178 Rev. A. M. Norman on Recent Eryontidae. 



more remarkable than those upon tvhich specific and even 

 generic characters are usually based. 



This is now so fully proved that it may be laid down 

 plainly as an axiom. As instances of extraordinary diver- 

 gence in form and structure between the sexes it will suffice 

 to name the order Cumacea, the Isopoda (Bopyrus, Phryxus, 

 G-yge, lone, Cryptotheria, Anceus, Tanais, Apseudes, Ncesa, 

 &c), the parasitic Copepoda {Chondr acanthus, Brachiella, 

 Lernanthropus, Lernodopoda, Anchorella, Strabax, Medisecaste, 

 Diocus, and their allies), the Cirripedia (Ibla and Scalpellum). 

 It would be a long task to attempt to enumerate the multitude 

 of instances in which the different sexes of Crustacea have 

 been described as different species, very often belonging to 

 different genera. 



These are the grounds on which the conclusion was arrived 

 at that the chelate or achelate character of the last legs of 

 those recent Eryontida? which I had seen and of those which 

 had been described was indicative of the sex to which the 

 individual belonged and had no further importance. 



Have Mr. Spence Bate's papers changed my views ? In 

 his first paper, I repeat, there was not a statement to make 

 me modify it in the slightest degree, inasmuch as the sex of 

 the specimens described was in no case given, while forms 

 were described which apparently closely corresponded in all 

 particulars save in those which he regarded as generic, but 

 which I took to be sexual. I therefore considered that his 

 paper, so far from weakening, gave strength to the opinion 

 which I entertained. 



In his second paper, however, in which he was good enough 

 to reply to my inquiries, Mr. Bate has given us the sexes of 

 the species he has examined ; and a first glance at the list 

 might seem to argue that he had completely proved his pointy 

 and that the divergences were generic and not sexual. 



To those, however, who know much of the life-history of 

 the Crustacea the statement will not seem necessarily to be 

 quite conclusive. 



There are certain rules which should be borne in mind in 

 the study of the Crustacea which seem now to be fully esta- 

 blished, and which I will proceed to lay down. 



a. Among the Crustacea, for every female we must be pre- 

 pared to find, and shall often find, two forms of males. 



b. One of these males may be expected to resemble the females 

 more or less closely, and will perhaps only be distinguishable by 

 examination of the sexual organs themselves; the other form 

 will exhibit varied and very marked points of divergence from 



