68 Traces of Unity, &c. 



the spines might be protruded and retracted so as to 

 hide or expose the tips of the spines, an arrangement 

 strikingly like that met with in the armed sucker on the 

 spatulose extremities of one of the ten-footed cephalo- 

 pods, for here in the same way the central hooks may be 

 hidden or exposed by the protrusion or retraction of the 

 soft parts surrounding it. Or the moveable spine of the 

 sea urchin may be made to supply another illustration 

 to the same effect, for here the moving power has its 

 seat in the external capsule of the ball and socket joint 

 at the base of the spine. Or, lastly, a still better 

 illustration may be found in the cephalic cartilages of a 

 cephalopod, which are deeply buried under muscle- 

 like structure, and which, according to the fancy of the 

 observer, may be classed with equal propriety under the 

 head of annellus or under that of vertebra. 



In a word, the vertebra and the annellus, in spite of 

 all their differences and these are legion are found to 

 have so much in common as to necessitate the conclu- 

 sion that both are framed upon a common plan that 

 there are in both the same unmistakeable traces of 

 unity. 



