146 AN INTRODUCTION TO ZOOLOGY 



in the mosquito takes some days, and so if a person is stabbed inside 

 this time, unless the mosquito was previously infected, no harm 

 follows, and this has been shown experimentally. 



Various precautionary measures are adopted to prevent the 

 spread of these diseases. Firstly, the number of mosquitos in an 

 area may be reduced by removing all small puddles and accumula- 

 tions of water in old cans, barrels, etc., for it is only in still water 

 that the eggs of the mosquito are laid and the larvae live. Larger 

 areas of stagnant water can often be dealt with by pouring a little 

 petrol upon them ; this spreads out over the surface and forms a 

 thin film that effectively prevents the mosquito larva or pupa from 

 coming to the surface to breathe, and so kills it. Various ointments 

 may be smeared over the exposed parts, which tend to reduce the 

 number of bites, and, most important of all, an infected man must 

 be isolated at once and kept in curtained rooms from which mosquitos 

 are rigidly excluded. 



The two parasitic protozoa we have just studied illustrate several 

 of the main characteristics of parasites in general. In the first place, 

 owing to the peculiar conditions under which they live, they are for 

 the most part devoid of all adaptations for a free-living and food- 

 seeking type of life : they are inactive, save for certain limited 

 periods, and consequently lack organs of locomotion ; their food 

 is already in an assimilable form, so they do not possess cytostome, 

 cytopharynx or food vacuoles. The result, therefore, is a greater 

 or less degree of simplification, or better, perhaps, degeneration, of 

 structure as compared with free-living forms. Lastly, their immo- 

 bility, although satisfactory in some ways, is a distinct bar to their 

 reinfecting fresh hosts, and so maintaining the species. In Mono- 

 cystis the transference of the spores to another worm is largely a 

 matter of pure chance, and even in Plasmodium chance plays a large 

 part, and it is necessary to ensure that any mosquito biting an 

 infected man should itself become infected, and should subsequently 

 reinfect in turn. Thus it is not merely necessary to reproduce, but 

 also desirable that the mode of multiplication should also serve as a 

 means of dispersal. The element of chance iit both cases is met by a 

 typically parasitic phenomenon, the production of an enormous 

 number of young. In Monocystis also, in order to withstand the 

 weather changes, the spores are provided with a tough resistant 

 envelope that can protect the sporozoites and keep them alive. 

 Some such protected stage is often met with in parasites, but not 

 invariably, for in some, as in Plasmodium, where the parasite never 

 lives outside the body of its hosts, the second host that has been 

 acquired obviates the necessity of providing against the inclemencies 

 of the weather. 



