CHAPTER THIRTY-FIVE 



ARTHROPODA 



THE phylum Arthropoda (from the Greek, meaning Antiquity of 

 "with jointed feet") includes more species of animals 

 than all the other phyla combined. It is of immense 

 antiquity, with representatives in the Cambrian rocks, 

 laid down not less than 30 millions of years ago. A 

 terrestrial form, a scorpion, is known from the Silurian 

 strata, and is more than 20 millions of years old. Un- 

 like some of the ancient groups, the arthropods have 

 continued to flourish up to the present time, producing 

 in all ages vast numbers of genera and species, adapted 

 to almost every conceivable condition of life. 



The earliest known arthropods were marine, and must Evolution of 

 have been derived from some primitive type of seg- Arthr P da 

 mented worm. The characteristic feature is the ex- 

 ternal more or less hard covering (exoskeleton), the 

 essential basis of which is the hornlike substance 

 chitin, though there may be also a deposit of carbonate 

 of lime, as in the larger Crustacea (crabs and their rela- 

 tions). In the segmented worms paired appendages 

 (parapodia) are frequently well developed. These are 

 muscular projections from the body, and often bear 

 remarkable chsetae or bristles, which may be jointed. 

 The arthropod has such paired appendages still further 

 developed, the majority being jointed and serving as 

 legs. They commonly arq bristly or hairy, and bear 

 one or two claws at the extremity. With the develop- 

 ment of a hard surface, segmentation is necessary to 

 permit flexibility ; so not only the body but also the 

 appendages are jointed at intervals. Among the vast 

 numbers of arthropods known, species will be found 

 which do not agree with the general definition of the 



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