EXCAVATORS. 365 



power of excavating the canals, and other spaces, 

 which it usually occupies. My own intimate know- 

 ledge of the species has led me to a contrary con- 

 clusion. When located in oyster, or other, shells it 

 usually fills entirely the cavities between the two 

 surfaces, but when the canals excavated in the lime- 

 stones extend to the depth of two or more inches, it 

 frequently occurs that the sponge terminates at the 

 depth of less than an inch, and the remaining part of 

 the canal is quite empty and clean, without the 

 slightest indication of having been ever occupied by 

 sponge ; and in one of these perforated stones from 

 Tenby, which I broke through the centre, although 

 it abounded with the sinuous canals, none of them 

 presented the slightest traces of having ever contained 

 sponge ; and occasionally, oyster shells, full of per- 

 forations, may be found in the same condition. These 

 facts militate strongly against the idea that the ex- 

 cavations are produced by the sponge ; and, in 

 addition to them, we must bear in mind that the 

 dermal membrane is quite smooth, and that there are 

 no mechanical appliances, or organs visible, by which 

 such a power of attrition could be exerted." l This 

 author contends that the perforations must have been 

 made by worms (annelids), and the cavities subse- 

 quently occupied by the sponge. 



1 "A Monograph of the British Spongiadae," by J. S. Bower- 

 bank (Ray Society, 1866), vol. ii. p. 219. 



