JAMES DWIGHT DANA 247 



traca), the second and third pairs of maxillipeds are represented 

 by legs, so that these creatures have only six cephalic (sensory and 

 oral) segments, and seven, instead of five, locomotive segments. 

 In still lower Crustacea (Entomostraca), the number of function- 

 ally cephalic appendages is still less, even the antennae becoming 

 sometimes organs of locomotion or adhesion. Moreover, in the 

 crabs (Brachyura), which form the highest division of the Deca- 

 poda, the posterior part of the body is greatly reduced in size, and 

 most of its segments are destitute of appendages. The whole 

 body seems almost, so to speak, absorbed into the head. The 

 larger number of appendages appropriated to cephalic functions 

 in the higher Crustacea is naturally correlated with a greater 

 development of the cephalic ganglion. It was natural that the 

 contemplation of facts like these should suggest to a mind so fond 

 of generalization as was that of Dana the broad principle that, as 

 "anteroposterior polarity" characterizes animals in distinction 

 from plants, so the grade of different animal forms in comparison 

 with each other is shown by the " degree of structural subordina- 

 tion to the head and of concentration headward in body structure." 

 The principle is an important and valuable one. Certainly, 

 as we pass from the lower, and in general the earlier, types of ani- 

 mal life, to the higher, and in general the later, types, there is a 

 tremendous advance in cephalization. From a protozoan, desti- 

 tute even of a mouth (the earliest cephalic feature to be developed), 

 or from a sea-anemone, whose symmetry is radial rather than 

 bilateral, and in which therefore there is but faint indication of an 

 anteroposterior axis, to man, with his immense brain, there is a 

 tremendous advance in "degree of structural subordination to 

 the head." In Dana's application of the principle of cephalization 

 to zoological classification, there was much of ingenuity. But it 

 cannot be denied that he sometimes gave undue weight to mere 

 analogies. A notable example of this is his argument for the ordi- 

 nal distinctness of man from other mammals, on the ground that 

 his anterior limbs, instead of being locomotive in function, are 

 used for prehension and manipulation. They are, according to 

 his conception, cephalic organs organs appropriated to the imme- 



