vii SENSORY ORGANS 381 



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 num- 

 ber of delicate spatulate setae, borne on the external 



op.ff'V 



FIG. 97. A, Longitudinal section of eyestalk ; B, a single ommatideum ; a, vitreous 

 body ; b, retinula ; en, cornea, continuous with cnt, cuticle of eyestalk ; 

 m, muscles ; <?;/;, ommatidea ; op.gn, optic ganglion ; op, n, optic nerve. (After 

 Howes.) 



flagellum, 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 seg- 

 ment, 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 



