2 THE ANIMAL PARASITES OF MAN 



In consequence of their mode of life all these forms live on 

 the external surface of the body of their host, though more rarely they 

 take up their abode in cavities easily accessible from the exterior, 

 such as the mouth, nose and gills. They are therefore frequently 

 called EPIZOA or ECTOPARASITES ; but these designations do not 

 only cover the temporary parasites, because numerous epizoa (as 

 for instance the louse) are parasitic during their entire life. 



In contradistinction to these temporary parasites, the permanent 

 parasites obtain shelter as well as food from their host for a long 

 period, sometimes during the entire course of their life ; they do not 

 seek their host only when requiring nourishment, but always remain 

 with him, thus acquiring essential protection. The permanent para- 

 sites, as a rule, live within the internal organs, preferably in those 

 which are easily accessible from the exterior, such as the intes- 

 tine, with its appendages ; nevertheless, permanent parasites are 

 also found in separate organs and systems, such as the muscular 

 and vascular systems, hollow bones, brain, &c. : some live on the 

 outer skin. Here, again, the terms ENTOZOA and endoparasites do 

 not include all stationary parasites ; to the latter, for instance, 

 appertain the lice, which pass all their life on the surface of the body 

 of their host, where they find- shelter and food and go through their 

 entire development. The ectoparasitic trematodes, numerous insects, 

 crustaceans, and other animals live in the same manner. 



All " HELMINTHES," however, belong to the group of permanent 

 parasites. This term is now applied to designate certain low worms 

 which lead a parasitic life (intestinal worms) ; but they are not 

 all so termed. For instance, the few parasitic TURBELLARIA are never 

 classed with the helminthes, although closely related to them. The 

 turbellarians, in fact, belong to a group of animals of which only a 

 few members are parasitic, whereas the helminthes comprise those 

 groups of worms of which all species (Cestodes, Trematodes, Acantho- 

 cephala), or at least the majority of species (Nematodes), are para- 

 sitic . Formerly the Linguatulidse (Pentastomum) were classed with 

 the helminthes because their existence is also endoparasitic, and 

 because the shape of their body exhibits a great similarity to that of 

 the true helminthes ; but since the study of the development of 

 the Linguatulidse (P. J. van Beneden, 1848, and R. Leuckart, 1858) 

 has demonstrated that they are really degenerated arachnoids, they 

 have been separated from the helminthes. 



It is hardly necessary to emphasise the fact that the helminthes 

 or intestinal worms do not represent a systematic group of animals, 

 but only a biological one, and that the helminthes can only be dis- 



