160 



LIMULUS. 



each of which forms a special lens. Underneath each 

 of these secondary lenses is a group of large, elongated 

 pigmented cells, arranged round a central space, and 

 the lens with their outer ends, while the 



touching 



Fig. 109. — Diagram of a vertical section through a portion of the lateral eye of Limulus 

 polyphemus, showing some of the conical lenses, and corresponding retinulse (after 

 Lankester and Bourne), a, Cuticle ; hb, cuticular lens ; cc, hypoderm ; Rn, 

 retinula ; m, nerves. 



inner ones are continued into the optic nerve. These 

 nerve-end cells form the " retinula," while their sides, 

 which face one another, are thickened, and coalesce 

 into a rod, the rhabdom, which is hollow at the end 

 nearest the lens, but solid towards the nerve. The 

 central eye is very different. It possesses a single 

 lens, like that of an ordinary ocellus, underneath 

 which is a layer of cells not differiDg much in appear- 

 ance from those of the hypoderm, and below which 

 again is another layer of large nerve-cells, which, how- 

 ever, are so irregular as to suggest the idea that the 

 central eye of the king crab may have partially lost its 

 function. The king crab, then, so remarkable in other 

 ways, is also very interesting in reference to the peculiar 



