228 MANUAL OF ZOOLOGY sect. 



Sensory organs. — The eyes differ entirely in structure 

 from those of any animal that has been described hitherto. 

 Each is a compound structure, being made up of a large 

 number of distinct elements termed the ommatidca. The 

 chitinous cuticle covering the distal end of the eye-stalk is 

 transparent, divided by delicate lines into square areas or 

 facets, and constitutes the cornea. Each facet of the 

 cornea marks the position of the outer end of an omnia- 

 tideum, optically separated from its neighbours by black 

 pigment. 



The antennules contain two sensory organs, to which are 

 assigned the functions of smell and hearing respectively. 

 The olfactory organ is constituted by a number of extremely 

 delicate olfactory setce, borne on the external flagellum. The 

 auditory organ is a sac formed by invagination of the dorsal 

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

 tion with the surrounding water by a small aperture. 



Reproduction. — The crayfish is dioecious, and presents a 

 very obvious sexual dimorphism. 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 (Fig. 124,9); anc ^ the 

 reproductive aperture is situated in the male on the proximal 

 podomere of the fifth leg, in the female on that of the third. 



The testis (Fig. 130, B, /, it) lies in the thorax, just 

 beneath the floor of the pericardial sinus, and consists of 

 paired anterior lobes (/) and an unpaired posterior lobe («). 

 From each side goes off a convoluted vas deferens (vd), 

 which opens on the proximal segment of the last leg. The 

 sperms are curious non-motile bodies produced into a num- 

 ber of stiff processes ; they are aggregated into vermicelli- 

 like spermatophores by a secretion of the vas deferens. 



The ovary (A, ov, u) is also a three-lobed body, and is 



