Vertebra and Annellus. 67 



instances unite in such a manner as to form distinct, 

 arches, would seem to be rudiments of a spinal column 

 indeed, between these calcareous arches of the 

 crustacean and the bony belts which take the place of 

 the bodies of the vertebrae in the carapace of the 

 chelonian the resemblance is evidently that which is 

 more than merely accidental. 



In every vertebrate animal also there are vertebrae 

 which are transitional to the annelli in a degree which is 

 only less perfect than that which is met with in the 

 carapace of chelonians, and these are to be found in 

 places where any such meeting would seem to be least 

 likely, even in the cephalic region. For here, the broad 

 ring-like vertebrae, articulated by the edge with their 

 fellows, the tendency to the formation of a simple cavity 

 by the disappearance of the floor of the skull, and the 

 rudimentary development of muscle outside the bone, are 

 certainly features which, when taken in connection with 

 those that have been already noticed, may easily serve as 

 bridges to any one who wishes to get across from the 

 side of the vertebra to that of the annellus, or vice versa. 



On the other hand, the annellus, in being more or 

 less covered by soft irritable tissues may move, as it 

 were, from the outside to the inside, and so take up the 

 internal position ordinarily occupied by the vertebra. 

 The annellus of the ray of a starfish, for example, 

 which is known curiously under the name of ' vertebra,' 

 is covered by the coriaceous layer which has to do the 

 work of muscle : and in some cases, as in that of a 

 rare starfish which not many days ago I saw taken by 

 trawling off the coast of Arran by the Duke of Argyle, 

 this covering was thick enough and soft enough to make 

 the likeness to an external layer of muscle very obvious. 

 In this case, indeed, the sheath of soft structure around 



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