II.] THE CRAYFISH AND LOBSTER. 19I 



p. The a?tus; a longitudinal slit beneath the 

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

 the adjacent integument {anal valves). 



y. The paired ge?iital 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 .f 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 -^ the 

 second pair are normal, the first are either very 

 small or absent. 



3 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 {appendicidar portion); it is for the most part 

 calcified, but it remains soft and flexible where freedom 

 of motion is required. 



