532 BIOLOGY OF THE PROTOZOA 



restricted to any particular group. Comparatively harmless types 

 are lumen-dwelling parasites of different organs, particularly of the 

 digestive tract (Gregarinida, Actinomyxida, Cryptosporidium and 

 Eimeria mitraria among Coccidia, etc.); more pernicious types 

 are cytozoic (Coccidia, karyozoic in Cyclospora, and Microsporidia) 

 and hematozoic (Hemosporidia) for these involve the destruction 

 of cells and impairment of function. Histozoic forms (Myxospor- 

 idia, Sarcosporidia) are likewise pernicious through the formation 

 of great tumor-like cysts in muscles and skin. The massing of cysts 

 in celozoic types often impedes normal activities of the endothelial 

 cells as in the seminal reservoirs of earthworms w T hich frequently 

 contain nothing but cysts, thus virtually effecting castration. 



Transmission of Sporozoa from host to host for the most part is 

 by the contaminative method. Enteric parasites develop resistant 

 spores which are passed out with the feces and are ingested sooner 

 or later by other hosts of the same species. Or in some few cases 

 such enteric forms are ingested by hosts of an entirely different 

 animal type. Porospora, for example, is a quite harmless intestinal 

 gregarine of the lobster which forms so-called " gymnospores, " 

 either singly (P. gigantea) or during pseudo-conjugation (P. legeri). 

 These are taken into the digestive tract of the mussel (Mytilus 

 edulis) where fertilization occurs. This peculiar history involves 

 some difficulty in classification, for if these gymnospores are gametes 

 as is indicated by Porospora legeri then the genus belongs in the 

 Eugregarinida, as is advocated by Reichenow-Doflein; if, on the 

 other hand, they are equivalent to merozoites (agametes) as appears 

 to be the case in P. gigantea, then the genus should be classified 

 with the schizogregarines. Until further knowledge is forthcoming 

 we adopt the latter course. 



When spores are formed in celomic or body cavities the mode of 

 transmission is less obvious. They may, indeed, be passed out 

 through nephridia or by way of sperm and oviducts or, like copro- 

 zoic forms, they may pass unaltered through the digestive tracts 

 of animals which feed upon the normal hosts, to be cast out ulti- 

 mately with the feces. Minchin suggested that birds may be the 

 main disseminating agent for spores of earthworm gregarines, but 

 it is also probable that dissemination occurs through death of the 

 host or by pinching off infected portions of the organism which then 

 disintegrate. In all such cases and in the great majority of all 

 Sporozoa infection is brought about by swallowing spores, the 

 resistant spore cases of which are dissolved by digestive juices and 

 the germs liberated. These spore coverings for gregarines, coccidia 

 and Cnidosporidia are special adaptations which are undoubtedly 

 useful for protection during the exposed periods in the life cycle. 

 With blood-dwelling parasites such capsules would be fatal, for 

 there is no chemical in the blood to dissolve off the coverings and 



