PHYLUM PROTOZOA 45 



within organs located in the coelom. Monocystis from the 

 sperm sacs of oHgochaet worms is the typical genus. 



The Coccidia (Fig. 28) dwell typically within the epithelial cells 

 and are wholly lacking in powers of locomotion. Both verte- 

 brates and invertebrates are subject to attack and one species of 

 the genus Isospora parasitizes man. 



The Haemosporidia live chiefly in the red blood corpuscles of 

 vertebrates and are thus different in habits from the lumen- 

 dwelling gregarines and the epithelial tissue-inhabiting Coccidia. 

 As a typical example of the group stand the organisms {Plasmo- 

 dium vivax, P. malariae, Laverania falciparum) which are the 

 causal agents of malaria. The life cycle involves both schizogony 

 and sporogony. 



The amoeboid organisms of malaria, called schizonts (Fig. 29 ; 

 1 to 7), occur in the erythrocytes of persons afflicted with 

 the disease. Each schizont grows until it almost fills the 

 corpuscle (6). Then spore formation takes place and upon rup- 

 ture of the corpuscle numerous spores or merozoites are liberated 

 into the blood stream, ready, if they escape the attack of the- 

 leucocytes, to enter new erythrocytes and thus continue an auto- 

 infection through an indefinite period. The chills characteristic 

 of the disease are coincident with the periods of sporulation. 



Following a series of these asexual generations, the spore upon 

 entering a corpuscle produces a gametoblast (9a, 96). In later 

 development, each gametoblast develops into either a single 

 female gamete or a group of male gametes. Further develop- 

 ment of these gametoblasts depends upon their being taken into 

 the body of a mosquito of the genus Anopheles which secures 

 them with the blood of a malarial patient. The mosquito is not 

 able to transmit malaria to a new human host immediately follow- 

 ing the introduction of the organisms into the body of the 

 mosquito. It is only upon completion of the sexual cycle (13 

 to 27) of the parasite within the tissues of the insect that the 

 bite of the mosquito may transmit the disease. In the stomach of 

 the mosquito, each microgametoblast gives rise to a number of 

 motile microgametes (156). Each of these upon fusion with a 

 macrogamete (16) forms an ookinete or zygote (17). The zygote 

 penetrates the wall of the stomach and becomes encysted (18). 

 While in this condition sporoblasts are formed and each of these 

 gives rise to numerous spindle-shaped sporozoites (25). These 

 are liberated into the body cavity of the mosquito (26) and are 



