PURPLE EGG-URCHIN. l7l 



if you recorded the fact should they increase, the date 

 of their introduction to this side of the island, 18'36, may 

 thus be known. 11 



That these Urchins live in cavities of rocks was ob- 

 served long ago by Rondeletius, who figures some in 

 that situation. That these cavities are formed by the 

 animals, and are not mere depressions from some oilier 

 cause, is the opinion of those who have attended most to 

 their history, of Dr. Leach, Mr. Bennett, Mr. Ball, and 

 Mr. Thompson. How the boring is effected is another 

 question, and one not as yet settled. The probability is 

 that the animal forms the first perforation by means of 

 the teeth, and softens the rock by some secreted solvent. 

 The circumstance of its burrowing in limestone only, fa- 

 vours this view, especially as when found on soft clay 

 slate they do not bore, though such animals as the 

 pholas, which bores by mechanical means, perforate slate 

 — a rock mineralogically similar. 



The subject of boring animals is still open to the zoolo- 

 gist and chemist. Much has been written on it but little 

 satisfactory. Dr. Drummond has suggested that some of 

 these creatures may have the power of decomposing the 

 sea-salt as their wants may require, and applying the 

 liberated muriatic acid to the solution of the calcareous 

 rock. Mr. Thompson has some interesting observations 

 on the subject in a paper on the Teredo 7iavalis and Lim- 

 noria terebrans published in the Edinburgh New Philo- 

 sophical Journal for January 1835. 



