20 /. D. Dana on the Classification of Crustacea. 



nor, in the reverse order, should we from the Macroural reach 

 the Brachyural structure. In the remarks above, we speak only 

 of the comparative actual conditions of the species as regards 

 centralization. 



The Macroura and Brachyura belong to independent yet cor- 

 related and subordinate types of structure, each perfect in itself, 

 and admitting of wide modifications, and having its own system 

 of degradations. We add a few words on these degradations 

 among the Macroura. We have seen that, in the Brachyura, the 

 powerful prehensile legs are those of the first pair, these acting 

 for the collection of food, and so contributing to the mouth. In 

 the Macroura, there are species of high rank that have the an- 

 terior legs strong-handed, like the Macroura. There are others, 

 in which the second or third pair is the strong-handed pair ; 

 others having all the legs weak appendages, with only rudi- 

 mentary hands or none. The several marks of degradation are 

 as follows : — 



First. The outer maxillipeds pediform. 



Second. The maxillipeds next anterior pediform. 



Third. Second pair of legs cheliform and stouter than the first. 



Fourth. The third pair of legs cheliform and stouter than 

 either of the preceding. 



Thus as we descend, we find one and even two pairs of mouth 

 appendages beginning to pass from the mouth series to the foot 

 series, and the cephalic portion is thus losing its appendages and 

 high centralized character. Moreover, the power belonging to 

 the first pair of legs in the higher species is transferred to the 

 second pair of legs, as in the Palaemons ; or, to the third pair, 

 as in the Penaeidae ; indicating a further decrease of that cen- 

 tralization so remarkable in the Brachyura. Still lower among 

 the species, as in the Sergestidae, all the legs are weak, and the 

 posterior pair may be short or obsolete,— the same deterioration 

 that occurs in the lower Brachyura. 



As we descend farther, there is an increased obsolescence of 

 organs, and every step is one of marked imperfection as well as 

 degradation. 



Fifth. The branchiae become external and small. 



Sixth. The branchiae become wholly wanting, or part of the 

 abdominal appendages. 



Seventh. The last two pairs of thoracic legs become obsolete. 



Eighth. The abdominal appendages become obsolete. 



Ninth. The eyes and antennae have separate segments, and the 

 abdomen is very long and large. 



The fifth point of degradation is seen in the Euphausidce ; the 

 sixth, in the Mysidoe and other Anomobranchiates ; the seventh 

 is found in several genera of the same group ; the eighth in cer- 

 tain Mysidae. The Anomobranchiates are thus degraded Ma- 



