416 BIOLOGY OF THE PROTOZOA 



protected against drought and other external conditions by resistant 

 spore membranes or capsules which are opened or dissolved only in 

 the digestive tract of a new host. In the majority of cases such 

 new hosts are individuals of the same species and infection is brought 

 about by eating contaminated food. In many forms, however, the 

 life cycle involves a change of hosts the metagamic spores developing 

 in one type of animal and the sexual phases of the parasite develop- 

 ing in another tA pe belonging to an entirely different group of the 

 animal kingdom. Thus vegetative stages of the genus Aggregata 

 develop in the crab (Portinivs dejmmtor) and the sexual stages in 

 the cephalopod {Sepia officinalis); vegetative stages of the malaria 

 organisms, Plasmodium, Larerania, and Hcemoproteiis develop in 

 the blood of man or birds and the sexual stages in the mosquito. 

 In these blood infesting Sporozoa a further adaptation is noted in 

 the loss of the characteristic capsules of the metagamic spores which 

 are inoculated with the bite of the mf)squito directly into the blood. 

 Spore capsules here would make an imjiossible barrier to develop- 

 ment and such forms are obligatory parasites in all phases of their 

 life history. In some cases parasites reach the blood by way of the 

 digestive tract and infection is contaminative. The rat parasite 

 Ilcpmogregarina (Hcpato^odn) perniciosa (according to Miller, 1908) 

 forms its metagamic spores in the rat mite {Lelaps echidnimis). 

 Such infected mites are eaten by the rat and the spores develop in 

 liver cells through some agametic generations, the agamic spores 

 finally entering the blood where they are taken up by leukocytes in 

 which the parasites encyst. Such encysted spores are taken with 

 the blood into the gut of the flea where sexual phases take place 

 and metagamic spores are formed (Fig. 177). 



Although wide differences exist in the life cycles of the various 

 kinds of parasites included in the Sporozoa there is a sufficient 

 general resemblance in all to indicate a fundamental common type. 

 A special nomenclature has grown up for these parasitic forms which 

 is generally limited to the Sporozoa although there is no reason why 

 the terms, if useful should not be applied with equal right to the 

 stages in the life histor\' of free-living forms. We shall use here the 

 terminolog\' suggested by Hartmann as modified by Doflein with 

 such changes as will make the terms applicable to both free-living 

 and parasitic forms. These terms are: 



1. Sporo^oite. A spore or germ produced as a result of fertili- 

 zation. 



2. ^igamont. x\n asexual individual reproducing without fer- 

 tilization (also called a schizont, or a trophozoite). 



3. Scki'soiitocyie. A special form of agamont which breaks up 

 into a number of agamete-forming centers by multiple division. 



4i Agamogony (schizogon\'). Asexual or agamic reproduction 

 by equal, unequal or multiple division. 



