No. 1.] EYES OF MOLLUSCS AND ARTHROPODS. -jj 



able one. The flanged walls of the inner ends of the retinophorse 

 unite at the same level with those of neighboring cells, producing 

 a sharp division between the rods and the inner ends of the cells. 

 This pseudo-membrane was named the ''sieve membrane'' by 

 Carriere. A delicate wall, the terminal membrane, separates 

 the retinophora from its rod ; on the edge of the retina are many 

 slender and rodless retinophorae. 



The rods are cylindrical, and consist of a refractive cap or 

 sheath, surrounding a pyramidal, axial core. The axial nerve 

 is continued through the distal end of the rod, and immediately 

 divides into two branches, one of which unites with a similar 

 branch from a neighboring rod, while the other is bent over and 

 distributed in fibrillae over the surface of the rod. I have 

 called these nerves the axial nerve-loops. That portion of the 

 axial nerve within the rod gives rise to successive etages of 

 radiating fibrillae which unite with the nerve-fibres upon the sur- 

 face of the rod. These cross fibrillce constitute the retinidium, 

 which is composed of fibrillae similar to those seen in the rods 

 of Haliotis, but arranged in a more systematic manner. There 

 are also circular fibrillae arranged around the axial core of each 

 rod, and connecting the radiating fibrillae. 



Above the retinophorae is the outer ganglionic layer, which 

 consists of large ganglionic cells {g. c}'^), terminating at either 

 end in a varying number of fibrous prolongations ; those of the 

 outer end are continued into the ganglionic branch of the optic 

 nerve, while those of the opposite extremity extend along the 

 walls of the retinophorae to the rods, over the surface of which 

 they form a net-work of fibres. This layer contains cells in 

 instructive stages of ganglionic perfection. 



The inner ganglionic layer consists of a single row of minute 

 ganglionic cells {g. c}), which, when seen at all by previous 

 writers, have been mistaken for the nuclei of the retinophorae. 

 Each cell, which is nearly filled by its nucleus, is provided 

 with several fibrous prolongations, one of which is directed out- 

 wards, passing into the ganglionic nerve-branch, while five or 

 six others extend inwards to help form the net-work of fibres on 

 the surface of the rods. Many of the nerve-branches from both 

 ganglionic layers terminate upon the walls of the retinophorae in 

 one of two ways, — either a single fibre impinges directly upon 

 the cell-wall, and there divides into several short fibrillae, con- 



