Rev. A. M. Norman on Recent Eryontidse. 179 



the female , sometimes in general contour , but more especially in 

 the structure of the limbs. 



c. Different limbs will be thus sexually modified in different 

 groups or in different genera ; and these modifications become 

 valuable for the classification of families and genera, while the 

 exact mode of the modification of the particular limb is of 

 much use as specific character. 



d. In the case of dimorphous males, the males most closely 

 resembling the females are either immature or, in some in- 

 stances perhaps, where the males are as large as the adult 

 females, sterile. The second form of male, which is widely 



divergent from its female, is the fully developed sexual indi- 

 vidual ; and the alterations of structure are exhibited sometimes 

 in a very different form of carapace, but always in specialized de- 

 velopment of certain appendages, which may be either antennae, 

 mouth-organs, gnathopods, pereiopods, pleopods, or uropods, 

 which become indirectly subservient to the discharge of the 

 function of the impregnation of the female. It is often only 

 at the final moult of the male that these peculiar specialized 

 characters of sex are assumed. 



e. From the time of the assumption of the complete sexual 

 character, in some [rare) cases the discharge of the sexual 

 office would seem to be the only thing remaining for the life of 

 the individual, the function of alimentation in part or even 

 wholly ceasing. This extreme case, however, has, I believe, 

 as yet been observed only among the order Isopoda, where we 

 find instances in Anceus (Hesse and others), Tanais (F. Miiller 

 and A. M. N.), Apseudes (Gr. 0. Sars, in Aspeudes ano- 

 malies). 



Dimorphous males are known to us in genera of Macrura, 

 Cumacea, Isopoda, Amphipoda, Copepoda, Ostracoda, and 

 perhaps Cirripedia ( u complemental male," Darwin). 



One instance may now be referred to, because it has refe- 

 rence to that group of genera, the Astacidse, to which it was 

 stated, in my former notes, that I agree with Heller in con- 

 sidering Polycheles to be perhaps most nearly related. 



The freshwater crayfish, Astacus, of Europe has its closely 

 allied counterpart in North America in the genus Cambarus. 

 In this genus, though not as far as is known in Astacus, two 

 forms of the male occur. 



The following passage from Hagen will speak for itself : — 

 " The discovery that every species of Cambarus possesses 

 two different forms of males was made by Prof. L. Agassiz, 

 and kindly communicated to me. 



" The existence of a second form of the male, if it were no 



