STRUCTURE OF CRAYFISH 



327 



Sensory system. — A skin clothed with chitin is not 

 Ukely to be in itself very sensitive, but some of the setae 

 are, and some observers describe a peripheral plexus of 

 nerve-cells beneath the epidermis. The setae are not 

 mere outgrowths of the 

 cuticle, but are con- 

 tinuous with the living 

 epidermis beneath ; and 

 though some are only 

 fringes, both experi- 

 ment 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 a smelling 

 function. 



Other specialised setae 

 have sunk into a sac at 

 the base of the anten- 

 nules, 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 contains a 

 gelatinous fluid and 

 small " otoHths," which 

 appear to be foreign 

 particles. This " ear " 

 seems to be an equili- 

 brating organ, con- 

 cerned with directing the animal's movements. In some 

 other Crustaceans the auditory hairs are lodged in an 

 open depression ; this has become an open sac in the 

 crayfish, a closed bag in the crab. Small setae on the 

 upper lip of the mouth have been said to have a tasting 

 function. 



Fig. 174. — An ommatidium of a 

 compound eye. 



CO., cornea; cgn., corneagen layer; n., nuclei; 

 c.c, crystalinfe cones; O.P., outer pigment; 

 7.P., inner pigment; iJZi., retinule cells ; RH., 

 rhabdom ; N.^ optic nerve. 



