ARTHROPODA. 103 



The visceral nerves are concerned with the regulation of the 

 movements of the alimentary canal. As small ganglia are devel- 

 oped upon their course in front, they acquire a certain inde- 

 pendence. 



10. The different sense organs have a certain community in 

 structure. They all consist of elongated " end-organs," belonging 

 to the ectoderm, and connected directly or indirectly with the 

 cerebral ganglion by sensory nerve-fibres. 



(1) Tactile Organs. The numerous setae scattered over the 

 body, and the slender ectodermic processes with which they are 

 connected, serve as end-organs of touch. The antennae and, to a 

 less extent, the antennules, are of great use to the animal in 

 informing it as to the position of external objects, and well 

 deserve the name of " feelers." 



(2) Olfactory Organs. Each joint of the exopodite of the 

 antennule bears below two tufts of flattened, spatula-shaped setae, 

 which probably serve as organs of smell, and are denominated 

 olfactory setce. 



(3) Auditory Organs. The organs of hearing are two small 

 auditory, sacs, lodged one in the basal joint of each antennule, and 

 opening on its upper surface by a three-cornered aperture fringed 

 with setae (Fig. 27, A, au). Each is somewhat pear-shaped, with 

 a backwardly directed narrow end, into which an auditory nerve 

 passes. The floor and posterior wall of the sac bear numerous 

 auditory setw, which project into a gelatinous mass, in which are 

 imbedded numerous foreign particles (sand, &c.), the otoliths, 

 introduced from the exterior. The auditory nerve branches in 

 the wall of the sac, one of its fibres becoming continuous with the 

 granular axis of each auditory seta. 



(4) Visual Organs (Fig. 30). The crayfish possesses a pair of 

 eyes, each of which is placed on a two-jointed stalk traversed by 

 an optic nerve, that dilates at its extremity into a rounded optic 

 ganglion. On the end of the stalk is an oval area, the cornea, 

 where the cuticle is transparent and uncalcified. Owing to under- 

 lying dark pigment it appears black. The corneal surface is 

 divided into a large number of small square facets, each of which 

 corresponds to one of the elements which build up the deeper 

 part of this so-called " compound " eye. These visual elements 

 (oinmatidia, visual pyramids) are the slender, four-sided bodies 

 whose bases are applied to the inner sides of the corneal facets, 



