/. D. Dana on the Classification of Crustacea. 15 



of gradation, as we descend from the highest Brachyura to the 

 lowest Lernasa. And while we so readily distinguish these gra- 

 dations, we as plainly see that they are not steps of progress fol- 

 lowed by nature in the production of species ; but, simply suc- 

 cessive 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 

 classification 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 Eeport. But we here bring together the facts in a con- 

 nected view, in order to state the principles more definitely, and 

 exhibit the full extent of their bearing. We leave out, how- 

 ever, a large part of the details, which may be found elsewhere 

 in the work. 



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

 various distinctions of structure among the species is, the higher 

 centralization 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 exem- 

 plified among Crustacea. 



This centralization is literally a cephalization of the forces. In 

 the higher groups, the larger part of the whole structure is cen- 

 tred in the head, and contributes to head functions, that is, the 

 functions of the senses and those of the mouth. As we descend, 

 the head loses one part after another, and with every loss of this 

 kind, there is a step down in rank. This centralization may be 

 looked for in the nervous cords ; but the facts are less intelligi- 

 bly studied there, than in the members, the production and po- 

 sition of which measure the condition of the forces : — -just as we 

 can better measure the forces of a galvanic battery by the work 

 done, than by the size or external appearance of the plates 

 which constitute it. 



In the Crustacea type, there are normally twenty-one seg- 

 ments to the body, and correspondingly twenty-one pairs of 

 members, — as laid clown by Milne Edwards,' — the last seven of 

 which pertain to the abdomen, and the first fourteen to the ceph- 

 alothorax. Now, we may gather from an examination of the 

 crab, or Macroural Decapod, acknowledged to be first in rank, 

 what condition of the system is connected with the highest cen- 

 tralization in Crustacea. 



In these highest species, nine segments and nine pairs of 

 appendages out of the fourteen cephalothoracic, belong to the 

 senses and mouth, and only five pairs are for locomotion. Of 

 these nine, three are organs of senses, six are the mandibles and 



