552 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
(fig. 78, Vetr.). The white body and optic ganglion are 
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, Se.). 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 Hledone. 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. Grenacher 
and Hesse made very careful examinations of the retina 
of /. moschata. ‘The structure (as given by Hesse) of this 
retina appears to agree with that of H. cirrosa, except in 
one point, which will be mentioned below. The retina 
consists of a single layer of cells (see Pl. X, fig. 86), 
which are of two kinds, retenal and limiteng cells. The 
former are long slender cells, alternating in position with 
the rather shorter Lmiting cells (Pl. X, fig. 86, Ret. C.; 
Iim.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, 
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