EXCAVATORS. 357 



Harbour, and which was in excellent condition three 

 years previously, had been subsequently destroyed 

 by an accumulation of mud. The shells of the dead 

 oysters, which were of large size, and in great number, 

 in the course of two years had been so completely 

 riddled by the boring sponge (Cliona) that they might 

 be crushed with the utmost ease, whereas without the 

 agency of this sponge, the dead shells might have 

 remained in their soft, muddy bed, devoid of sand 

 and pebbles, undecomposed, perhaps, even for a 

 century." J 



In reference to the mode by which this burrowing 

 is accomplished, Dr. O. Schmidt says : " One would 

 first think of the siliceous needles as the cause, but 

 we soon see that we must abandon the notion that 

 this is the boring apparatus, since it must be borne 

 in mind that such apparatus must be operated. 

 Even though the protoplasm executes delicate fluc- 

 tuating movements, so that in Cliona, as in many 

 other sponges, the needles are drawn into bundles, 

 rows, or series in particular directions ; in any case, 

 the force so exerted would not be sufficient to scrape 

 or erode the lime rock with their points. This mode 

 of distribution and extension of the sponge would 

 rather indicate that a process of chemical solution 



1 Leidy, in " Proceedings of Academy of Natural Sciences of 

 Philadelphia," vol. viii. p. 162. 



