32 Journal of the Mitchell Society [December 



the different parts of the abdomen. And the chitinous envelope of 

 these appendages has broken into joints in the same manner in which 

 the body itself became annulated. Hence the name of arthropods 

 which is given to these animals. 



What a difference from the vertebrates. Their skeleton becomes 

 an internal framework. The organism is thus allowed to attain great- 

 er dimensions; the segments are able to fuse to a greater degree and 

 to lose more or less their independence; all of which results in the re- 

 duction of the number of limbs to only two pairs. 



Now, the relative independence of the segments and the multi- 

 plicity of the appendages have as a corollary the differentiation of 

 these structures, each of which plays a special part in the organism. 

 As Bergson remarks, the various appendages of arthropods are as it 

 were natural implements, which differ from each other in structure as 

 well as in function. Their specialization may be carried so far as to 

 have each part of a single organ perform a separate function. This 

 is clearly seen in the bee, in which the first tarsal joint of the hind legs 

 is transformed into a brush, the tibia into a pollen basket, while the 

 two joints, by the contact of their edges, act as pincers which take up 

 the flakes of wax secreted under the abdomen. It is an admirable 

 instrument wonderfully adapted to the performance of its particular 

 tasks. As a general rule, apart from the changes which they may 

 undergo in the course of specific evolution, the appendages of arthro- 

 pods are unchangeable in the individual and are narrowly adapted to 

 certain purposes; they are the tools for instinctive work, and in this 

 they differ from the less specialized but more generally useful limbs 

 which serve as implements to the vertebrates, at least to the higher 

 vertebrates. With these latter, as Bergson expressed it, the two pairs 

 of limbs "perform functions much less strictly dependent upon their 

 forms," acquiring complete independence in man, whose hand can do 

 any kind of work. 



"It seems, then, that the extraordinary preponderance of instinc- 

 tive activity among the arthropods has as its essential reason the dif- 

 ferentiation and the multiphcity of the appendages, in other words, 

 the chitinization of the integument and the formation of joint lines 

 which results from it. From the beginning these animals were doomed 

 to use organic instruments, and they made the best use possible of 

 these. Their main psychical task consisted in engraving upon their 

 memory and in instinctively repeating the acts to which these organs 

 were adaptable." (Bouvier.) 



