SHA-FKISHBERIES LABORATORY. Ao 
has the appearance of being formed of two membranes 
placed together, and which are here slightly separated. 
the nerve passes intact over the septum to the middle, 
where its several fibrils penetrate the membrane, 
diverging at the same time in all directions to make their 
connections with the outer layer of retinal cells. The 
peripheral edge of the septum appears to be a direct 
continuation of the sclerotica (fig. 29, Se/.). 
Retina.—The Retina and its two underlying layers 
have received, at different times, varying names. I have 
adopted in the main those of Hesse, with some alterations ; 
the chief synonyms will be referred to in the text. In 
sections the retina appears as a thick band lining the 
inner wall of the optic vesicle, but separated from it 
by two other concentric layers—the argentea and the 
fapetum (fie. 29, Arg. and 7.). The retina further 
resembles in a remarkable way that of the vertebrate eye, 
in the fact that the light rays must pass through it to the 
argentea, and then the nervous reactions aroused pass 
back again through the various elements to the optic 
nerves which innervate the retina on its distal surface. 
This arrangement is only approached outside the verte- 
brates in the Planaria (Hesse, 32), and there the structure 
is much simpler. Im most figures a space is shown 
between the retina and the argentea. ‘his appearance, 
however, is due to contraction on fixing, and normally 
retina, argentea and tapetum are in contact. The retina 
has been the source of all the disagreement with regard to 
the structure of the Pecten eye, and I have endeavoured to 
examine the various views held and compare them with 
the appearances of my own preparations; but a complete 
elucidation has not yet been arrived at. he retina can 
be divided into three main layers, viz. : — 
(a) An outer layer lying against the septum, and 
