Eyes of Molluscs and Arthropods. GG9 



otherwise exposed soft parts. T li c o m m a t e a 1 1 a y c r , a 1 1 h o u g li d i - 

 videdwitli great distinetuess iiito several zones, consists 

 (excludiug nerve fibres) iu the strongest sense of the word 

 of a single layer, since each cell extends from the basal 

 membrane to the corneal eells. The great thickness of the om- 

 mateal layer is probably due to the additioual strength and firmness 

 thus obtaiued. The bacilli may be regarded as Supports for the 

 pigment cells, and the style, as a similar modification of the retiuo- 

 phorae, whose bacilli have fused to form a lioUow tube for the protec- 

 tion of the axial nerve. 



If we were asked to arrange a fiat layer of cells in the most 

 economic manner, of course with certain limitations, and without 

 changing the size of the outer ends of the cells , we should have to 

 arrange them in a hemispherical mass . the inner ends being reduced 

 in size to fibres. The problem has been solved in the same manner 

 by the Arthropod eye. The greater the number of ommatidia, 

 the greater is the curvature of the surface and the depth of 

 the layer. 



It cannot be saidthatGRENACHER, or any of bis predecessors, per- 

 haps with the exeption of Schultze who has represente d the fibrous 

 markings ou the style and calyx , have succeeded in demonstrating 

 anything like nerve endings in the Arthropod eye. To assume, having 

 seen nerve bundles penetrate the basal membrane, that they terminate 

 in this or that place, is a pure supposition and of absolutely no value, 

 until oue at least is able to draw some reasonable comparison between 

 the presumable nerve end cells and those in the retina of some other 

 group, where it is knowu what the sensitive cells really are. That both 

 these criteria have been wanting in the Arthropods is well known. My 

 observations h-ave shown that it is only in the most simple forms of 

 ommatidia that the retinulae produce rods. In all other cases they are 

 the product of the colorless cells or retinophorae. In Arthropods it 

 Ì3 the same; the colorless cells with their various parts are 

 the essential Clements, the pigment cells, or retinulae of 

 Grenacher, having nothing to do with the formation of the 

 so- call ed rhabdom. One might discuss the subject, and offer ob- 

 jectious to Grenacher's theory, indefinitely, and never come to any 

 more decisive conclusion. Grenacher's whole work hinges upon 

 the supposition that the rhabdoms are secreted by the retinulae which, 

 according to him, form a layer of cells, distinct from those of the crys- 

 talline cones. Ibelieve we have proved that the inward con- 



