REMARKS ON THE STRUCTURE OF GREGARIN^. 347 



Gregarinida presents us with a series of gradual differentia- 

 tion in various species^ from a perfectly homogeneous condi- 

 tion to that in which both circular and longitudinal fibrils 

 are present. 



The series would be represented by, a. the homogeneous 

 cortical substance of Monocystis sipunculi or M. lumbrici ; b, 

 that of M. nereidis [pellucida, Koll.), with longitudinal 

 fibrillation of its anterior expansion ; c, that of M. telepsavi 

 with very fine circular and more distinct longitudinal fibrilla- 

 tion, though I must confess that Stuart's mode of making his 

 statement is very inconclusive ; d, that of G. gigantea, Van 

 Ben., with its perfectly distinct hoop-like circular fibrils and 

 subjacent homogeneous substance. 



Though it possibly thus forms, but one of the furthest 

 points attained in the differentiation of the primarily homo- 

 geneous cortical substance, I must refer back to what has 

 been already said against the assumption that Van Beneden's 

 layer of circular fibrils forms part of the muscular or corticle 

 substance, and that it is not merely cuticular. 



Monocystis sijmnculi, Koll. — Among sjiecimens of Sipun- 

 culus nudus Avhich the fishermen at Naples brought me in 

 great abundance last winter and spring I frequently, whilst 

 working at the anatomy and histology of these worms, came 

 across individuals infested with Gregarinae. They usually 

 occurred as Avhite spherical bodies, often of considerable size 

 (fig. 3 h), frequently enclosed in a cyst formed by the peri- 

 toneal membrane, which everywhere lines the perivisceral 

 cavity of Sipunculus, and is altogether very remarkable his- 

 tologically. This membrane is ciliated at intervals, and 

 those growths Avhich invest Gregarinse are frequently cihated 

 also. In fig. 3 c a portion of one of these sjiherical Gre- 

 garinse, with its nucleus and investing growth of peritoneum 

 with ciliated patches is represented. Such cysts as these, 

 when detached, rotate slowly in consequence of the action of 

 the cilia, and lead, at first, to the notion that they are ciliated 

 organisms. 



On one occasion, on slitting up a large Sipunculus, and 

 alloAving its abundant pink perivisceral fluid to run into a 

 glass dish, my attention was attracted by two white flakes of 

 about an eighth of an inch in length, which were swimming 

 actively in the liquid. Their movement was like that of 

 some planarians, and seemed to depend on the undulation of 

 their lateral margins, which were plainly seen to be in a state 

 of vibration. These white flakes turned out to be specimens 

 of the Monocystis sipunculi, differing entirely from the 

 spherical encysted forms in shape, but, like them, having the 



