124 



THE SEA SHOBE 



knife, and retains its natural appearance to perfection if preserved 



in methylated spirit. Its horny skeleton is of a very compact 

 nature, and the spicules are minute siliceous 

 needles pointed at both ends. 



Rambling on the sea beach we frequently 

 meet with old oyster and other shells per- 

 forated by a number of circular holes about 

 the size of a pin's head or less, and chalk 

 and limestone rocks also are seen similarly 

 bored. On breaking into or grinding down 

 the substance we find that the openings are 

 the ends of channels that form a network of 

 canals and chambers, some of which are so 

 near the surface that they are covered by 

 an exceedingly thin layer of the calcareous 

 substance. These canals and chambers form 



the home of the Boring Sponge (Cliona), which, although a very 



soft-bodied animal, has itself excavated them. 



FIG. 74. SPICULES OF 

 Halichondria, 

 NIFIED 



FIG. 75. AN OYSTER SHELL BOEED BY Cliona 



The manner in which the Cliona excavates such a complicated 

 system of passages in so hard a material has naturally raised a 

 considerable amount of curiosity, and those who have studied the 



