260 



METAZOAN PHYLA 



Ommaticiium 

 Corneoi 



■Basement 

 membrane 



294. Eyes and Vision. — The crayfish possesses com-pound eyes. 

 Each eye is hemispherical in form and is covered by a transparent 

 cornea which represents a modified portion of the cuticula (Fig. 166). 

 The cornea is divided into rectangular facets, each one of which is the 

 outer end of a rodlike unit known as an ommatidium. These ommatidia 

 — of which there are approximately 2500 — are radially arranged rods 

 tapering toward the base, which causes their axes to converge toward 

 a common center. From the ommatidia lead nerve fibers which, together, 

 make up the optic nerve. When an ommatidium is observed carefully 

 it is seen to consist of a corneal facet at the outer end (Fig. 167) beneath 

 which are corneagen cells which secreted it. Inward from these are four 

 elongated cells, mtrellae, which form a crystalline cone near their distal 

 ends. The distal and proximal ends of the vitrellae are enveloped by 

 several retinular cells, sensory in function, from which nerve fibers extend 



inward to optic gangha. The rhabdom, 

 a refractive body secreted by the proxi- 

 mal retinular cells, lies in the axis of the 

 ommatidium at its inner end. The cor- 

 neal facet, vitrellae, crystalline cone, and 

 rhabdom are all transparent. The re- 

 tinular cells cover the surface of the 

 ommatidium and contain pigment. In 

 bright hght this pigment is distributed 

 throughout the length of the omma- 

 tidium; in dim light, however, it con- 

 tracts toward the respective ends leaving 

 much of the surface without this dark covering. 



A compound eye sees just as many Httle images as there are omma- 

 tidia, and since these images together make up the whole of the picture 

 received by the animal, the picture has been termed a mosaic image. 

 There is, however, some overlapping of the separate images. The pro- 

 duction of a separate image for each ommatidium results from the fact 

 that each of the ommatidia is long and slender, and since the pigment 

 along the sides absorbs all the rays which it receives, only those rays 

 reach the bottom and stimulate the retinula which are practically in fine 

 with the axis of the ommatidium. In dim hght the withdrawal of pig- 

 ment from the wall of the ommatidium permits all of the rays entering 

 it to be reflected inward, increasing the amount of hght falhng upon 

 the retinula and thus giving a stronger stimulation. This probably 

 does not result in a clear image but enables the animal to distinguish 

 between light and dark. A compound eye has a great disadvantage 

 when compared with such an eye as the vertebrate eye in that the animal 

 cannot focus with it, thus hmiting the distance to which vision is possible. 

 It has, however, the great advantage that it more readily perceives move- 



FlG. 



Opf/c nerve 



166. — Diagrammatic representa- 

 tion of the eye of a crayfish. 



