INVEBTBBRATA. 421 



15. Comparison of Monocystis and Coccidium. It will 

 be seen that the two chief peculiarities of Coccidium as 

 compared with Monocystis are (1) the occurrence of 

 schizogony, (2) the differentiation of the gametes. Mono- 

 cystis does not multiply by asexual division within the 

 same host, Coccidium does. In Monocystis the gametes 

 are similar, there is no distinction of naicrogametes and 

 megagametes. In Coccidium, on the other hand, the 

 microgametes are much smaller than the megagametes, 

 and are furnished with flagella. 



16. Characteristics of Parasitic Protozoa. As the 

 Protozoa in many cases correspond to single cells of the 

 multicellular Metazoa, it is natural that the parasitic 

 forms should be intracellular, that they should be parasites 

 within other cells, just as a Metazoan parasite like 

 Distomum is parasitic within the body of the Metazoan 

 host. The rule, however, is by no means universal. The 

 parasite of malaria lives within the blood corpuscles, as 

 Coccidium lives within epithelial cells, but on the other 

 hand the ciliate Opalina lives free in the cavity of the 

 rectum of the frog. The special characteristics of 

 parasitic forms are to a certain degree similar in Protozoa 

 and Metazoa. In the former the power of locomotion is 

 much reduced ; it is entirely absent in many stages, as in 

 the schizonts of Coccidium, and is exhibited only in those 

 stages which migrate from one host cell to another, as in 

 the sporozoites and merozoites, and in the microgametes 

 of Coccidium. Cilia are absent in all the intracellular 

 parasites of the class Sporozoa, although flagella are 

 present in certain stages, as in the microgametes. Food 

 is never ingested in the form of solid particles, as in free 

 Protozoa such as Amoeba and Paramecium. This is one 

 of the most characteristic features of parasitism. The 

 parasite living within another animal takes for its own use 

 the food material which has been obtained and digested by 

 its host, and loses in consequence all the special organs and 

 activities which an independent animal employs in seeking 

 and digesting its food. Parasitic protozoa absorb their 

 food in liquid form at their surface, but whether they 



