THE MALARIAL PARASITE 185 



into the blood cavities of the mosquito. They travel along in 

 the blood stream, and by some means not fully understood 

 are collected in the salivary glands, whose secreting cells they 

 penetrate. Hence when an infected mosquito sucks blood 

 and, as described above, injects a drop of saliva into the 

 wound made by its proboscis, it discharges a swarm of 

 sporozoites into the capillaries of the individual whom it has 

 attacked. Each sporozoite promptly attacks and penetrates 

 a red blood corpuscle and forms the starting point for a cycle 

 of schizogonous generations. With the infection of the 

 human blood corpuscle the life history of the parasite is 

 brought round to the point at which it began. 



It is a remarkable instance of the close adjustment of 

 parasitic organisms to a strictly limited set of life conditions 

 that, if a human malarial patient is bitten by a gnat of the genus 

 Culex, the latter, no less than an Anopheles, sucks into its 

 stomach all stages of the malarial parasite, but they are 

 speedily digested, gametocytes and all. On the other hand 

 the gametocytes of avian malaria are digested by Anopheles, 

 but undergo further development in the stomach of Culex. 



The Sporozoa that infest the blood corpuscles of various 

 kinds of vertebrate animals are placed in a special order 

 Haemosporidia. In all those whose life history is accurately 

 known, the trophozoite inhabits a red blood corpuscle, at 

 least for a part of its existence, and becomes a schizont, which 

 multiplies itself asexually by schizogony ; the process is 

 repeated for several generations till the host is thoroughly 

 infected by the parasite. Then follows a sexual generation, 

 consisting of gametes which conjugate and form zygotes. 

 The zygotes are motile and seek out a suitable position in 

 which to encyst. The encysted forms or oocysts then divide 

 into a large number of minute sporozoites, which are always 

 naked and motile, never enclosed in sporocysts. 



Different kinds of Haemosporidia are found in all classes of 

 Vertebrata, Fishes, Amphibia, Reptiles, Birds and Mammals ; 

 but, with one doubtful exception, they do not occur in In- 

 vertebrata. Those that attack cold-blooded Vertebrates do 

 not appear to have any harmful effect upon their hosts, but 

 those infesting Birds and Mammals produce all kinds of 

 illness. Thus of the genus Piroplasma, P. bigeminum is the 

 cause of the fatal "red-water" fever in cattle, and is trans- 



