CHAPTER Y. 

 OETHONECTID.E AND DICYEMID^. 



The DicyemidjB were discovered as early as 1839 by Krohn, 

 the Orthonectidse in the sixties by Kefkkstein and McIntosh. 

 They were more than once after that the object of investiga- 

 tion (v. KoLLiKER, G. Wagener), but a more thorough 

 knowledge of their structure and development was not ac- 

 quired until recent times. Our knowledge of the latter 

 division of these most simply constructed, parasitic creatures 

 is due principally to the exertions of A. Giard, Metschnikoff, 

 and JuLiN, whereas the Dicyemidte have been thoroughly 

 studied by E. van Beneuen and Whitman. 



I. ORTHONECTID/E. 



Systematic : There are only two species known : — 



(1) Bhopalura Giardii Metschn. (Bh. opMocomce Giard., Tato- 

 shia gigas Giard.), from Amphiura squamata ; 



(2) Bhopalura lutoshii Metschn. (probably synonymous with 

 Intoshia Linei and LeptoplauK Giard.), from Nemertes lacteus. 



The OrthonectidfB, which live parasitically in Ihirhellarians, 

 Nemerteans, and Ophiurans, exhibit a striking sexual dimor- 

 phism. Male and female differ both in form and size (Fig. 

 97 A and B). The organization is very simple. The females 

 are composed of only a peripheral cell-layer and a central 

 cell-mass (Fig. 97/4). They are spindle-shaped and covered 

 on the surface with vibratile cilia. However, two forms 

 are distinguishable : those with a cylindrical body (formes 

 cylindriques of Julin) and those with a flattened body 

 (formes aplaties). Both forms exhibit a kind of external 

 segmentation. They probably migrate out of the body of 

 the Ophiuran (Amphiura squamata) which they inhabit, in 



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