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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 



