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. Insome 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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