BRACHYURA. 59 



the succession of parts forming the body, the similarity in the modes 

 of aeration, in the organs of the senses and the mouth, are evidences 

 of a common type, and a general resemblance of habit or mode of life ; 

 and a study of the embryology of these Crustacea, explains this close 

 typical affiliation. The value of any characters is hence to be ascer- 

 tained by a direct study of their bearing among the various species. 



It should be remembered, moreover, that the characteristics men- 

 tioned in a description are not always the fundamental differences, 

 though as far as possible they should so be. The several fundamental 

 differences may be indicated perhaps by a mere angle in the shell; 

 and hence, when this is found to be the case, the peculiarity of this 

 angle is often mentioned, and the fact it indicates left untold. It is, 

 therefore, a general truth, that external characters are often of value, 

 not for what they are, but rather for what they indicate. In one 

 division, a character may be of the very lowest importance, dis- 

 tinguishing, possibly, only species, when it separated families or 

 tribes in another department. The pointed front of the Maioid 

 Crustacea is characteristic; for there is a range of peculiarities at once 

 suggested by it. But this form was allowed in the early stages of 

 the science, to gather many species into the same division with the 

 Maioidea, which have since been shown to be widely separate. The 

 character is valuable for what it indicates, and not for itself alone. 

 The long anterior legs of the Parthenope group afford an obvious cha- 

 racteristic for the group; but this is not the important characteristic, 

 but only an external mark of actual peculiarities. The long posterior 

 legs of certain Maioid species have been allowed to have the same 

 value in Taxonomy; yet in fact, this character in itself indicates no 

 other difference of any moment, and is, therefore, of little real value 

 as a source of distinction. Yet if, in any subdivision of the Maioidea, 

 this peculiarity should be found to be a regular attendant upon other 

 important peculiarities, it would become a convenient and useful 

 means of characterizing the group or groups. 



In searching for characteristics of the natural groups among the 

 Brachyura, we should, perhaps naturally, look first to the nervous 

 system. Yet, it is generally true, that this system does not undergo 

 variations correspondent with the minor subdivisions in Zoology. It 

 has its several types of structure, and under these types it is accommo- 

 dated in its character to the various forms of species, rather than to 

 differences in other functions or in habits. 



