THE SPOROZOA 191 



observed by Caullery and Mesnil [Sb] l in a species of Selenidium 

 from Spio fuliginosa. In the Schizogregarinae, on the other hand, 

 schizogony is of constant occurrence, as their name implies, and 

 takes the form of multiple fission during the free extracellular 

 phases of the life-history. 



CLASSIFICATION. 



The systematic arrangement of the Gregarinida that follows is taken 

 from Labbe's " Sporozoa " [4], for the most part, but with some additions 

 or modifications necessitated by recent advances in our knowledge of the 

 group. 



SUB -ORDER I. SCHIZOGREGARINAE, Le'ger (Amoebosporidia auct). 

 Gregarinida in which schizogonic reproduction takes place during the 

 extracellular phase of the trophozoite, in addition to the ordinary 

 sporogony. 



The forms composing this sub-order have been regarded until recently 

 as a very problematic group. Their position in the Sporozoa and their 

 affinities with other members of the class have been considered doubtful 

 and altogether uncertain. Up to the end of the nineteenth century the 

 group was represented only by two species of Ophryocystis, except for the 

 fact that the supposed cancer-parasite has been referred to it by some 

 authorities. The misconception which has prevailed with regard to the 

 natural position of these forms appears to be largely due to the fact that 

 the species of Ophryocystis were originally described as amoeboid, and this 

 character was supposed to be diagnostic of the order represented by them, 

 hence termed Amoebosporidia. 



The recent investigations of Leger [20, 21, 25], however, have not 

 only made known an allied form, Schizocystis, which has a fixed body-form 

 like other Gregarines, but have demonstrated that even Ophryocystis is not 

 amoeboid, as originally described, but has a definite orientation of the 

 body, the apparent pseudopodia being merely stiff processes of attach- 

 ment (Fig. 21). There can be no question that the natural position of 

 the group is amongst the Gregarines ; indeed it is difficult to find any 

 constant diagnostic character, except the mode of reproduction, separating 

 Ophryocystis and Schizocystis, the only two genera known at present, from 

 the rest of the order. 



Genus 1. Schizocystis, Leger, 1900, for 8. gregarinoides, Leger, from 

 the intestine of a dipterous larva, Geratopogon sp. The trophozoites 

 are cylindrical and elongated, about 150 //. in length, with an anterior 

 clearer region, and occur fixed to depressions in the intestinal wall. They 

 resemble a Monocystid in general appearance, but while uninucleate in 

 the youngest stages, the full-sized individuals may have as many as sixty 

 nuclei. The body then divides up to form a number of merozoites, which 

 become trophozoites of the second generation. The latter are uninucleate, 

 and when full -sized they associate and become encysted, giving rise to 

 gametes which conjugate and produce octozoic spores, exactly after the 



1 Whose figures, however, are far from convincing. 



