2 TROPICAL MEDICINE AND HYGIENE 



Other parasites, as the Ascaris lmnbricoides,a.re parasitic 

 in man only, and though their eggs or embryos are passed 

 externally, development is limited until they are reintro- 

 duced into man. Others, again, as the Ankylostoma, 

 develop to a considerable extent as free-living larvae. 

 This development is dependent on temperature and other 

 simple meteorological conditions, but the completion of 

 the development requires that such larvae should again 

 have access to man. 



A knowledge of the different modes of development of 

 different species is a necessary preliminary to the under- 

 standing of the measures required for the prevention of 

 infection and therefore of the diseases caused by these 

 parasites. 



Forms of Parasites. Parasitism may be permanent, and 

 in that case bodily change of the parasite results. Even 

 with ectoparasites, which do not live in the interior of 

 the body, special modifications occur, for instance, wings 

 are often absent or rudimentary, as in the fleas, sheep- 

 keds, and pediculi. The claws (ungues), on the other 

 hand, are frequently exceptionally well developed. 



With internal parasites still further modifications occur, 

 so that they become quite unsuited for external life. 

 Many of these cease to be actively motile during 

 some periods of their parasitic life ; in others the 

 alimentary canal is rudimentary or lost, as in the Filaria 

 medinensis, or Guinea-worm. Hermaphroditism is com- 

 mon in parasites, though not limited to them. 



In many cases the males die early, and the females, 

 after fertilization, become little more than living receptacles 

 of ova. In such instances every vital function is sub- 

 ordinate to the development of the fertilized ova. 



It is not uncommon for parasites to feed on waste 

 products of their host or on superfluous food ; such 

 parasites are known as commensal parasites. The bird- 

 lice, for instance, live merely on epithelial debris, and, 

 therefore, may be actually beneficial to their hosts. 



Parasites may be confined to one class of host, and 



