PEOTOZOA 25 



and leaves the frog in the encysted condition. The cyst yields 

 many small spores, which if swallowed by tadpoles grow into 

 the adult Opalina found in the frog. 



Sporozoa. A large number of parasitic Protozoa are 

 grouped under this title, and are better known as gregarines 

 and coccidia. A common coccidium affects the liver of the 

 rabbit. It causes a white-spotted condition of the liver, 

 and the disease is called coccidiosis ; this species is Coccidium 

 oviformis. Coccidiosis is common in all vertebrates, and 

 coccidia are found in several invertebrates. 



Gregarines are found only in certain invertebrates and in 

 Ascidia, not in any of the higher classes. 



Monocystis. The gregarines may be illustrated by 

 Monocystis, the spore masses of which are so common in the 

 seminal vesicles of the earthworm. In describing the history 

 of such parasites as Monocystis it has been found convenient 

 to introduce the term Trophozoite for the members of the 

 trophic or feeding phase, and Sporozoite for those resulting 

 from spore formation. 



The Monocystis trophozoite is a single elongated cell with 

 a small nucleus. It inhabits the mother cell which produces 

 the sperms of the earthworm and grows there by imbibing 

 the juices. When it is fully grown it escapes and is now a 

 gametocyte. Two individuals come together and a mutual 

 cyst is formed about them. The nucleus in each divides to 

 form a large number of nuclei arranged peripherally, and they 

 are freed with cytoplasmic investments. Fusion takes place 

 among these cells or gametes in pairs, and it is evident that 

 the pairs arise respectively from the two original cells, for 

 they colour differently in Leishman's stain. The cell resulting 

 from the fusion is invested in a spore case and divides to 

 form eight sporozoites. How these are spread is not known. 



Plasmodium. Plasmodium (Haemamoeba), the cause of 

 malaria in man, and its allies are parasites of the blood 

 corpuscles of vertebrates and are spread by invertebrates. 

 They are necessarily very small. The trophozoite phase of 

 Plasmodium is a small amoebiform cell which, entering a red 

 corpuscle, feeds and grows until it practically fills the corpuscle. 

 A vacuole is present during the early growth, but it gradually 



