STRUCTURE AND DEVELOPMENT 49 



which is also turned under the anterior part of the body. 

 The internal organic systems are constructed upon a 

 worm plan with modifications. Nearly every one of 

 the segments bears one pair of appendages, which can 

 be referred by their forked nature to the two-parted, 

 oarlike flaps of sandworms, but the appendages of 

 Crustacea have departed from their prototypes in 

 functional respects and in details of structure. They 

 are variously feelers, jaws, legs, pincers, and swimming 

 paddles, evolved to serve different purposes, just as 

 the limbs of the vertebrates we have described have 

 become variously arms, wings, flippers and paddles in 

 apes, bats, seals, and whales. 



Butterflies, beetles, bees, and grasshoppers seem at 

 first sight to be entirely different, even though they 

 agree in being more or less segmented. But all of them 

 have heads with four pairs of appendages of the same 

 essential plan, middle thoracic regions of three segments 

 more or less united, bearing three pairs of legs and 

 usually two pairs of wings, while the hinder part is a 

 freely jointed abdomen without real limbs. In these 

 respects the countless varieties of insects agree so that 

 they also like Crustacea of various kinds seem to have 

 been derived from wormlike animals with more simply 

 segmented bodies. Indeed spiders and scorpions and 

 their relatives of the group arachnida prove for similar 

 reasons to be derivatives of the same original stock, 

 and own cousins of the insects. 



In nearly every one of the invertebrate branches we 

 find representatives which interest us chiefly because 

 they appear to have reached their present condition 

 by retrograde evolution. Barnacles are really crus- 



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