184 



PARASITISM 



While most of the examples cited above are to be found among the 

 clearly defined forms of unquestioned systematic position, quite a 

 variety of endogenous variations have been described in the lesser 

 known parasites. Here, especially in the recently created group of 

 haplosporidia and in the sarcosporidia, the former, including parasites 

 mainly of annelids, Crustacea, and fish, the latter, mainly of mammals, 

 the method of asexual spore formation is much more primitive than in 

 the better-known parasites, and, as in selenidium, all of the cell con- 

 tents are used in the formation of the reproductive elements. Some 

 of these forms are cytozoic (HaplosporidiuTn heterocirri, H. vejdow- 

 skli); some are coelozoic (H. marchouxi), and some combine the 



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-if,..; - - '-VJ.'-ijP 



Schewiakovella in the body cavity of cyclops. (From Minc.hin, after Schewiakofl.) -4, free 

 ameboid form; B, encysted ameba; C, sporulation of ameboid stage; D, plasmodial stage; 

 E, sporulation stage of Plasmodium. 



intracellular with lumen dwelling life; but all agree, according to 

 Caullery and Mesnil ('05), in having an endogenous and an exoge- 

 nous cycle, although the full life history in no species is known. In 

 all cases the trophozoite begins as a uninucleated cell, similar to a 

 young form of Plasmodiophora hrassicce, and develops into a multi- 

 nucleated ameboid form which fragments into as many germs (mero- 

 zoites ?) as there are nuclei. Conjugation processes are quite unknown, 

 although Caullery and Mesnil suspected a fusion of nuclei (autogamy) 

 comparable with that in plasmodiophora or that in the more closely 

 allied group of actinomyxidee. The haplosporidia are forms of con- 

 siderable theoretical interest, as indicating a possible close connection 



