8 J. D. Dana on the Classification of Crustacea. 
nor, in the reverse order, should we from the Macroural reaeh 
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 1n 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 Maeroura, there are species of high rank that have the an- 
terior legs strong-handed, like the Maeroura. "l'here 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. 'lhe several marks of degradation are 
as follows :— 
J'irst. 'The outer maxillipeds pediform. 
Second. ''he maxillipeds next anterior pediform. | 
Third. Second pair of legs cheliform and stouter than the first. 
Jourth. 'l'he 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 cephalie 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 Palzmons; or, to the third pair, 
as in the Pensidse; indicating a further decrease of that cen- 
tralhzation so remarkable in the Brachyura. Stilllower among 
the species, as in the Sergestidsz, 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 branchize become external and small. 
Sixth. ''he branchie become wholly wanting, or part of the 
abdominal appendages. [ 
Seventh. 'l'he last two pairs of thoracic legs become obsolete. 
Jghth. 'The abdominal appendages become obsolete. 
ANinth. 'T'he eyes and antennse have separate segments, and the 
abdomen is very long and large. 
The fifth point of degradation is seen in the Zuphauside ; the 
sixth, in the 7Myside and other Anomobranchiates; the seventh 
is found in several genera of the same group; the eighth in cer- 
tain Myside. 'lhe Anomobranchiates are thus degraded Ma- 
