140 AN INTRODUCTION TO ZOOLOGY 



as in some Sporozoa. For this reason they are termed Isogametes, 

 and the process of their union is spoken of as Isogamy. 



To all appearances Monocystis is quite a harmless parasite, and 

 does not seerii to have any evil effect upon its host ; indeed, it is so 

 widely spread that almost all worms are infected to a greater or 

 less extent. The only parts affected are the sperms, and these are 

 produced in such quantities that even in the case of a heavy infection 

 there are still sufficient healthy sperms to do the work of fertilisation. 



Parasitic Protozoa ii. PJasmodium, the Malarial 

 Parasite. 



Three species of the genus Plasmodium, causing in man 

 three distinct diseases, malaria and two kinds of ague, are known : 

 P. immaculatum (or P '. falciparum) , producing pernicious or tropical 

 malaria ; P. vivax, producing tertian ague ; and P. malaria, re- 

 sponsible for quartan ague. The diseases are very widespread over 

 the tropical and temperate parts of the world, and were at one time 

 common in the fenlands and low-lying districts of England, where 

 now, fortunately, they have practically disappeared. Often tracts 

 of country are devastated by their ravages, for they may be deadly, 

 as England found to her cost in the Walcheren expedition in 1809. 

 In this force, out of 39,219 men, 4175 died, the number who suffered 

 from the disease was nearly 27,000, and even when, the troops 

 were recalled and reached England, 11,500 were suffering from 

 " Walcheren sickness," as it came to be called. It was long asso- 

 ciated with very damp soils, hence its name of marsh-fever, and also 

 the word malaria, mat' aria, which means the bad air of marshy 

 places, and which was supposed to be poisonous. 



The discovery in 1882 that the disease was caused by minute 

 sporozoa in the red blood corpuscles we owe to A. Laveran, a 

 French military medical officer, and the satisfactory working out 

 of its complex life history may well be regarded as one of the bio- 

 logical triumphs of the end of the nineteenth century. The life 

 history, practically the same in the three species, is more com- 

 plicated than that of Monocystis, since it is intimately connected 

 with two distinct animals, man and a blood-sucking mosquito of 

 the genus Anopheles. We term the species in which the sexual 

 part of the life cycle is gone through, in this case the mosquito, the 

 principal host, and that in which the asexual period occurs, here a 

 man, the secondary host. These two terms, principal and secondary, 

 are applicable generally to the life histories of all parasites, but in 

 the case of malaria and certain other diseases it is often customary, 

 especially in medical works, to entirely reverse the terms. In 



