SENSORY ORGANS 



367 



usually assigned the functions of smell and hearing respec- 

 tively. The olfactory organ is constituted by a number of 

 extremely delicate olfactory seta;, borne on the external 

 flagellum, and supplied by branches of the antennulary 

 nerve. The auditory organ is a sac formed by invagination 

 of the dorsal surface of the proximal segment, and is in 



op.g/v. 



Fig. 90. — h, Longitudinal section of eyestalk ; B, a single ommatideum ; a, vitreous 

 body ; ^, retinula ; cw, cornea, continuous with cnt, cuticle of eyestalk ; 

 11/, muscles ; 0111, ommatidea ; pp. gn, optic ganglion ; fp. «, optic nerve. (After 

 Howes.) 



free communication with the surrounding water by a small 

 aperture. The chitinous lining of the sac is produced into 

 delicate feathered auditory setce, supplied by branches of 

 the antennulary nerve, and in the water which fills the sac 

 are minute sand grains, which take the place of the otoliths 

 (p. 188) found in most auditory organs, but which, instead 

 of being formed by the animal itself, are taken in after each 

 ecdysis when the lining of the sac is shed. Many of the 



