48 Elementary Zoology. 



here. The olfactory organs appear to be represented by certain 

 hairs upon the antennules. Other hairs which cover the body 

 and the appendages seem to be of a tactile nature ; the auditory 

 organ is a sac in the anterior of the basal joint of the anten- 

 nules, with an opening to the exterior, and a lining of delicate 

 auditory hairs, between which float tiny particles — the otoliths. 

 The crayfish is of two sexes : there are males and females. 

 Externally, the two sexes can be readily distinguished. In the 



^^a^^^^ .HEART 



Fig. 25. — Diagrammatic transverse section of Crayfish. (After T. J. Parker.) 

 CCEL, cavity of gonad ; M, muscles ; INT, mesenteron with liver appended. 



female the abdomen is broader and deeper in its excavation 

 below ; this serves to pack away the eggs, which when extruded 

 are attached to the swimmerets. In the two sexes the first two 

 pairs of abdominal appendages are modified in the way that 

 has been already described. In the male the reproductive 

 organs open on to the last pair of thoracic appendages ; in the 

 female on to the last but two. The "gland" itself in both 

 sexes is a Y-shaped mass, white in the male and the immature 

 female, brown in the more mature female. It is a hollow 

 structure, and the cavity communicates with the lumen of the 



