CHAPTER V. 



OETHONECTID^ AND DICYEMID^. 



The DicyemidEe were discovered as early as 1839 by Krohn, 

 the Orthonectidge in the sixties by Kefkrstein and McIntosh. 

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

 tion (v. KÖLLIKER, G. WaCxENEr), 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 Dicyemidae have been thoroughly 

 studied by E. van Beneden and Whitman. 



I. ORTHONECTID>e. 



Systematic : There are only two species known : — 



(1) Rhopalura Giardii Metschn. (Bh. ophiocomce Giard., Into- 

 shia gigas Giard.), from Ämphiura sqtiamata ; 



(2) Rhopalura Intoshii Metsckn. (probably synonymous with 

 Intoshia lÄnei and LeptoplancB Giard.), from Nemertes lacteus. 



The Orthonectidfe, which live parasitically in Turhellarians, 

 Nemerteans, and Ophiuraus, 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 A). 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 Jülin) 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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