FORE AND HIND LIMBS 



in the lizards, and by the internally placed " sternal 

 plate " in scorpions, etc. The proper way to put the 

 difference is that in all Chordata a supporting rod wholly 

 or partly underlies the central nervous system, and 

 that in all vertebrata the entire central nervous system 

 is completely sheathed in a tube of bone or cartilage, 

 which partly or entirely replaces that axial supporting 

 rod ; and that in no " Invertebrate " is this the case. 



All vertebrates possess two pairs of limbs ; these are 

 the paired (not the unpaired dorsal and ventral) fins 

 of fishes and the legs and arms of frogs, reptiles and 

 mammals, and of course the wings and legs of birds. 

 No vertebrate possesses more than these two pairs, 

 and it is nearly invariably the case that both pairs are 

 present, or that, if missing, there is evidence that absence 

 is due to disuse and to degeneration. Thus the whales 

 have no hind limbs, but they possess in most cases 

 tiny bonelets which can be identified as the rudiments of 

 the missing limbs. In some snakes the otherwise entirely 

 missing limbs are represented by minute fragments in the 

 neighbourhood of the vent, which can also be identified 

 with limbs. There is only one type of vertebrate life 

 which seems to have no traces of limbs and that is the 

 lampreys. Undoubtedly, these fishes lie near to the 

 base of the vertebrate series, and it possible that they 

 have, so to speak, not yet acquired the typical verte- 

 brate limbs ; or, on the other hand, it is possible that 

 they have lost them entirely, as the snakes are pre- 

 sumed to have lost their fore limbs. 



Many invertebrate creatures have limbs ; the insects for 

 examples. An exact comparison, however, between 

 these appendages and the limbs of vertebrata is im- 

 possible, and in any case the number is not limited to 

 two pairs as in the group which we are considering here. 



The next point of importance is the condition of the 

 respiratory organs in the vertebrata. In all the 



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