PROTOZOA OR SIMPLEST ANIMALS. 14r, 



Tho invudiui; euciuy is attacked and partly held in check hy the 

 white blood cells, which devour and desiroy many of the enhitnio- 

 spores before they become ensconced inside the helpless red blood cells. 



If the malaria parasite had to depend on the aljove asexual mode 

 of propagation by simple division, it would soon become extinct ; 

 the host would die, and the ])arasite also. For the mosquito wholly 

 digests ama'buhc, rosettes, and enlucmospores along with its meal of 

 blood, and accordingly is unable to convey them alive to another 

 host having a fresh supply of blood cells. 



After a certain number of generations of asexual ilivision, 

 however, the full-grown amrrbuliE, instead of dividing up into 

 euhasmospores, become sausage-like crescents (Fig. I of,/), some of the 

 crescents being male (Fig. lOF, //) and others female (Fig. lOF, //), 

 the sexes being distinguished by the mode of distribution of the 

 dark granules (see figures). No further changes take place in the 

 human host, but if an Anopheles now sucks the blood, and takes in 

 the malaria germs, the male and female crescents and spheres are not 

 digested, but become spherical and develoj) as follows : the male 

 spheres suddenly push out projections which lengthen (Fig. IOf, /') 

 and very rapidly become free as wriggling spermatozoa ; the female 

 cell attracts a spermatozoon, probably by chemical allni-ement 

 (Fig. 10F,y) ; the two elements fuse and the resulting fertilised ^y:,^^ 

 or zygote becomes an actively motile vermicular cell (Fig. KiF, k), 

 which burrows through the stomacli wall of the mosquito, where it 

 forms a sphere (Fig. IOf, /). The sphere nowgrow'sto a considerable 

 size (Fig. IOg, showing many spheres on the wall of the stomach), and 

 its contents break up into sporoblasts, which again give rise to 

 countless spindle-shaped exotospores (Fig. Kif, m). The sphere bui-sts 

 and the exotospores are conveyed into the tissues and organs of the 

 mosquito, many of them coming to rest in the salivary glands, 

 whence they are finally inoculated into a human being when the 

 mosquito stabs the skin ; and we now arrive at the point whence we 

 started. 



Tlie terrible Bleeping Sickness of man, and theXagana or Tsetse 

 disease of domestic animals are both due to the presence in the blood 

 of different species of a minute corkscrew-like flagellate organism 

 Trypanosoma — which though mentioned here must Itechussed with the 

 group Flaoellata. The body of the Tri/pmiosoiud (Fig. Inn) is 

 provided with a lin-like extension, and with a flagelhim. As in the 

 case of malaria, there is an intermediate iiost, the blood-sucking 

 Tsetse fly, which inocidates the 7\-i//i(iiiosoiti(i into the blood of its 



