138 



TEETH OF ECHINUS 



small, and take to itself a better ; and to add to 

 the difficulty/ there is no supply of arteries and 

 veins ramifying through these plates, as is the 

 case with the bones of a vertebrate animal, but 

 each plate is dense and dead. 



In order to overcome these apparently in- 

 superable difficulties, a very beautiful arrange- 

 ment takes place. The delicate living membrane 

 with which the entire surface of the body is 

 covered insinuates itself between the edges of 

 these plates, and continually deposits round the 

 margin of each particles of calcareous matter ; so 

 that each plate simultaneously increases round 

 its edge, and the original form of the shell is 

 preserved. 



If we still keep before our eyes the image of 

 the rolled-up star-fish, we shall see that as the 

 mouth is precisely in the centre of the disc, it 

 would also be found in the centre of the Echinus 

 shell. And such a mouth as it is could hardly be 

 conceived. If a human being, say a man of six 

 feet in height, were to be possessed of a similar 

 mouth, it would be about the size, and very much 

 the shape, of an ordinary wooden pail, the teeth 

 being as long as the staves of the pail, only they 

 must be made very sharp at the top, and but five 

 in number. The teeth of the Echinus may be 



