84 



(fig. 78, Vitr.). The white body and optic ganglion arc 

 also contained in a thin-walled sac, which encloses a venous 

 blood sinus — this blood, therefore, bathing these organs. 



The optic vesicle is covered posteriorly by a 

 tough semi-cartilaginous sclerotic (fig. 78, ScL). This is 

 iridescent, and reaches to the external border of the ciliary 

 body, which supports the lens. Posteriorly it is pierced 

 by numerous fine holes, which allow the passage of optic 

 nerve strands to the retina. The internal wall of the iris 

 is very darkly pigmented, and raised anteriorly into a 

 circular ridge. While arterial blood is supplied to the 

 eye by two arteries which are given off by the anterior 

 aorta soon after bifurcation, the venous blood is drained 

 off into the above-mentioned venous sinus, and thence 

 passes by a vein through the ventral wall of the skull to 

 the anterior vena cava. As in all Cephalopods, the eye 

 may be adapted for near and distant vision by variation 

 of the distance between the lens and the retina. 



The Retina is the most complex part of the eye 

 of Eledone. Anteriorly it is continuous with the ciliary 

 body and internal layer of the cornea, and it forms the 

 posterior wall of the optic vesicle. It is very deeply 

 pigmented with a dark brown retinal pigment. Grenadier 

 and Hesse made very careful examinations of the retina 

 of E. mosehata. The structure (as given by Hesse) of this 

 retina appears to agree with that of E. cirrosa, except in 

 one point, which will be mentioned below. The retina 

 consists of a single layer of cells (see PI. X, fig. 86), 

 which are of two kinds, retinal and limiting cells. The 

 former are long slender cells, alternating in position with 

 the rather shorter limiting cells (PI. X, fig. 86, Ret. C . : 

 Liin.C). There are three regions in the retinal cell: — 

 (1) The innermost and longest region, where the rods are 

 situated (fig. 86, Rod.) ; (2) the central shortest region, 



