Physiological Studies on Certain Protozoan Parasites. 151 



Very little could be made out of its structure, owing to its small size, rapid 

 movements, and its peculiar tendency to disintegrate with almost explosive 

 suddenness when kept under observation on a microscopic slide. In 

 addition to these four characteristic forms, several other ciliates were 

 observed more rarely, and on one occasion an amceba. They were not found 

 frequently enough, however, to be considered in the experimental part of 

 the work. So far as the present observations go, the important group of 

 flagellates appears not to be represented by parasitic forms in Diadema. 



The parasites of Diadema seem to occur exclusively in the alimentary 

 tract, none being found in the body-fluid so long as the intestine is uninjured. 

 In the alimentary tract itself the number is not the same in the different 

 regions, some parts being more favorable than others. The greatest 

 numbers are found in the distended pouches of the second, or upper, coil 

 of the intestine. In the lower coil they seem to be almost or entirely absent, 

 even when present in great abundance elsewhere. It is interesting to note 

 in this connection that free-living forms {e. g., Foraminifera, etc.), which 

 are taken in with the food, are invariably dead before the second coil is 

 reached. The rectal portion of the intestine also shows very few protozoa, 

 form D being the one most frequently encountered there, though B and C 

 may occur rarely and in small numbers. On several occasions cysts, pos- 

 sibly of D, were found in this region. It seems quite probable that cysts 

 thus formed may pass out and eventually infect other Diademas. It does 

 not always happen that the different protozoa present show the same 

 distribution, thus, A, for example, may be quite absent in some of the 

 pouches and present in others, while B, C, and D show similar irregularities. 

 It is usual to find several of the forms together, but there are no constant 

 combinations; any form may occur associated with any of the others. 



The above remarks concerning the abundance and distribution of the 

 parasites apply only to adult Diademas. As might be expected from our 

 knowledge of other animals, young individuals contain either no parasites 

 at all or at most but a few. The youngest ones examined (0.5 inch in 

 diameter) contained nothing. Somewhat older ones (1.5 inches in diameter) 

 contained sometimes nothing at all, sometimes large numbers of a form not 

 found in any of the adults examined, and in a few cases small numbers of 

 C and D. It is rather striking that form B, which appears to be present 

 in all adults, should be so constantly lacking in younger individuals. 

 Whether this is because conditions for its existence are unsuitable or simply 

 because no opportunity for infection has occurred could not be determined. 



A search for the parasites in question was also made in sea-water and in 

 other animals, related and unrelated to Diadema. The only situations in 

 which any of them were found was in the alimentary tract of other sea- 

 urchins, where they may occur in varying numbers. B, for example, was 

 found rarely in Toxopneustes and Echinometra, and C on a number of 

 occasions in Toxopneustes, while D seems to be present in every adult 

 Toxopneustes, and rarely in Echinometra. A was never found outside of 



