CHAP, ix.] 



OF THE SENSE OF SIGHT. 



423 



of the arthropods is, schematically, composed of a crystalline 

 bacillum, wrapped in a pigmentary mass, and covered exteriorly 

 with a transparent chitinous lamina. This lamina connects with 

 the general tegnmentary envelopment, and sometimes bombates 

 within so as to form a plano-convex lens. Sometimes this 



FIG. 79. 



Schematic horizontal section of the eye of a sepia, ; KK, cephalic cartilage ; C, cornea ; L, 

 crystalline humour ; ct, ciliary body ; R, internal layer of the retina ; p, pigmentary layer ; 

 o, optical nerve ; go, ganglion of the optical nerve ; fc', cartilage of the globe of the eye ; 

 ik, cartilage of the iris ; w, white body ; ae, external argentine layer (after Hensen). 



chitinous crystalline substance bombates also outwards, and then 

 it is bi-convex. This simple eye, with a simple bacillum, exists 

 in the inferior crustaceans. 



In the arachnida many bacilla group themselves, cling 

 together ; they are habitually covered with one and the same 



