60 CRUSTACEA. 



We next look to the nutritive system. But this system, among the 

 Brachyura, is very uniform in character. The mandibles, the organs 

 which should be the first to exhibit any fundamental distinctions, are 

 of one type; having a simple cutting edge and but slight variations 

 in form, or in the character of the jointed appendage. These organs 

 are the earliest in embryologic development, preceding even the an- 

 tennas, and this fact would give to their distinctions, if there were 

 such, a high value in classification. 



With regard to the organs following the mandibles, the maxillae 

 and maxillipeds, there is no a priori ground for giving to their cha- 

 racters a primary rank. They are related normally to the legs which 

 follow, and are not a necessary part of the nutritive system, and 

 moreover, are subsequent to the antennas and eyes in embryological 

 development. In those of their peculiarities which have especial refe- 

 rence to tlie functions of the mouth, their variations of form are mostly 

 of little value. 



The exterior maxillipeds may vary widely in the same family or 

 even genus, or may have the same characters through very different 

 groups. The variations of the widest importance are in the articula- 

 tion of the fourth segment with the third, which may take place at 

 the inner apex of the latter, its summit margin, or outer apex. A 

 large division of the Brachyura is characterized by the first of these 

 three modes, and another group of natural limits (Grapsoidea), by the 

 second and third. 



It should be observed, that there is a liability to error in referring 

 some examples to the second of these modes. It not unfrequently 

 happens that the third joint is much elongated along the inner side, and 

 is also obliquely truncated, as usual towards the summit on this side; 

 and in consequence, the summit margin, instead of being horizontal or 

 nearly so, slopes very much outward, or may even be nearly, longitu- 

 dinal. In such a case, the fourth joint, to the view, is articulated with 

 the summit of the third joint, although normally with the inner apex, 

 the summit and inner apex being really the same part. There are 

 many cases of this kind. Eurypodius is an example of this elongated 

 third joint; while in the genus Oregonia, to which it is very nearly 

 related, the third joint has the ordinary shorter form, and the arti- 

 culation with the inner apex is distinct. 



The relative lengths of the second and third joints of the outer 

 maxillipeds afford distinctions often of generic importance, and so also 



