172 



COMPARATIVE ANATOMY 



CHAP. 



two-fold origin, however, always remains evident, its equatorial plane 

 being traversed by the double lamella of the corpus epitheliale. 



A new circular fold grows over the eye, forming a fresh cavity 

 over it ; this is the secondary cornea of the dibranchiate eye, which 

 must not be confounded with the primary cornea of the optic vesicle 

 here represented by the corpus epitheliale. In most forms the circular 

 fold (cornea) does not altogether close over the eye ; an aperture 

 remains through which the water can enter the anterior chamber of 



H 



FIG. 148. Section of the eye of Sepia officinalis, somewhat diagrammatic (after Hensen). 

 1-8, As in Fig. 147 ; 1+3, corpus epitheliale ; 9, anterior chamber of the eye opening outward at 

 10; 11, cartilaginous capsule; 12, ganglion opticum= retinal ganglion; 13, nervus opticus ; -2c., 

 pigment layer of the retina. 



the eye. In some animals, however, the secondary cornea closes com- 

 pletely. 



We thus obtain, ontogenetically, some idea of the general structure 

 of the dibranchiate eye. A few details of the structure of the adult 

 eye are given below (Figs. 148 and 149). 



1. The retina (Fig. 149) consists of two kinds of cells (1) pig- 

 mented visual or rod cells, and (2) limiting cells. Since the nuclei 

 of the visual cells form, with relation to the centre of the vesicle, an 

 outer, and the nuclei of the limiting cells an inner layer, and since, 

 between these two layers, a limiting membrane traverses the inter- 

 stices between the retinal cells, the retina appears to be laminated, 

 whereas it in reality consists of one layer of cells. The rods of the 



