IX. PROTOZOAL PARASITES. PROTOZOA (part of.)--This miscellaneous assemblage of minute creatures embraces a number of parasites of very low organisation. In the present work it is neither desirable nor necessary to hazard any statements respecting their precise zoological position. It is sufficient to say that the parasitic protozoa are for the most part entozoal in habit, not a few of them possessing vegetable affinities. The microscopic _Bacteridæ_, _Gregarinidæ_, and _Psorospermiæ_, comprise a multitude of organisms which are strictly parasitic in their habits, whilst amongst the _Infusoria_ we find numerous forms which, though dwelling in the intestinal canal of their hosts, do not derive nourishment in a direct manner from their bearers. Of this kind are _Paramecium_ and _Balantidium_. The separation of the psorospermiæ and gregarinæ into genera is attended with difficulty; nevertheless, I have for convenience long recognised various types under titles corresponding with the names of the observers who first discovered them (_Hesslingia_, _Gubleria_, _Lindermannia_, and so forth). Of necessity, the protozoal parasites will only be incidentally noticed in this work. In this category I place the falsely so called “cattle-plague bodies.” The micrococci and bacteria hardly come within the province of the helminthologist. Without prejudice to the foregoing restrictions I must at the same time observe that the varied characters presented by the above-mentioned groups show how impossible it is to treat the subject of parasitism adequately, if one is obliged to confine his remarks to the internal parasites or helminths proper. Many creatures possessed of entozoal and ectozoal habits are parasites in every legitimate sense of the term, and yet they do not belong to the class _Helmintha_ in its common zoological acceptation. _That_ class taken by itself may still be allowed to stand pretty much as I represented it in 1864; but in the present work I cease to speak of the Entozoa as in any sense the zoological equivalent of the _Helmintha_. I prefer to employ the term Entozoa in its popular and wider acceptation. It conveniently stands thus, moreover, in direct contradiction to the term Ectozoa.