vn 



SENSORY ORGAN'S 



381 



like the rods and cones of the vertebrate eye (p. 185), 

 covered by a layer of nerve-fibres, &c., through which 

 the light must first penetrate. 



Each antennule bears two sensory organs, to which 

 the functions of smell and hearing have been respectively 

 assigned. The " olfactory " organ is constituted by a 

 number of delicate spatulate setae, borne on the external 



C,n, 



ds 



FIG. 97. A longitudinal section of an eyestalk (x 12) ; B, a single ommatideum 

 (X 30). a, vitreous body ; 6, retinula ; en. cornea, continuous with cat. cuticle 

 of eyestalk ; m. muscles ; om, ommatidea ; op. gn, optic ganglion ; op. n, optic 

 nerve. (After Howes.) 



feeler, and supplied by branches of the antennulary 

 nerve. The " auditory " organ, which is better described 

 as an organ of equilibration, or statocyst (p. 315), is a 

 sac formed by invagination of the dorsal surface of the 

 proximal segment, and is in free communication with 

 the surrounding water by a small aperture. The 

 chitinous lining of the sac is produced into delicate, 

 feathered, sensory setae, supplied by branches of the 

 antennulary nerve, and in the water which fills the sac 



