[Illustration: FIG. 5.--LIFE-HISTORY OF THE MALARIA PARASITE. _a_, malarial germ, or sporozoite, as introduced into the blood by the mosquito; _b_, sporozoite after entry into blood-corpuscle; _c_, growth of sporozoite into an amœbula; _d_, division of amœbula to form merozoites; _e_, liberated merozoites; _f_, growth of merozoites into a crescent at expense of corpuscle; _g_, male, and _h_, female crescent; _i_, male cell with projections, which lengthen and are set free as spermatozoa; _j_, fertilisation of ovum by spermatozoon; _k_, fertilised egg as the active motile vermicule; sphere formed from the _l_, enlarged vermicule, after this has bored through the stomach-wall of the mosquito; _m_, segment of sphere at final stage of development, containing countless needle-shaped spores, which, when it bursts, escape as sporozoites into the organs of the mosquito’s body and pass through the salivary glands into the proboscis, and so infect a man bitten or pricked by the mosquito. ] The House-Fly--_see_ enlarged models of the perfect insect and its preliminary stages,--although not provided with a piercing proboscis, and consequently incapable of biting, sometimes plays an important part in spreading deadly diseases, such as cholera and enteric (typhoid) fever. In this case the disease-causing organisms are carried by the insect either in its intestine or adherent to the outer surface of its body, and thus House-Flies may spread disease by contaminating food. Plague, which is a disease of rats and a few other rodents, especially the bobac Marmot, is communicated to man by the bite of the flea known as _Xenopsylla cheopis_, one of several species of fleas with which rats are infested. When a rat dies, the fleas that it has been harbouring seek another host, and may bite human beings, in which case, if the rat itself was suffering from the disease, an epidemic of plague may be the result. The model of the Plague-Flea exhibited is enlarged 200 times (linear). The Arachnida, a group which includes the Spiders and can generally be distinguished from Insects by the number of their legs (four pairs instead of three pairs), include also the Ticks, which are responsible for the transmission of many deadly diseases which attack Man and Domestic animals. Enlarged models of a disease-carrying Tick are in course of preparation. [Illustration: FIG. 6. _Trypanosoma gambiense_, the parasite of Sleeping-sickness, very highly magnified. The occurrence of three kinds of individual, as shown in the figure, appears to be characteristic of this and certain other species of _Trypanosoma_. (After Col. Sir D. Bruce.)] [Sidenote: African Elephant.]