Eyes of Molluscs and Arthropods. 653 



exceeded by that of tlic nerve fibres themselves , is shown by the Con- 

 stant ' abseuce of granular pigment in all retinophorae , from the sim- 

 plest ißolated ones, to tlie uiost specialized. 



It is not inipossible that the retinophorae originated from simple 

 gland eells , which had the advantage over the pigmented ones in that 

 the rays of light would be reflectcd outwards again, by the colorless 

 and refractive Contents underlying the retiaterminalia; pigment would 

 absorb, not reflect, light. But I am inclined to regard the refractive 

 globules as being secondarily acquired by the colorless cells , since in 

 the Coelenterata they are absent. The remarkable constancy with 

 which similar gland-like cells are associated with the scuse hair papillae 

 of the Mollusca, the sense organs of the lateral line, and organs of 

 taste in Vertebrates, indicates that these gland-like cells have some- 

 thing more than a casual connection with organs of special sense. 



In the invaginated forms, the shape of the eye determines, to a 

 certain degree, the direction of the rays of light, and we should expect 

 to find in the simplest eyes of this type that both retinulae and retino- 

 phorae contribute nearly equal parts to the formation of the retinidia ; 

 and, indeed, such is invariably the case in the simpler forms ofMolluscan 

 eyes. Accordiug as these invaginate eyes become more highly devel- 

 oped, — a process accompanied by the narrowing of the pupil together 

 with the addition of some refractive body, as the lens, — the direction 

 of the rays of light becomes definitely fixed , and the fibrillae of the 

 retinidia assume a more perfect radial arrangement, in overlying planes, 

 around the larger nerves tìbres. 



In the simplest Molluscan eyes, the arrangement of the fibrillae is 

 not of grcat moment so that both retinophorae and retinulae may pro- 

 duce rods; but in the more perfect organs, as those of Cephalopods, 

 Heteropods and Pecten, the retinophorae, owing to the advantageous 

 arrangement of their axial nerves, alone develop rods. 



The visual organs of the Mollusca may be divided, according to the 

 arrangement and modification of the ommatidia, into four types: the 

 diffuse, the invaginate, the faceted, and the pseudo-lenti- 

 culate type, representing the three modifications of light sensitive 

 surfaces, i. e., a retineum, ommateum and a retina. 



The Mollusca is the most interesting group of animals we know 

 of, as far as visual organs are concerned, since it contaius ali types of 



' In Cephalopods, Nautilus and Sepia, there appears tobe an exception, in 

 that the so-called sense cells are somewhat pigmented. 



