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PART II. 

 HELMINTHES. 



THE term Helminthes (intestinal worms) includes all the parasitic 

 worms. These, though belonging to widely different zoological 

 classes, yet possess striking similarities of structure and development, 

 the result of modifications brought about by the conditions under 

 which they live. To this group belong the Trematodes (sucking- 

 worms), the Cestodes (tapeworms), Nematodes (threadworms), and 

 the Acanthocephales (hooked- worms) . Formerly it also included the 

 Cysticerci (bladder- worms) and the Linguatulidse (tongueworms) , but 

 the former are now recognized as a developmental stage of the 

 Cestodes, while the latter are modified Arthropods. The Hirudinea 

 (leeches), which live more in a state of brigandage and attack both 

 man and the lower animals, are certainly parasitic in their mode of 

 life, though they are not usually classed as Helminthes. The group, 

 moreover, does not include other parasitic Metazoa, whether occa- 

 sional or permanent in their attachment ; these should be classified 

 according to their distinguishing characteristics. 



The majority of the intestinal worms live as endoparasites in the 

 interior of the body of their host. Their usual habitat is the bowel 

 and its appendages, though certain varieties, such as the mono- 

 genetic Trematodes, are to be found upon the surface of the body, 

 in hollow organs which are readily accessible from the exterior (oral 

 cavity, urinary bladder). The Trematodes, Cestodes and Acantho- 

 cephales are entirely parasitic in their manner of life. Free-living 

 stages occur among the Cestodes, and, in certain Nematodes (Angio- 

 stomides), free-living and parasitic generations alternate. There are 

 also true free-living Nematodes which may occasionally live as 

 parasites (facultative parasitism), while a large number of Nematodes 

 never become parasitic at all. These are described as " free-living 

 Helminthes." 



The number of the varieties observed in man has largely increased 

 in later years. In all, eighty-three species are recognized ; of these 

 sixteen are Trematodes, twenty Cestodes, forty-five Nematodes, and 

 two Acanthocephales. A certain number only are specific, e.g., 



