ARTHROPODA. 



The word. Arthropoda means "jointed foot/' and is very 

 characteristic of all that immense series of forms which, 

 like the grasshopper and the crayfish, have an external 

 skeleton which only permits of motion by a thinning or 

 jointing at intervals. In this way both body and limbs 

 have this jointed appearance, but with the body this joint- 

 ing or segmentation of the external surface is associated 

 with features of internal structure which must have a 

 moment's attention. This external jointing of the body 

 divides it into a series of essentially similar rings or somites, 

 and in each of these we find parts of all the internal organs. 

 That is, the segmentation is not confined to the external 

 surface, but is characteristic of all parts. 



In an ideal arthropod each of these segments would be 

 an exact repetition of its fellows, but in nature we find that 

 certain segments or parts of certain segments become over- 

 developed (hypertrophied), and this produces an under- 

 development (tendency towards atrophy) in others. Thus 

 every segment in our ideal arthropod would bear a pair of 

 jointed appendages, but our studies have shown us that 

 these appendages are frequently atrophied on some of the 

 segments. Again, there is a tendency in some regions, 

 and especially in the head, for a more or less complete 

 fusion of segments, so that the number can only be ascer- 

 tained by the appendages or by the features presented in 

 development. 



226 



