II.] THE CRAYFISH AND LOBSTER. 191 



/?. The anus; a longitudinal slit beneath the 

 telson. It is bounded by two flap-like anal 

 valves. 



y. The paired genital apertures : in the male, on the 

 basal joints of the last pair of thoracic appendages: 

 in the female, on those of the antepenultimate 

 pair. Examine the bases of the adjacent thoracic 

 appendages, for possible traces of supernumerary- 

 genital orifices (cf. p. 186). 



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 9 exceeds, at its widest 

 part, that of the cephalothorax ; it is the reverse 

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

 pendages are especially modified for purposes 

 accessory to reproduction, in the t ; in the 9 the 

 second pair are normal, the first are either very 

 small or absent. 



8. The excretory apertures; each on the summit of a 

 tubercle, borne upon the under surface of the 

 basal joint of the antenna. 



e. 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 portion); it is for the most part 

 calcified, but it remains soft and flexible where freedom 

 of motion is required. 



