108 HISTOEY OF CRUSTACEA. Chap. X. 



On the other hand, what do we know of the physio- 

 logical significance of the number of segments, and all 

 the other matters which we are accustomed to regard 

 as tyjDical peculiarities of the different organs, and 

 to which we usually ascribe the highest systematic 

 value ? 



" Those jpecuUarities ivMch first a])]pear, sliould he more 

 higlihj estimated than those ivhich a])])ear subsequently. A 

 system, in order to he true and natural, must agree with 

 the sequence of the organs in the development of the 

 embryo." If the earlier manifested peculiarities are to 

 be estimated more highly than those which afterwards 

 make their appearance, then in those cases in which 

 the structure of the adult animal requires one position 

 in the system, and that of the larva another, the 

 latter and not the former must decide the point. As 

 the Lerncece and Cirripedes, on account of their ISTauplius- 

 brood, were separated from their previous connexions 

 and referred to the Crustacea, we shall, for the same 

 reason, have to separate Peneus from the Prawns and 

 unite it with the Copepoda and Cirripedia. But the 

 most zealous embryomaniac would probably shrink 

 from this course. 



A " true and natural system " of the Crustacea to 

 be in accordance with the sequence of the phenomena 

 would have to take into account in the first place the 

 various modes of segmentation, then the position of 

 the embryo, next the number of limbs produced within 

 the egg and so forth, and might be represented some- 

 what as follows : — 



