130 HISTORY OF CRUSTACEA. CHAP. XII. 



all Zoese, and what may easily be explained as being trans- 

 ferred back from a later into this stage, the Zoeae of the 

 Crabs, for example, agree with those of Pagurus and 

 Palcemon in no single peculiarity of structure which 

 leads us to suppose a common inheritance. Conse- 

 quently we may apparently assume, without hesitation, 

 that when the Brachyura and Macrura separated, the 

 primitive ancestors of each of these groups passed 

 through a more complete metamorphosis, and that the 

 transition to the present mode of development belongs 

 to a later period. With regard to the Brachyura, it 

 may be added that in them this transition occurred 

 only a little later and indeed before the existing families 

 separated. The arrangement of the processes of the 

 carapace, and, still more, the similar number of the 

 caudal setae in the most different Zoe'aB of Crabs (figs. 

 19-23) prove this. Such an accordance in the number 

 of organs apparently so unimportant is only explicable 

 by common inheritance. We may predict with cer- 

 tainty that amongst the Brachyura no species will 

 occur which, like Peneus, still produces Nauplius- 

 brood. 3 



As we have already seen, Mysis and the Isopoda 

 depart from all other Crustacea very remarkably by 

 the fact that their embryos are curved upwards, instead 



3 I must not omit remarking that what has been said as to the 

 development of the Crabs applies essentially only to the groups 

 Cyclometopa, Catometopa and Oxyrhyncha, placed together by Alph. 

 Milne-Edwards as " Eustomes." Among the Oxystomata> as also among 

 the " Anomura apterura," Edw., which approach so nearly to the Crabs, 

 I am unacquainted with the earliest young states of any of the species. 



