THE EVIDENCE OF THE ORGANS OF VISION 85 
the long cylindrical cells. We see also smaller nuclei in among 
these larger ones, which look like nuclei of intrusive connective 
tissue, or may perhaps form a distinct layer of cells, situated between 
the cells of the anterior wall and the terminations of the nerve 
end-cells already referred to. 
These elongated cells are in exactly the same position and present 
the same appearance as the cells of the corneagen layer of any median 
eye. Like the latter they are 
free from pigment and never 
show with osmic staining any 
sign of the presence of trans- 
lucent rhabdite-like bodies, 
such as are seen in the termi- 
nation of the retinal cells, and 
like the latter their nuclei are 
at the base. The resemblance 
between this layer and the 
corneagen cells of any median 
eye is absolute. Between it 
and the terminations of the 
tetinal cells there exists some 
ill-defined material certainly 
containing cells which may 
well correspond to Patten’s 
pre-retinal layer of cells. 
Retina, corneagen, nerve, 
optic ganglion, all are there, all 
in their right position, all of 
the right structure, what more 
is needed to complete the 
picture ? 
In order to complete the dioptric apparatus a lens is necessary, 
Where, then, is the lens in these pinealeyes? In all the arachnid eyes, 
whether median or lateral, the lens is a single corneal lens composed 
of the external cuticle, which is thickened over the corneagen cells. 
This thickened cuticle is composed of chitin, and is not cellular, 
but is dead material formed out of the living underlying corneagen 
cells. Such a lens is in marked contrast to the lens of the lateral 
vertebrate eye, which is formed by living cells themselves. This 
— as 
Fic. 37.—Pingau Eyr or AMMOC@TES, 
WITH ITS Ganglion Habenule. 
