CLASSIFICATION OF CRUSTACEA. ^397 



This centralization is literally a cepJialization of the forces. In the 

 higher groups, the larger part of the whole structure is centred 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 intelligibly studied there, than in the 

 members, the production and position 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 segments, and 

 correspondingly twenty-one pairs of members, as laid down by Milne 

 Edwards, the last seven of which pertain to the abdomen, and the 

 first fourteen to the cephalothorax. Now, we may gather from an 

 examination of the crab, or Macroural Decapod, acknowledged to be 

 first in rank, what condition of the a system is connected with the 

 highest centralization 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, thr.ee are organs 

 of senses, six are the mandibles and maxillae. Moreover, these organs 

 are clustered into the smallest possible space, so that the six pairs of 

 mouth organs hardly occupy more room than the first pair of legs. 

 The organs are all small, the antennae exceedingly short, the maxillaa 

 small lamellar organs sparingly jointed. The vegetative powers of 

 growth have had but little play. The inner antennae are rather large 

 as regards the basal joint, which is devoted to one of the senses, but 

 the rest is nearly rudimentary, and the whole is snugly boxed away, to 

 be extruded at the will of the animal. The exterior maxillae (or 

 outer maxillipeds) cover exactly the other pairs, and shut closely 

 down over the mouth, like a well-fitting operculum to the buccal area. 



We hence learn, that the condition of highest centralization in 

 Crustacea, is where the cephalic part embraces the largest portion of 

 the normal structure of the cephalothorax, and the whole is con- 

 tracted within the smallest compass, with the least vegetative growth 

 or elongation of the parts. The forces are concentrated in the more 

 perfectly developed senses and the higher functions of the animal 

 not in giving size to the organs of the senses, but acuteness to the 



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