104 HISTOKY OF CRUSTACEA. Chap. X. 



single adult Amphipod ; we know it only in tlie young 

 of tlie genus Brachyscelus, which therefore in this 

 respect undoubtedly depart more widely than the 

 adults from the type of their order. This applies also 

 to the young males of the Shore-hoppers (Orchestia) 

 with regard to the second pair of anterior feet (gnatJio- 

 jooda). In like manner no one will hesitate to accept 

 the possession of seven pairs of feet as a "typical" 

 peculiarity of the Edriophthalma, which Agassiz, on 

 this account, names Tetradecapoda ; the young Isopoda, 

 which are Dodecapoda, are also in this respect further 

 from the " type " than the adults. 



It is certainly a rule, and this Darwin's theory would 

 lead us to expect, that in the progress of development 

 those forms which are at first similar gradually depart 

 further from each other ; but here, as in other classes, 

 the exceptions, for which the Old School has no ex- 

 planation, are numerous. Not unfrequently we might 

 indeed directly reverse the proposition and assert that 

 the difference becomes the greater, the further we go 

 back in the development, and this not only in those 

 cases in which one of two nearly allied species is di- 

 rectly developed, and the other passes through several 

 larval stages, such as the common Crayfish and the 

 Prawns which are produced from Nauplius-brood. 

 The same may be said, for example, of the Isopoda 

 and Amphipoda. In the adult animals the number of 

 limbs is the same ; at the first sight of a CyrtopJiium or 

 a Dulichia, and even after the careful examination of a 

 Tanais, we may be in doubt whether we have an 



