THE ORTHONECTIDA 153 



of D. moschatum, one or even two somewhat deformed infusoriform 

 embryos, which he believes had penetrated into the nematogen. He is 

 therefore inclined towards van Beneden's view. In that case, the 

 nematogens will be females, and the germ cells true ova, which will be 

 fertilised by the infusoriform embryos. It is, further, probably of some 

 importance that in young Cephalopods nematogens predominate, while 

 rhombogens are in excess in older hosts. But this is entirely conjectural. 

 Nothing is accurately known about the fate of the infusoriform embryo, 

 nor as to how new hosts become infected. 



The view held by van Beneden that each species of Cephalopod has 

 its own peculiar species of Dicyemid is not true. Whitman has noted 

 both that two or more species of the parasite may occur in one Cephalopod, 

 and that the same parasite may occur in different hosts ; for example 



Dicyema typus. v. Ben. 1 . ~ . 7 



T, V in Octopus vulqans. 



D. schulzianum, v. Ben. J 



D. moscftatom Whitm. \ 



Dicyemennea eledones, WagenJ 



D. truncatum, Whitm. | fc 



D. gracile, Wagen. / 



f in Rossia macrosoma. Sepia elegans* 

 D. truncatum, Whitm. | & officinalis. 



D. schulzianum, v. Ben., in S. biserialis, Oct. vulgaris. 



CLASS ORTHONECTIDA, Giard. 1 



This group includes certain small ciliated organisms parasitic in some 

 low invertebrates. These parasites are built up of an outer layer of flat 

 ciliated cells, arranged in rings round the animal, and thus giving the 

 appearance of segmentation, and of a central mass of polyhedral cells, 

 which become ova or spermatozoa. Between the ectoderm and the central 

 mass are muscular nbrillae, having an obliquely longitudinal course. 

 The sexes are separate and dimorphic, the males being very much smaller 

 than the females, from which they diifer structurally. The females may 

 themselves be dimorphic one being cylindrical, the other flattened. 



The group contains a single genus, Rhopalura, Giard, of which only 

 three species are accurately known ; R. giardii, Metschn. ( = R. ophiocomae, 

 Giard ; + Intoshia gigas, Giard), occurs in the peritoneal cavity of Amphiura 

 squarnata. R. intoshii, Metschn. ( = Intoshia linei, Giard), in the body 

 cavity of Nemertes lacteus. R. pterocirri, St. J., 2 occurs in the Polychaete, 

 Pterocirrus macroceros. Further, a parasite was described by Keferstein 

 in Leptoplana tremellaris, which no doubt belongs to the group. 3 



fertilised eggs ; while females, i.e. vermiform embryos arise parthenogenetieally. 

 Further, the same individual dicyemid may be at first a nematogen, and later a 

 rhombogen. 



1 Giard, Journ. anat. et physiol 1879, xv. p. 449 ; and Q. J. Mic. Sci. xx. 1880, 

 p. 225. 



2 St. Joseph, Bull. Soc. Zool France, xxi. 1896, p. 58. 



3 Caullery and Mesnil have described a genus Staechartrum, from the Annelid 

 Scoloplos (C. R. Ac. Sc. Paris, cxxviii. pp. 457 and 516, 1899). 



