63 
in some of the humbler parts of their organization, are very inferior in vertebra- 
tion and nervous energy. 
But while the absolute effect of the muscles or mechanical organs of the more 
highly developed animals is less, upon the whole, than that of the lower, there are 
counterbalancing advantages; for the internal skeleton is, if we may be allowed the 
term, much more disposable, that is, capable of much more varied action in a 
single articulation, than the external crust. We may take the Crustacea and 
Arachnida , of which the common crab and the garden spider may be taken as 
types, as expressive of the highest mechanical structure of invertebrated animals ; 
and we may take the human body, in consequence of the universality of its appli¬ 
cation, as the most characteristic of the vertebrated ones. In these, if any one 
examines the pincer-claw of the crab in the articulations of its crust, and the 
skeleton of the human arm in the articulations of the bones, he will not fail to be 
struck at the very limited range of motion which the former possesses to that 
possessed by the latter. In the claw, the hard parts which are moved are external 
of the muscles which move them; and, therefore, if there is an articulation of one 
part of the crust upon another, there must be two centres, and an axis of motion 
passing through those centres. But two points determine, and fix the position of 
a line, so that it cannot by possibility vary, if the points themselves are fixed ; as, 
for instance, a line on the earth’s surface, passing through a fixed point at Bir¬ 
mingham and another at London, would be determined until it girded the earth as 
a great circle, and could not by possibility deviate a single inch to the right hand 
or to the left, even at the remotest distance from those two fixed points. The two 
centres of motion in the articulation of the crusted animal are two fixed points in 
the crust; and therefore the axis of motion, which must pass through them, can 
have no angular play, and the motion must be confined to one plane, from which 
it cannot deviate a single hair’s breadth. Such a joint must act with the most 
perfect precision ; and it will be found that in all the hinge joints of the crab’s 
claw there is not the least lateral motion. If, therefore, the limb of an inverte¬ 
brated animal is jointed by crust articulated upon crust, a great number of joints 
is required, in order to produce even a very limited variety of motion ; and no 
number of joints could produce the variety which the articulations of the human 
arm can communicate to the point of the finger. A more varied motion is ob¬ 
tained, by uniting the extremities of the two pieces of crust by a certain portion of 
cartilaginous matter, as we find in those joints which unite the crab’s claw to the 
body of the animal, and also in the joints of the smaller claws, or walking legs. 
This mode of union, for it is not strictly an articulation, allows of bending in any 
direction, in proportion to the extent and flexibility of the cartilage that joins the 
two portions of crust. This, however, has a limit, and a very narrow one, because 
a very little extent or increased flexibility of the cartilage would render the limb 
so feeble and unsteady that it would not be efficient for any one purpose. Any 
