2 THE ANIMAL PARASITES OF MAN 



do not cover only 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 it, thus acquiring substantial 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 intestine, with 

 its appendages. Nevertheless, permanent parasites are also found in 

 separate organs and systems, such as the muscular and vascular 

 systems, hollow bones and brain, while some live on the outer skin. 

 Here again, the terms ENTOZOA and ENDOPARASITES do not include all 

 stationary parasites ; to the latter, for instance, the lice belong, which 

 pass all their life on the surface of the body of their host, w 7 here they 

 find shelter and food and go through their entire development. The 

 ectoparasitic trematodes, numerous insects, Crustacea, 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 lowly 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 (Cestoda, Trematoda, Acantho- 

 cephala), or at least the majority of species (Nematoda), are parasitic. 

 Formerly the Linguatulidae (Pentastoma) 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. Since the study of the development of the Linguatulidae 

 (P. J. van Beneden, 1848, and R. Leuckart, 1858) has demonstrated 

 that they are really degenerate arachnoids, they have been separated 

 from the helminthes. 



It is hardly necessary to emphasize 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- 

 cussed in the same sense as land and water animals are mentioned, 

 i.e., without conveying the idea of a classification in such a grouping. 

 It is true that formerly this was universally done, but very soon the 

 error of such a classification was recognized. Still, until the middle 

 of last century, the helminthes were regarded as a systematic group, 

 .although C. E. v. Baer (1827) and F. S. Leuckart (1827) strenuously 

 opposed this view. Under the active leadership of J. A. E. Goeze, 



