2o8 ZOOLtDGY FOR MEDICAL STUDENTS chap, v 



throughout its existence completely parasitic, passing from one individual 

 host to another simply by bodily transference in the encysted condition. 



II. In Ascaris, Trichocephalus, Oxyuris, the eggs pass to the exterior 

 and lead for a time a free existence but" they complete their development 

 only if taken in by a second individual host. 



III. In Ancylostoma not merely the egg stage, but the young worms 

 themselves are for a time free-living, but a return to the host is necessary 

 for them to complete their development. In Dracimculus a similar 

 condition is found, with the additional complication however that the 

 free stage makes its way into a second or intermediate host, of a species 

 different from the principal host : while in Filaria bancrofti, F. loa, and 

 F. Persians, the whole of the part of the life-history free from the prin- 

 cipal host is passed within the body of the secondary host. 



IV. In Ascaris nigrovenosa, a common parasite in the lung of the 

 ordinary Frog, the young which pass to the exterior become sexually 

 mature and one or more generations of free-living individuals occur 

 before the return to the host. A somewhat similar life-history occurs 

 in the human parasite Strongyloides stercoralis. 



V. In Anguillula and its allies — small nematodes which occur com- 

 monly in soil, in flour-paste, in vinegar (exhibited sometimes by showmen 

 under the microscope as " eels " in paste or vinegar) — the parasitic phase 

 has become completely eliminated from the life-history. 



BOOKS FOR FURTHER STUDY 



Brumpt. Precis de Parasitologie. 



Manson. Tropical Diseases. 



Castellani and Chalmers. Manual of Tropical Medicine. 



