ORGANIC RESPONSE 285 



what the appearance of a tiny amoeba, but in the 

 sexual phase the parasite withdraws to one side 

 of the blood corpuscle in a characteristic crescent- 

 like shape. It is now called the gametocyte. If 

 now an Anopheles mosquito bites a man suffering 

 from malaria, the parasitized blood is drawn up 

 into the stomach of the mosquito, where the gametes 

 free themselves and conjugate. The active zygotes 

 then penetrate the stomach wall and become en- 

 cysted, giving rise to an enormous number of motile 

 spores. These find their way to the salivary gland 

 of the mosquito, and thence into the blood of the 

 man whom the mosquito bites. Once introduced, 

 they attack the red blood-cells, and the cycle begins 

 again. 



Parasitism in Worms. The heterogeneous group 

 of " worms " includes several grand divisions that 

 are almost wholly parasitic in their mode of life 

 and have become correspondingly modified in struc- 

 ture. An American species that has recently be- 

 come notorious is the "hookworm" (Necator 

 americanus) that is widely distributed over the 

 Southern states. The larvae of this microscopic 

 worm lives in the ground and finds entrance through 

 the skin of its human victim. It then makes its 

 way through heart and lungs to the alimentary 

 canal, where it attaches itself to the wall, feeds 

 on the blood of its host, and reproduces prolifically. 

 As a result, the host soon begins to show the effects 

 of malnutrition and anaemia. This takes the form 



