PHYLUM V. 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, somites, 

 or metameres, and in each of these we find parts of all the 

 internal organs. That is, the segmentation is not con- 

 fined 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, much as they are in 

 the annelids (p. 183), but in nature we find that certain 

 segments or parts of certain segments become over-devel- 

 oped (hypertrophied) , and this produces an under-develop- 

 ment (tendency towards atrophy) in others. Thus every 

 segment in our ideal arthropod would bear a pair of 

 jointed appendages, but these 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 



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