CRA YFISH. 



273 



- ''.:- 



CO- 



\~d.r 



iluid and small " otoliths, : ' which appear to be foreign 

 particles. This " ear " seems to be an equilibrating organ, 

 connected with directing the animal's movements. In 

 some other Crustaceans the audi- 

 tory hairs are lodged in an open 

 depression ; this has become an 

 open sac in the crayfish, a closed 

 bag in the crab. Small hairs on 

 the upper lip of the mouth have 

 been said to have a tasting func- 

 tion. 



The stalked eyes, which used to 

 be regarded as appendages, arise 

 in development from what are 

 called " procephalic lobes " on the 

 head. They are compound eyes, 

 that is, they consist of a multitude 

 of elements, each of which is 

 structurally complete in itself. On 

 the outside there is a cuticular 

 cornea, divided into square facets, 

 one for each of the optic elements ; 

 beneath this lie, as in other parts 

 of the body, the nucleated epi- 

 dermal cells. Then follows a focus- 

 sing layer, consisting of many 

 crystalline cones. Each crystalline 

 cone is composed of four crystalline 

 cells, which taper internally, and 

 externally secrete a firm crystalline 

 body. The bases of the crystal- 

 line cones are surrounded by the 

 retinula cells. Each retinula con- 

 sists of five elongated cells arranged 

 about a central axis. Distally, 

 this axis is formed by the crystal- 

 line cone, proximally by a little 

 rod or rhabdom. The rhabdom consists of four little 

 red rods closely apposed together, and connected by a 

 nerve-fibre with the optic ganglion, which lies at the end of 

 the optic nerve. The proximal ends of the retinal cells are 

 18 



FIG. 126. A single eye ele- 

 ment or omraatidium of 

 the lobster. After G. H. 

 Parker. 



c.. Cornea; c./i. , corneal hypu- 

 dermis ; cf>., cap of crystalline 

 cone; co., crystalline cone 

 and body ; d.r., distal retinula 

 elements; />.r., proximal re- 

 tmula elements; 1\., rhab- 

 dom ; IV., nerve-fibre. 



