supplies 

 of food from the surrounding medium, might acquire the habit 

 ultimately of obtaining its nourishment from the living sub- 

 stratum upon which it has planted itself. Examples of truly 

 parasitic epizoic forms are the Flagellate Costia 7iecatrix and the 

 Infusorian Ichthi/ophihirius, both of which live on the skin of 

 fishes and injure the epidermis. 



In like manner, there are many entozoic Protozoa which 

 inhabit the bodies, and more especially the intestines, of other 

 animals, but which are not to be considered as true parasites ; 

 they simply feed on various substances to be found there, such 

 as waste particles of food or faecal matter, or on other organisms, 

 such as Bacteria in short, on material which, from the point 

 of view of the host, is superfluous or even noxious. Thus entozoic 

 Protozoa may be quite harmless, perhaps even useful as 

 scavengers to their host ; but such forms may also readily acquire 

 truly parasitic habits and become a danger to their hosts. A 

 good example of this is seen in the amoebae found in the intestine 

 of man. One species. Amoeba coll, which occurs commonly in 

 man in all countries, is believed to be a harmless entozoic form. 

 In the tropics, however, other species occur which find a more 

 abundant source of nutrition in the living body which harbours 

 them ; they attack the epithelium and tissues of the intestine, 

 devouring cells and blood-corpuscles and giving rise to a virulent 

 form of dysentery ; and they penetrate into the liver, where they 

 destroy and corrode the tissues, producing most dangerous 

 abscess?s. It is often very diflicult to draw the line between 

 harmless and harmful entozoic Protozoa, but all such organisms 

 must be considered suspect, and it is the safest course from the 

 practical point of view to hold them guilty until they have been 

 proved innocent. 



The entozoic Protozoa which are truly parasitic are found 

 inhabiting a variety of situations in the bodies of their hosts. In 



