.] THE CRAYFISH AND LOBSTER. IQI 



/?. The anus; a longitudinal slit beneath the 

 telson. It is bounded by two flap-like folds of 

 the adjacent integument {anal valves). 



y. T\\Q paired genital apertures : in the male, on the 

 basal joints of the last pair of appendages of the 

 thorax: in the female, on those of the ante- 

 penultimate pair. 



The two sexes may be at once distinguished 

 from each other, apart from these apertures, by 

 the following characters. The transverse diameter 



of the abdomen of the ° exceeds, at its widest 

 part, that of the cephalothorax ; it is the reverse 



for the ^. The first two pairs of abdominal ap- 

 pendages are especially modified for purposes 



accessory to reproduction, in the ^', in the -f the 

 second pair are normal, the first are either very 

 small or absent. 



5 The apertures of the excretory organs or green 

 glands; each on the summit of a tubercle, borne 

 upon the under surface of the basal joint of the 

 antenna. 



€. The auditory apertures; on the flattened upper 

 surfaces of the basal joints of the antennules. 



These can be better examined when the ap- 

 pendages on which they are situated have been 

 removed. 



B. The exoskeleton and appendages. 



Note that the exoskeleton forms a continuous invest- 

 ment for the whole body (axial portion) and its ap- 

 pendages {appendicular portiofi) \ it is for the most part 

 calcified, but it remains soft and flexible where freedom 

 of motion is required. 



