4 THE VOYAGE OF H.M.S. CHALLENGER. 



velum by a narrower cylindrical portion (_/), which only lay in part within the plane 

 of the sections. This connecting portion, changing its direction, could be followed 

 across the muscular mass throughout the whole of the adjacent portions which I felt 

 justified in removing for the purpose of making sections. 



The central mass and its prolongation are surrounded by a layer which exhibits 

 numerous nuclei, is especially delicate in front of the central mass, and forms a sheath 

 which completely separates the latter from the surrounding tissues, that is, from the 

 muscular mass (g) of the cephalic velum. 



How, then, are we to interpret these different parts of the eye of GuiviUea, espe 

 cially the homogeneous sub-epithelial layer and the central mass with its prolongation ? 

 for the epithelium and the reticulated connective tissue which separates the central 

 mass from the more superficial tissues do not at first sight present anything special 

 or inexplicable. 



1. The sub-epithelial homogeneous layer. This is the sub-epithelial connective 

 tissue, which, like the epithelial portion which covers the eye in Molluscs, has become 

 transparent, and forms the layer to which Hensen gave the name "pellucida." It 

 surrounds the whole ocular mass in front and at the sides, and forms the deep portion 

 of the cornea. 



2. The central mass and its prolongation. There can be no doubt as to the import 

 of the prolongation which is given off from the central portion of the eye, and is con- 

 tinued with a constant diameter into the depths of the cephalic mass ; it is the optic 

 nerve. 



The central mass, continuous with this last, is the retina, whose anterior margins 

 have joined after the complete disappearance of the refractive parts (the crystalline and 

 the vitreous body). This is a modification somewhat similar to that which has taken 

 place in Proteus} 



Finall}^, the common envelope (c/) of the central mass and its prolongation is 

 simply the sheath of the optic nerve continued round the ball of the eye to form a 

 " choroid." 



The non-functionality of the organ of sight has quite changed the structure of these 

 parts of the eye of GuiviUea. The cellular elements have completely disappeared from 

 the mass of the retina, and the transformation of the latter has extended to the adjoin- 

 ing portion of the optic nerve, in which the fibrillar structure no longer exists. The 

 percipient elements then have entirely disappeared, both from the retina and from the 

 terminal portion of the optic nerve ; the connective substance between these elements 

 alone persists, and forms the entire homogeneous and structureless mass of that part of 

 the organ of sight in GuiviUea. 



' Semper, Die natUrliche Esistenzledingungeii iler Tliiere, Bd. i. fig. 20, p. 9C ; transl. Animal Life, Intemat. 

 Sci. Series, p. 78 



