EEPOET ON THE NEMERTEA. 117 



Even less remains to be said when we consider the blood-spaces of the Challenger 

 Nemertea, be they the closed longitudinal and the metameric transverse vessels of the 

 Hoplonemertea and of the posterior body-region of the Schizonemertea, or the circum- 

 oesophageal and circumrhynchodaeal lacunae of the latter, or the two spacious lateral 

 longitudinal cavities of Carinina. 



All these points have been dwelt upon at length by Oudemans (XXVIl), and the 

 Challenger material furnishes a general confii-mation of his results. Doubt cannot be any 

 longer entertained that these spaces are all clothed by an epithelium, — at any rate by a 

 special and continuous cellular coating, either applied against the muscular tissue when 

 this surrounds (PI. IV. figs. 5, 6 ; PL XIII. fig. 6 ; PI. XIV. fig. 4), or traverses (PI. 

 VII. fig. 10) the cavities or against the gelatinous tissue, when it is in this that the 

 vessel takes its course (PL X. figs. 1, 8). In the latter case the structure of the vessel 

 is still more complicated, so that, as described by Oudemans, there is constantly a tubular, 

 denser layer of the homogeneous tissue just outside the inner epithelial coating ; and 

 outside this tubular layer, which might be termed the basement layer of the vessel, we 

 find a second layer of generally more spherical ceUs, amongst which a layer of fibres, specially 

 belonging to the blood-vessel, may be seen to make its appearance (PL XV. fig. 1, dv). 



This same description holds good for the longitudinal vessel, as far as it takes its 

 course inside the proboscidian sheath (PL X. fig. 8). The gradual narrowing of the 

 circumcesophageal lacunar space into the two ventral vessels shows the passage of the 

 epithelium of the one into that of the other very clearly. 



The diff'erence in the distance along which the medio-dorsal vessel is enclosed in the 

 proboscidian sheath was mentioned and figured by Oudemans (xxvil) for different 

 species ; my own observations on the Challenger specimens fit into the same general 

 outlines ; a few additional data concerning these points are contained in the systematic 

 description of the species. 



GENERATIVE ORGANS. 



Certain not unimportant additions to our knowledge of the generative organs 

 of the Nemertea are due to the Challenger specimens. Among these facts I wish 

 successively to record : 



{a) The irregular distribution in certain species of very numerous generative sacs 

 enclosed in the gelatinous tissue, and having each its separate external opening, which 

 are consequently neither paired nor metameric. 



{h) The comparatively late period at which the definite external opening is formed, 

 although long before that time the sac is characterised by a pointed projection reaching 

 between the muscular tissue and foreshadowing the definite openings, dehiscence of 

 the body- wall being certainly not the normal way of exit of the generative products. 



