342 THE CRAYFISH LESS. 



divided by delicate lines into square areas or facets, and 

 constitutes the cornea. Beneath each facet of the cornea is 

 an apparatus called an ommatideum, consisting of an outer 

 segment or vitreous body having a refractive function, and 

 an inner segment or retinula forming the actual visual 

 portion of the apparatus. The ommatidia are optically 

 separated from one another by black pigment, so that each 

 is a distinct organ of sight, and the entire eye is called a 

 compound eye. 



The antennules contain two sensory organs, to which are 

 usually assigned the functions of smell and hearing respec- 

 tively. The " olfactory" organ is constituted by a number 

 of delicate olfactory setce, borne on the external flagellum and 

 supplied by the antennulary nerve. The "auditory " organ 

 or statocyst is a sac formed by invagination of the dorsal 

 surface of the proximal segment, and is in free communi- 

 cation with the surrounding water by a small aperture. The 

 chitinous lining of the sac is produced into delicate feathered 

 auditory seta, 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 or ear-stones 

 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 setae on 

 the general surface of the body have a definite nerve-supply, 

 and are probably tactile organs. 



The crayfish is dioecious, and presents a very obvious 

 sexual dimorphism or structural difference between male 

 and female, apart from the actual organs of reproduction. 

 The abdomen of the female is much broader than that of 

 the male : the first and second pleopods of the male are 

 modified into tubular or rather spout-like copulatory organs ; 

 and the reproductive aperture is situated in the male on the 



