V NEMATODE PARASITES 207 



FiLARIA PERSIANS 



This is a common parasite in Western Tropical Africa^ the Congo, 

 Uganda, Algeria, Tunis and in British Guiana, inhabiting the con- 

 nective tissue, particularly in the meseijtery and lining of the body- 

 cavity. The female measures 70-80 mm. in length, the' male about 

 45 mm. The young reach the blood as in the two preceding species 

 but can at once be distinguished by their smaller size (200-230 //,) and 

 by their being without a sheath. There is no periodicity in their appear- 

 ance in the superficial blood-vessels, and this, as well as the lesser 

 frequency of this parasite in natives who wear no clothes, suggests that 

 possibly the transmitting insect in this case may be a louse or a flea 

 (Christy). 



Strongyloides 



Female individuals of S. stercoralis — small worms slightly over 2 mm. 

 in length — occur as parasites in the lining of the intestine of man in 

 many of the warmer parts of the world. In some cases of intestinal 

 trouble the parasites are apt to be particularly numerous though it is 

 not believed that they are the actual cause of disease. The eggs, which 

 apparently develop without being fertilized (parthenogenesis), give rise 

 to minute larvae which make their way into the cavity of the intestine 

 and after growing to about -75 mm. in length pass out to the exterior 

 in faecal matter. If conditions are favourable (moisture and a tem- 

 perature of 25"-35° C.) they develop into Rhabditis forms with separate 

 sexes. The eggs from these develop into larvae which are at first Rhab- 

 ditis-like but which gradually take on the form of the parasitic Strongy- 

 loides. If now they reach the intestine of man directly through the 

 mouth, or possibly also as in Ancylostoma by boring through the skin, 

 they develop into adult females like those from which the life-cycle 

 started. 



The combination of features that we find in the typical Nematodes — 

 the thick cuticle, the absence of colour, the relatively inefficient move- 

 ments, the absence of sense-organs, the simple alimentary canal, the 

 large numbers of eggs — make it clear that they constitute a group which 

 has been evolved as parasites. A survey of the group shows however 

 that many of its members have emancipated themselves to a less or 

 greater extent from simple parasitism within a single type of host. 



I. Amongst the forms dealt with in this chapter Trichina remains 



