218 A MONOGRAPH OF THE 



Guliot caves at Sark. Sections at right angles to the sur- 

 face from the massive part of the sponge on the inner surface 

 of the Pectunculus valve, presented all the characteristic 

 structures that are common to the genus Hymeniacidon, and 

 when compared with portions of the sponge taken from the 

 tortuous canals in the substance of the shell the one could 

 not be distinguished from the other. 



Some naturalists have promulgated the idea that this 

 species of sponge has the power of excavating the canals 

 and other spaces which it usually occupies. My own inti- 

 mate knowledge of the species has led me to a contrary 

 conclusion. When located in oyster or other shells it 

 usually fills entirely the cavities between the two surfaces, 

 but when the canals excavated in the limestones 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 its 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 perforations may be found 

 in the same condition, one such I have in my possession. 

 These facts militate strongly against the idea that the exca- 

 vations 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. Mr. Hancock, of Newcastle, believed that he 

 had discovered the organs by which the sponge effected the 

 perforations, and, with his usual kindness and liberality, he 

 presented me with a specimen of these supposed erosive 

 parts, which I at once recognised as cells secreting carbon- 

 ate of lime, which are found in abundance embedded in the 

 growing membranous tissues of the oyster-shells whence he 

 had obtained his specimens. The perforation of the shells 

 by the agent effecting that operation, would naturally 

 liberate abundant fragments of these membranes, and the 



