PROTOZOA OR SIMPLEST ANIMALS. 
14g 
The invading enemy is attacked and partly held in check by the 
white blood cells, which devour and destroy many of the enhaemo- 
spores before they become ensconced inside the helpless red blood cells. 
If the malaria parasite had to depend on the above asexual mode 
of propagation by simple division, it would soon become extinct ; 
the host would die, and the parasite also. For the mosquito wholly 
digests amoebulae, rosettes, and enhaemospores 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 division, 
however, the full - grown amoebulas, instead of dividing up into 
enhaemospores, become sausage-like crescents (Fig. 10 f ,/), some of the 
crescents being male (Fig. 10f, g ) and others female (Fig. 10f, h ), 
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 develop as follows : the male 
spheres suddenly push out projections which lengthen (Fig. 10f, i ) 
and very rapidly become free as wriggling spermatozoa ; the female 
cell attracts a spermatozoon, probably by chemical allurement 
(Fig. 10f,/) ; the two elements fuse and the resulting fertilised egg 
or zygote becomes an actively motile vermicular cell (Fig. 10f, Jc), 
which burrows through the stomach wall of the mosquito, where it 
forms a sphere (Fig. 10f, l). The sphere now grows to a considerable 
size (Fig. 10d, 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. 10f, m). The sphere bursts 
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. 
The terrible Sleeping Sickness of man, and the Nagana 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 be classed with the 
group Flagellata. The body of the Trypanosoma (Fig. 10h) is 
provided with a fin-like extension, and with a flagellum. As in the 
case of malaria, there is an intermediate host, the blood-sucking 
Tsetse fly, which inoculates the Trypanosoma into the blood of its 
