14 



each ray, opposite each of which a single ganglion is placed. 

 This arrangement gives each of the rays a power of motion 

 independent of that of the others. 



The repetition of nervous centres exhibited in this 

 instance is found in every other division of the animal 

 kingdom up to man, in whom we see two cerebral hemispheres , 

 and two halves of the spinal cord. In this double arrange- 

 ment of a tract of nervous fibres which connects the ganglia 

 of the trunk and their connecting cord with the cephalic 

 ganglia, in the articulated animals, impressions are first 

 couverted into sensations, and here we behold the first 

 example of a cephalic centre controlling and harmonising 

 the movements of the whole system. 



That sensation and volition reside in the cephalic 

 ganglia, is made manifest by separating them from those of 

 the trunk. The latter, in this case, are capable of producing 

 only the reflex motions — of which, indeed, they seem to be 

 the exclusive organs. 



The author next proceeded to give a slight sketch of the 



various systems of zoological classification, and, adopting 



Cuvier's, showed its lowest division, the Radiated Animals, 



to be composed of a vast number of creatures of very 



dissimilar form and structure. Wishing to consider the 



subject from two points of view, **. e. 1st, as perfecting a 



function, and 2d, as completing some modification of form, 



and pursuing it in the natural order of development, he 



stated that, with respect to the means of locomotory action, 



it will be found that two distinct principles prevail, viz. : — 



1st Where there is no organ exclusively appropriated 



to that purpose, but where the part producing it 



seems to have another function attached as its 



primary end ; and 



2d. Where the organ employed is the appropriate and 



exclusive instrument of the function. 



