Chap. XII.] THE CRAYFISH. 249 



nerve (anterior visceral), which breaks up into branches which 

 innervate the stomach. From the last abdominal ganglion a nerve 

 (posterior visceral) passes forwards to innervate the hind-gut. 



It is probable that the original, i.e. not merely responsive, 

 activities of the crayfish depend on the integrity of the supra- 

 cesophageal ganglion, which also probably possesses an inhibitory 

 and co-ordinating power over the lower centres. The sub-ceso- 

 phageal ganglion, however, may have some co-ordinating power 

 over lower centres. If both these higher centres be removed the 

 crayfish, as an organism, is probably dead ; but the individual 

 life and activity of the legs, chelae, and swimmerets, continues 

 vigorously but quite aimlessly. 



Special Senses. (1.) Touch. Scattered over the surface of the 

 body, or grouped in clusters at certain points, are a number of 

 setce (78, A. and B.) which are probably organs of touch. Each 

 is a hollow diverticulum of the body-wall and exoskeleton. 

 During life they are filled with delicate prolongations of the 

 epithelial layer of the integument. 



(2.) Smell. From the seventh or eighth to the last joint but 

 one of the exopodite of the antennule, there are, on the under side 

 of each joint, two bundles of curiously-modified setae (78, A. and 

 C.). These are regarded as olfactory. 



(3.) Taste. No special organs of taste have been discovered. 

 Such organs should be sought for on the metastoma, or perhaps 

 on the maxillary appendages. It is scarcely probable that this 

 sense is altogether wanting. 



(4.) Hearing. The auditory sacs are lodged in the basal joint 

 of the antennule. The external aperture is in the concave upper 

 face, its outer lip being guarded by closely-set setae. Within 

 the sac are delicate feathered auditory setae (78, D.) implanted 

 on either side of a ridge. Their tips are often imbedded in a 

 gelatinous mass in which there are minute particles of sand (s.) 



(5.) Sight. The eye has a convex cornea (78, E., co.) continuous 

 with the exoskeleton, and marked out into a number of facets, 

 which are square, except near the edges of the eye where they 

 become polygonal and eventually hexagonal. This is readily 



