1396 CRUSTACEA. 



the inner antennae may be very small, and retractile into fissures fitted 

 to receive them, or they may be very long organs, constantly thrown 

 forward of the head ; and descending but a single step, we come to 

 species of Decapoda without proper branchiae, some having the abdo- 

 minal legs furnished with branchial appendages, and others with no 

 abdominal members at all. 



When we consider, that these diversities occur in a class that may 

 not embrace in all over ten thousand species (riot half of which are 

 now known), we then comprehend the wide diversity in the distinc- 

 tions that exist. The series of species followed through, gives us an 

 enlarged view of those distinctive characteristics upon which the 

 limits and relations of groups depend. The network of affiliations, it 

 is true, is like that in other departments ; but it is more magnified to 

 the view. 



Moreover, the distinctions are obviously distinctions of rank. There 

 is no ambiguity as to which is the higher or superior group, as among 

 Insecta. The variations are manifestly variations in grade, and we 

 may readily trace out the several steps of gradation, as we descend 

 from the highest Brachyura to the lowest Lernaea. And while we so 

 readily distinguish these gradations, we as plainly see that they are 

 not steps of progress followed by nature in the production of species; 

 but, simply successive levels (grades of types), upon which species have 

 been multiplied. 



We, therefore, may consider the class Crustacea as especially well 

 adapted for instruction in some of the higher principles of classifica- 

 tion in Zoology ; and, if we mistake not, laws may be educed which 

 have not hitherto taken form in science. These have already been 

 partially alluded to in the previous pages of this volume. But we 

 here bring together the facts in a connected view, in order to state 

 the principles more definitely, and exhibit the full extent of their 

 bearing. We leave out, however, a large part of the details, which 

 may be found elsewhere in this Keport. 



The fundamental idea, which we shall find at the basis of the 

 various distinctions of structure among the species is, the higher cen- 

 tralization of the superior grades, and the less concentrated central forces 

 of the inferior, a principle which has been applied to the animal 

 kingdom in some of its larger subdivisions, but which has not been 

 followed out into all the details of structure exemplified among Crus- 

 tacea. 



