CHAP. III. MOEPHOLOGY OF CEUSTACEA. 15 



Crabs and Macrura, Amphipoda and Isopoda, in which 

 the last seven segments are always different from the 

 preceding ones in the appendages with which they are 

 furnished, could only be regarded as an inheritance from 

 the same ancestors. And if at the present day the 

 majority of the Crabs and Macrura, and indeed the 

 Stalk-eyed Crustacea in general, pass through Zoea-like 

 developmental states, and the same mode of transforma- 

 tion was to be ascribed to their ancestors, the same 

 thing must also apply, if not to the immediate ancestors 

 of the Amphipoda and Isopoda, at least to the common 

 progenitors of these and the Stalk-eyed Crustacea. Any 

 such assumption as this was, however, very hazardous, 

 so long as not a single fact properly relating to the 



to put forth limbs immediately after their own appearance, whilst the 

 segments of the hind-body often remain destitute of feet through long 

 portions of the larval life or even throughout life (as in many female 

 Diastylidse), a reason, among many others, for not, as is usual, regard- 

 ing the middle-body of the Crustacea as equivalent to the constantly 

 footless abdomen of Insects, The appendages of the middle-body 

 (pereiopoda) seem never, even in their youngest form, to possess two 

 equal branches, a peculiarity which usually characterises the appendages 

 of the hind-body. This is a circumstance which renders very doubtful 

 the equivalence of the middle-body of the Malacostraca with the section 

 of the body which in the Copepoda bears the swimming feet and in the 

 Cirripedia the cirri. 



The comprehension of the feet of the hind-body and tail in a single 

 group (as "fausses pattes abdominales," or as " pleopoda ") seems not 

 to be justifiable. When there is a metamorphosis, they are probably 

 always produced at different periods, and they are almost always quite 

 different in structure and function. Even in the Amphipoda, in which 

 the caudal feet usually resemble in appearance the last two pairs 

 of abdominal feet, they are in general distinguished by some sort of 

 peculiarity, and whilst the abdominal feet are reproduced in wearisome 

 uniformity throughout the entire order, the caudal feet are, as is well 

 known, amongst the most variable parts of the Amphipoda. 



