1402 



CRUSTACEA. 



for by its progress we should never reach the Macroural structure; 

 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 subordinate, yet correlated 

 types of structure, each perfect in itself, and admitting of wide modi- 

 fications, 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 anterior 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 rudimentary 

 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 ap- 

 pendages 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 

 Palsemons ; or, to the third pair, as in the Penseida? ; indicating a 

 further decrease of that centralization so remarkable in the Brachyura. 

 Still lower among the species, as in the Sergestidse, 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 abdo- 

 minal appendages. 



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

 Eighth. The abdominal appendages become obsolete. 



