112 



What is the Interest and Importance of Strongyloides ster- 

 coralis (syn. Rhdbdonema intestinale) ? 



The life - history shows alternation of generations or 

 metagenesis of the type known as heterogamy (see " Cate- 

 chism," Zoology, Part I., p. 13). A free-living rhabditiform 

 generation (stercoralis) in which the sexes are separate 

 alternates with a parasitic generation (intestinale), the 

 individuals of which are filariform and are either herma- 

 phrodites or parthenogenetic females. 



This parasite is found in the intestine of Man, and it is 

 the cause of a form of endemic diarrhoea in Indo-China, 

 Africa, Brazil, and certain parts of Europe. Infection may 

 be by the mouth or through the skin. The worm has been 

 found in association with Ancylostoma. 



What is the effect of Parasitism (1) on the Parasite, (2) on 

 the Host? 



1. Degeneration : organs essential for free life tend to 

 disappear through disuse ; and the ' body of the creature 

 may have undergone alteration so great that its identity 

 can only be established by reference to its development 

 (e.g., Sacculina. See account of its life history, page 127). 



The degeneration may be slight or considerable, according 

 to the degree of parasitism. In ectoparasites (temporary 

 or permanent) the changes are generally slight, as, for 

 example, the absence of wings in Melophagus, a fly which 

 has become a crawling sheep-parasite. In endoparasites 

 the organic changes are generally striking : locomotor 

 structures (e.g., cilia) are lost, in many instances (e.g. 

 tapeworms) there is no alimentary canal, the nervous 

 system is often poorly developed, eyes are usually absent ; 

 the whole body may be reduced to a generative sac. 



New features are frequently acquired, e.g., adhesive or 

 clinging organs, modification of mouth-parts (as in blood- 

 sucking insects), hermaphroditism, prolific reproduction 

 (" 60,000,000 eggs have been computed in a single Nema- 

 tode worm "). 



Shelter and a rich source of nutrition are the chief ad- 

 vantages gained by the parasite-. 



