118 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



The sonorous vibrations transmitted througli the 

 water in which the crayfish lives to the fluid and solid 

 contents of the auditory sac are taken up by the delicate 

 hairs of the ridge, and give rise to molecular changes 

 which traverse the auditory nerves and reach the cerebral 

 ganglia. 



The vibrations of the luminiferous ether are brought 

 to bear upon the free ends of two large bundles of nerve 

 fibres, termed the optic nerves (fig. 25, on), which proceed 

 directly from the brain, by means of a highly complex eye. 

 This is an apparatus, which, in part, sorts out the rays of 

 light into as many very small pencils as there are separate 

 endings of the fibres of the optic nerve, and, in part, 

 serves as the medilim by which the luminous vibrations 

 are converted into molecular nerve changes. 



The free extremity of the eyestalk presents a convex, 

 soft, and transparent surface, limited by an oval contour. 

 The cuticle in this region, which is termed the cornea^ 

 (fig. 28, a), is, in fact, somewhat thinner and less dis- 

 tinctly laminated than in the rest of the eyestalk, and it 

 contains no calcareous matter. But it is directly con- 

 tinuous with the rest of the exoskeleton of the eyestaUi, 

 to which it stands in somewhat the same relation as the 

 soft integument of an articulation does to the adjacent 

 hard parts. 



The cornea is divided into a great number of minute, 

 usually square facets, by faint lines, which cross it from side 



