INTRODUCTION 7 



species which for the most part content themselves with 

 devouring the feathers of birds and the hairs of quadrupeds. 

 In addition to these it may be added that some of the rat-tailed 

 larvae or Helophilus maggots (Syrphida) are parasitic in man 

 and quadrupeds, as are also the larvse of the churchyard beetle 

 (Blaptidrt). The closely allied Tenebrionidae and other coleo- 

 pterous families also supply various maggots possessed of para- 

 sitic habits. Fleas and bugs come under Van Beneden's category 

 of free parasites. This is equivalent to calling them non- 

 parasitic parasites, an expression which looks very like a con- 

 tradiction of terms. 



IX. PEOTOZOAL PARASITES. PROTOZOA (part of.) This mis- 

 cellaneous assemblage of minute creatures embraces a num- 

 ber 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 Bacteridae, Gregarinida, and Psorospermite, 

 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 psorospermias and gregarinae 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, 

 Gublericij 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 



