Eyes of Molluscs and Arthropods. 719 



in a single circle around each colorless or nervous one ; then, since the 

 nerve fibrillae are distributed over the walls of the colorless cells, they 

 would come in contact at all points with the pigment ofthesurrounding 

 cells. This might possibly account for the first appearance.of the om- 

 matidia: buti am inclined to regard the subsequent, progressive develop- 

 ment of the retia terminalia overying the colorless cells, as being due, as 

 already remarked, to the appearance of the refractive granules, in cou- 

 sequence of which the overlying nerve fibrillae , or the retinidium of 

 the retinophora, became doubly irritated, by the incident rays, as vrell 

 as by the same when reflected by the refractive plates or globules 

 beneath. The rods of the retinophorae owe their predomiuaut devel- 

 opmeut not only to the underlying reflectors, but also to the presence 

 of the axial nerve, which reached its position by the union of two 

 adjacent cells, and the disappearance, partial (Insects, Crustacea etc.) 

 or complete [Pecten, Cephalopods, etc.), of the apposed cell vralls. This 

 axial nerve permits the most economic arrangement in e tage s 

 of radiating fibrillae, which are always at right angles to the 

 direction of the rays of light. By the concentration of the latter 

 at varying levels, perfectly distinct impressions would be produced, a 

 result that could not be obtained by simple and vertical axial fibres. 



The retinophorae, without exception, seem to be double in the 

 Mollusca , and probably in the Worms as well. In the Coelenterates, 

 they appear to be single, but it is not improbable that a closer examin- 

 ation would in some cases show them to be double also. That the 

 retinophorae are double in the Vertebrates we have the very best rea- 

 son to believe , owing to the presence of the axial nerve fibre and the 

 two nuclei. 



A moments consideration will show that »rudimentary« eyes are 

 not the only dynamophagous organs of the lower animals. The numer- 

 ous otolithie sacs found in certain Coelenterates have been regarded 

 as rudimentary hearing organs ; but I think it would puzzle any one to 

 explain why it is necessary for a Medusa to hear. It is absurd to sup- 

 pose that these organs enable them to escape their enemies better , or 

 to obtain more food. If it is difficult to ascribe any adequate function 

 to these organs, which have, even in the Coelenterates, reached a high 

 stage of development , how much more difficult is it to discover any 

 sufficient explanatiou of how , and by what agency they reached their 

 present development, or how they were first brought into existence. 

 For although an organ for the transmission of» sound vibrations into 

 corresponding nerve sensations is very useful to the higher animals, 



