INVERTEBKATA. 427 



mosquito. Then they penetrate into the salivary glands of 

 the insect. Only the female mosquito sucks blood, and 

 when a specimen with sporozoites in its salivary glands 

 pierces the skin of a human being it injects into the 

 puncture a drop of saliva containing numbers of the 

 sporozoites, and in this manner the person becomes infected 

 with the so-called malarial disease. 



5. Other Haemosporidia. Haemosporidia are found 

 in various other vertebrates besides man. Drepanidium 

 ranarum lives in the corpuscles of the frog. In birds 

 occurs a genus called Haemoproteus, which is closely 

 similar to the malarial parasite of man. It has been 

 shown that this parasite of birds is transmitted by nios- 

 quitos or gnats of the genus Culex, which is the common 

 gnat in England. 



If Anopheles bites an infected bird all the parasites in 

 the blood swallowed by the insect are killed and digested, 

 when Culex takes the bird's blood all stages are digested 

 except the gametocytes, which go through the processes of 

 conjugation and sporogony. Conversely, if Culex bites an 

 infected person all stages of the malarial parasite are 

 digested, while in Anopheles the gametocytes survive. It 

 is a curious fact that ague was formerly common in the fen 

 districts of England, where it is now almost or quite extinct. 

 Yet gnats of the genus Anopheles still occur there, and it is 

 supposed that this race of the insect has become immune, 

 that is the gametocytes do not survive in them, so that 

 they no longer transmit the infection. 



