PROTOZOA. 



cell substance is divided into a firmer cortical layer and a 

 more fluid central substance. The protoplasm often presents 

 a delicate fibrillar appearance, suggesting that of striated 

 muscle. The nucleus is very distinct, but there are no 

 vacuoles. We may associate the absence of locomotor pro- 

 cesses, " mouth," and contractile vacuoles, as well as the 

 thickness of the cuticle and the general passivity, with the 

 parasitic habit of the Gregarines. It is not clearly under- 

 stood how these and other intestinal parasites have become 

 habituated to resist the action of digestive juices. 



Life History. — The young Gregarine is parasitic in one of 

 the lining cells of the gut ; it grows, and leaving 

 the cell, remains for a time still attached to it (^^ 

 by the cap (Fig. i8, a- yg.) ; later this is cast off, 

 and the individual becomes free in the gut, 

 while still increasing in size. Two individuals 

 often attach themselves together end to end, but 

 the meaning of this is obscure. Encystation 

 occurs, involving a single unit or two together, 

 and from the division of the encysted cell or 

 cells spores are formed. All the protoplasm 

 is not always used up in forming the spores, but 

 a residue may remain, which forms a network 

 of threads supporting the spores. The cyst is 

 sometimes (as in G. blattarum) complex, with 

 " ducts " serving for the e.xit of the spores, each 

 of which is surrounded by a firm case. Eventu- 

 ally the cyst bursts, the spore cases are 

 liberated, and from within each of these the 

 single spore emerges to become a cellular 

 parasite. The spore of G. gigantea is at first 

 non-nucleated ; it gives off two processes, one 

 of which becomes detached, vibratile, and 

 nucleated, while the other seems to come to 

 nothing (Fig. i8, sp-). The adult of this species 

 is sometimes three-quarters of an inch in length 

 — enormous for a Protozoon. 



Third Type — MoNOCVSTis. 

 Monocystis. — A type of those Gregarinida, or Sporozoa, in 

 which the cell is not divided into two parts by a partition. 



Fig. 17. — 

 End-to-eiid 



union of 

 Gregarines. 



(After 

 Frenzel. ) 



