Eyes of Molluscs and Arthropods. 593 



a cuticular secretion of the outer layer of the argeutea, and is ho- 

 mologous with thc cutieiüar rods secreted by the retinophorae. 



The argentea is formed by the modification of two cell layers into 

 refractive, laminated membranes, the outer being the most highly diffe- 

 reutiated. Eaeh membrane is composed of minute, Square plates, 

 whose edges are bevelled in such a manner that the outer faces are 

 smaller than the inner, which rest upon the undifferentiated, under sur- 

 face of the membrane by which all the plates are held together. In 

 passing iuwards, the membranes become thinuer, less distinct and re- 

 fractive, while the lamellated structure entirely disappears. 



In seetions, the retina is usually separated by a considerable space 

 from the two. folded layers of the argentea, which, in the living con- 

 dition , is perfectly smooth and concave ; its thicker outer layer, in the 

 adult condition, never contains nuclei, although one or two may occasion- 

 ally be found in the inner , less refractive and less dififerentiated one. 

 The argeutea is thicker in the centre of the eye, whence it gradually 

 diminishes to a thin layer, extending to the periphery of the retina, and 

 terminating at the place where the fibres from the axial brauch of the optic 

 nerve enter the ommateal sac. In the outer membranes of the argentea, 

 the Spaces separating the Squares from each other are not snperimposed, 

 but arranged so as tofall above the middle of the underlying Squares 

 ^Pl. 29, fig. 29) . In the inner membrane, where the Square plates are 

 either extremely thin , or entirely absent , it is impossible to recognize 

 this arrangement. When the argentea is »in situ«, it reflects a soft, 

 silvery light, like that from a highly polished mirror ; the refraction of 

 this light by the crystal-like lens gives rise to the well known, emerald 

 and violet colors first seen and described by Poli, By removal of the 

 lens, these colors disappear and we have instead the silvery light of the 

 argeutea and the colorless light from the lens. The iridescence produ- 

 ced by the lens , or argentea, depends greatly upon the condition of the 

 animai, and the position of the eye as regards the light. The emerald 

 and violet reflections are offen entirely absent. The isolated argentea 

 appears, by reflected light, like plates of polished silver. The trans- 

 mitted light is differently refracted in various parts, producing red, 

 orange, yellow, blue and pnrple lights. The optical properties of the 

 argentea are not easily understood, but it appears that in general, while 

 acting as a perfect reflector for incident rays passing through the lens, 

 it offers no great impediment to the entrance of light into the retina, after 

 passing through the colorless eye-stalk and red tapetum. 



Although the septum , sclerotica and tapetum stain deeply in hae- 



Mittheilniigen a. cl. Zoolog. Station zu Neapel, Bd. VI. 40 



