STRUCTURE OF THE CRAYFISH. 237 



food-canal, forming a complex visceral or stomato-gastric 

 system. Similarly from the last ganglia of the ventral chain, 

 nerves go to the hind-gut. If the brain be regarded as the 

 fusion of two pairs of ganglia, and so the development 

 suggests, and the sub-oesophageal as composed of six fused 

 pairs, then these, along with the eleven other pairs of the 

 ventral chain, give a total of nineteen nerve-centres, or a pair 

 for each pair of appendages 



Sensory Systetn. — A skin clothed with chitin cannot be 

 very sensitive, but it seems that some of the bristles are. 

 These are not mere outgrowths of the cuticle, but are 

 continuous with the living epidermis beneath, and though 

 some are only fringes, both experiments and histological 

 observation show that some are tactile. 



On the under surface of the outer fork of the antennules, 

 there are special innervated bristles which have been credited 

 with a smelling function. 



Other still more innervated hairs have sunk into a sac at 

 the base of the antennules, and are regarded 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 "otoliths" which seem to be foreign particles. 

 It is probable that this "ear" is somehow connected with 

 directing the animal's movements. In some other Crust- 

 aceans, 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 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. 



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. Then follows a focussing layer, 

 corresponding to the epidermis, consisting of many crystalline 

 cones. Each crystalline cone is composed of four crystalline 

 cells, which taper internally. Internal to each crystalline 



