THE EXCAVATING SPONGES. 343 



made these burrows in the one material, also made them in the 

 other ; of this there can be no doubt. And yet it would be very- 

 hard to believe that these hypothetical worms would be able to 

 derive much nutriment from limestone, however much they might 

 obtain from shell. This fact, indeed, sufficiently disproves the 

 nutritive theory, and the difficulty still remains, with those who 

 assume the agency of worms, to account for the great number and 

 vast extent of the excavations occupied by Cliona, and the almost 

 entire absence of worms. 



But there is another equally formidable obstruction in the way 

 of attributing these excavations to worms, and which appears to 

 be perfectly insurmountable. In all the excavations occupied by 

 Cliona there are numerous circular orifices, opening at the sur- 

 face of the shell or stone containing the parasite. Now, the 

 papillae or oscula of the sponge, communicating with the water, 

 always protrude through these apertures, and invariably corres- 

 pond to them in size, number, and position, leaving no orifice 

 unoccupied and exactly fitting each, whether there be upwards 

 of two hundred to the square inch as in C. vastifica, or only about 

 twenty-five, or less, as in C. celata. How are we to account for 

 such a fact as this, if it be maintained that these orifices were 

 drilled by worms ? Are we to consider that this complete cor- 

 respondence between these parts of the sponge and the orifices is 

 a mere chance coincidence (wonderful indeed if true!) or that 

 the worm made the openings purposely, in strict accordance with 

 the requirements of the sponge that on some future day might 

 take up its abode in the deserted excavation? or is it that the 

 sponge has the power of modifying the position, number, and size 

 of these important organs to meet the circumstances of its usurped 

 home? The two first propositions cannot be entertained for a 

 moment ; and the last is contradicted by the fact that C. vastifica 

 is never found with few and large papillae like C. celata ; nor in 

 the latter species are they ever very numerous and minute as is 

 the case with the former. And, indeed, the arrangement, size, 

 and number of the papilla? are good specific characters through- 

 out the species. 



If we now refer to what is stated in my former paper on the 



