101 



has the appearance of being formed of hvo 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, ScL). 



Retina. — The Hetina 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 

 tapetum (fig. 29, Arg. and T.). 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. In most figures a space is shown 

 between the retina and the argentea. This 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. The retina can 

 be divided into three main layers, viz. : — 



(a) An outer layer lying against the septum, and 



