260 



CRUSTACEA. 



not mere outgrowths of the cuticle, but are continuous 

 with the living epidermis beneath, and though some are 

 only fringes, both experiments and 

 histological examination show that 

 others are tactile. 



On the under surface of the outer 

 fork of the antennules, there are special 

 innervated setae which have been 

 credited with a smelling function. 



Other likewise specialised hairs have 

 sunk into a sac at the base of the 

 antennules, and are spoken of as 

 auditory. The sac opens by a bristle- 

 guarded slit on the inner upper corner 

 of the expanded basal joint, and con- 

 tains a gelatinous fluid and small 

 "otoliths" which seem to be foreign 

 particles. This "ear" is somehow 

 connected with directing the animal's 

 movements. In some other Crus- 

 taceans, the auditory hairs are lodged 

 in an open depression ; this has be- 

 come 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 

 function, but imagination is apt to 

 help conclusions as to the precise 

 nature of the sensitiveness of such 

 simple structures. 



p.r- 



R 



N 



FIG. 83. A single 

 eye element or omma- 



The stalked eyes, which used to be tidium of the Lobster - 

 regarded as appendages, arise in de- (After _ G ' H ; PARKER ' } 

 velopment from what are called " pro- 

 cephalic lobes " on the head. They 

 are compound eyes, that is, they con- 

 sist 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. Then follows a focussing layer, 

 corresponding to the epidermis, consisting of many crystalline 



c, Cornea ; c.h, corneal 

 hypodermis ; cp, cap of 

 crystalline cone ; co, crys- 

 talline cone ; d.r, distal 

 retinula elements ; p.r, 

 proximal retinula ele- 

 ments ; R, rhabdome ; 

 N, nerve fibre. 



