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ANIMAL BIOLOGY 



the execretory and reproductive systems. One has but to recall 

 that, on an earlier page, it was possible to describe in general 

 terms these systems as they exist throughout the Vertebrate 

 series — in forms as obviously different as Fish and Man. They 

 are all fundamentally the same. (Figs. 102-110, 121, 129, 141, 

 227, 228.) 



Turning to the Invertebrates, we may remind the reader that 

 all the appendages of the Crayfish are built on the same simple 



Fig. 228. - - Skeletons of Man and of Gorilla. (From Lull.) 



plan as exhibited in the swimming legs (swimmerets) of the ab- 

 domen. The highly specialized walking legs, great claws, jaws, 

 and feelers (antennae and antennules) are all reducible to modifi- 

 cations of the simple swimmeret type. In short, all are homologous 

 structures, though differing widely in function. This is a most 

 striking example of serial homology, though we have seen the 

 same principle exhibited in the Vertebrates where the fore limbs 

 and the hind limbs of each animal are homologous. Moreover, 



