THE SPOROZOA 



173 



briefly the variations exhibited by this order as compared with 

 the type selected for description. 



The body -form and external characters of the trophozoite 

 furnish sharp distinctions for classificatory purposes. The funda- 

 mental type of body-form may be described as a sphere or ovoid. In 

 many species this type of form is very nearly retained (Fig. 22), 

 especially in the non-motile coelomic forms, which often have a 

 great resemblance to ova. More usually, however, the body 

 becomes strongly elongated in one direction, a mode of growth 

 correlated either with attachment by one pole or with forward 



Epimerite. 



Protomerite. 



Deutomerite. 



FIG. 14. 



Scheme of development of a Gregarine from a sporozoite. a, free sporozoite ; b and c, stage 

 in the growth of the parasite within an epithelial cell ; d, the Gregarine beginning to protrude 

 from the cell ; e, segmentation of the body and emigration outwards of the nucleus, the iiitra- 

 cellular portion of the body remaining as the epimerite ; /, adult Gregarine with three- 

 chambered body. (From Wasielewski, after Schneider.) 



movement in a definite direction. In many cases the body is 

 extremely drawn out and attenuated, becoming vermiform in 

 character (Fig. 1). 



In the sub-order Acephalina, of which Monocystis is an example, 

 the body remains simple and is not subdivided into different 

 regions, whatever its form. The Cephalina, however, are, with 

 few exceptions, septate, that is to say, the body is divided by 

 septa or partitions into distinct chambers or segments, usually 

 three in number. The septate condition is brought about in the 

 following way. The sporozoite penetrates an epithelial cell (Fig. 

 14, b) and grows within it into an oval body, which at an early 

 stage cannot be distinguished in any way from a young Mono- 

 cystid, or even from a Coccidian parasite (Fig. 14, c). Very soon, 



