J. G. WAT.LER ON CLIONA CELATA, ETC. 259 



they lie parallel on the membranes, and exhibit no massiveness, 

 except at the oscula. Remarkable as these organs are in their 

 construction, their characteristics are not confined to this Sponge, 

 but are found so represented in others, without, however, the same 

 comparative importance, due to the fact, that in this Sponge these 

 orifices, for the most part, concentrate the necessary functions of in- 

 halation and exhalation. I should not have thought it necessary to 

 discuss the question of spicules taking any part in the " boring " if 

 there had not been still those who maintain the same. 



But I will now leave this part of my subject and proceed to 

 discuss the non-appearance of the Sponge in certain burrows or 

 parts of burrows. It was one of Mr. Hancock's arguments, in 

 favour of the Sponge being the excavator of its habitat, that it 

 always filled the excavations. That this is very important to that 

 argument needs scarcely to be noted. But I have already quoted 

 Dr. Bowerbank's testimony against the fact, and I shall again quote 

 him in another passage, and then take up the question of my own 

 experience. At page 221 of the former named volume he says, 

 '* I have in my collection several specimens of large Balani which I 

 took from the sides of the rocks forming the Guliot Caves in Sark, 

 which are perforated, in the usual manner, with numerous sinuous 

 canals, which I found filled with the living Annelids, the dried re- 

 mains of which still remain in them, and without the slightest 

 indication of the presence of H. celata, and I have also found living 

 Annelids in the deeply seated portions of the perforations in the 

 limestone boulders of Tenby, beyond the range of the Sponge, so 

 that I think it may be reasonably concluded that the Sponge occupies 

 the canals and cavities in shells and stones that have been excavated 

 by other animals, and that they have no power to make such 

 residences themselves." 



Such testimony as this cannot be passed over in silence, even if it 

 came from a less-known observer. But my own observations have 

 abundantly shown me analogous facts. Numerous instances have 

 presented themselves to me of small, clean excavations, which have 

 never had the Sponge within them, although it was showing all 

 around them. And the theory of *' washing out " will not here 

 answer, as I formed my observations on an oyster shell as it came from 

 the dredge with all its dirt, encrusting Zoophytes, Annelids, &c., 

 upon it. Besides, to provide a Detis ex machind for removing a 

 difficulty is, at best, but a poor and doubtful argument. 



