PHYLUM ARTHROPODA 



227 



cuticle covering the distal end of the 

 parent, 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 ommatideum, optically separ- 

 ated from its neighbours by black 

 pigment. 



Each of the antennules contains 

 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 

 communication with the surrounding 

 water by a small aperture. 



The Crayfish is dioecious, and 

 presents a very obvious sexual dimor- 

 phism. The abdomen of the female 

 is much broader than that of the 

 male ; the first and second pleopods 

 of the male are modified into tubu- 

 lar or rather spout-like copulatory 

 organs (Fig. 115, p); and the re- 

 productive 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. 121, B, t, u^) lies 



eye-stalk is trans- 



Er* 



FIG. 120. Nervous system 

 of Astacus fluviatilis. 

 bf. sub-cesophageal gang- 

 lion ; eg. commissural 

 ganglion ; g. brain ; j. vis- 

 ceral nerve ; sc. oesopha- 

 geal connective ; y. post- 

 oesophageal commissure ; 

 IV-VIII, thoracic gang- 

 lia; 1-6, abdominal gang- 

 lia. (From Lang's Com- 

 parative Anatomy, after 

 Vogt and Yung.) 



