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Fig. 19.—Dujardinia rotifera, with jointed tentacles and caudal appendages.—With some changes, after Quatrefages.

The origin of the joints or segments in the limbs of arthropods was probably due to the mechanical strains to which what were at first soft fleshy outgrowths along the sides of the body became subjected. Indeed, certain annelid worms of the family Syllidæ have segmented tentacles and parapodia, as in Dujardinia (Fig. 19). We do not know enough about the habits of these worms to understand how this metamerism may have arisen, but it is possibly due to the act of pushing or repeated efforts to support the body while creeping over the bottom among broken shells, over coarse gravel, or among seaweeds.

It is obvious, however, that the jointed structure of the limbs of arthropods, if we are to attempt any explanation at all of the origin of such structure, was primarily due mainly to lateral strains and impacts resulting from the primitive endeavors of the ancestral arthropods to raise and to support the body while thus raised, and then to push or drag it forward by means of the soft, partially jointed, lateral limbs which were armed with bristles, hooks, or finally claws.