188 FLORENCE BUCHANAN. 



nephridia) were present in the posterior twenty-two segments, 

 i. e. from the fourteenth to thirty-fifth inclusive. 



Nervous System. — There is a supra-oesophageal ganglion 

 nearly filling the prostomium (fig. 2,g?ig.), and probably sup- 

 plying the much-thickened epidermis of the anterior region of 

 the prostomium, the eyes, and the anterior pair of tentacles.' 

 From this a commissure goes down on either side to join the ven- 

 tral nerve-chain, which runs throughout the whole length of 

 the body as a double cord in the much-thickened epidermis of the 

 ventral surface (figs. 5, 6, 7, and 8, n. c). The two cords are 

 distinct from one another, although very close together. There 

 are no ganglionic swellings on them. Very minute giant-fibres 

 C neural canals," "fibres tubulaires," '^neurochords") may 

 be made out by careful staining in each cord on its dorsal and 

 inner side. In sections stained with hsematoxylin each 

 appeared as a hollow tube, containing a shrunken homoge- 

 neous mass inside (figs. 5, 6, and 7). In other sections, stained 

 with borax-carmine (fig. 8), the giant-fibres were more difficult 

 to distinguish from the rest of the nerve-cord, the homogeneous 

 mass not having shrunk away from its sheath. A similar posi- 

 tion for these structures has been noted in the anterior region 

 of Nerine foliosa, Sars, and in Scolecolepis vulgaris, 

 Johnst., amongst the Spionidae. In Prionospio there are also 

 two neural canals, but these are inferior in position.^ In forms 

 belonging to other families the same position of the giant-fibre 

 with regard to the nerve-cord is found, e. g.^ Arenicola (Telethu- 

 sidte), Trophonia (Chlorhsemidse), Sabellaria (Hermellidae). 



Affinities. — Hekaterobranchus I take to belong to the 

 family Spionidee on account of (1) the single pair of tentacles 

 containing a single blindly-ending vessel; (2) the branchia3, 

 each containing an afferent and efferent vessel not connected 

 with one another by capillaries ; (o) the very superficial posi- 



' Jacobi, «Polydorcu d. Kieler Bucht,' 1S83, p. 23. 



- M'liitosli, " Ou the Structure of the Body-wall iu the Spionidti;," ' Proc. 

 Hoy. Soc. Edin.,' vol. ix, pp. 123—129. 



^ See Cuuuiusham, " Some Points ia the Auatoniy of the Polychseta," 

 this Journal, vol. xxviii. 



