THE EXCAVATING SPONGES. 341 



pin-like spicula, these spicula equally pervade all the membranes ; 

 but I have not observed that they assume any definite arrange- 

 ment in the papillae, though in C. glolulifera (one of the species 

 described in the sequel of this paper), which has only the pin- 

 like form, they are occasionally arranged in a radiating manner 

 in the internal membranes. Thus it appears that in some in- 

 stances, at least, Cliona does show a limited degree of order in 

 the distribution of the spicula. 



It must not however be supposed, that the spicula supply the 

 only characters that distinguish the species : the colour of tbe 

 sponge must also be taken into account, as well as the size, num- 

 ber, and distribution of the papilla?, the general habit and mode 

 of branching, or, in other words, the size and character of the 

 burrows containing the sponge, are highly characteristic. But 

 here again we are unfortunately at issue with Dr. Bowerbank, 

 who asserts, that these burrows are made by "lithodomous anne- 

 lids,"* and consequently that the branching of the sponge is 

 accidental, being entirely dependent for its character on the form 

 of the cavities within which the sponge is lodged, being moulded, 

 in fact, in worm-burrows. I shall not here reiterate the facts 

 and arguments brought forward in my former paper to refute 

 such an opinion. It may, however, be asked, how it is that 

 while C. celaia is found in vast abundance on our coasts, inhabit- 

 ing excavations in shells and limestone, the worm or annelid 

 assumed to have made these cavities has never yet been deter- 

 mined. Surely, if these are worm-burrows, we ought naturally to 

 expect to find the makers of them, as frequently at least as Cliona, 

 in peaceable undisputed possession of their habitations. But 

 no, Cliona alone occupies these cavities ; no worm has yet been 

 taken that the naturalist can pronounce to have made them. 

 Then why not allow Cliona to be the fabricator, as it is the 

 constant possessor, of its excavated home ? 



Dr. Bowerbank indeed mentions " several" instances of having 

 found annelids occupying the "numerous sinuous canals" in 

 large balani from tbe Guliot caves in Sark ; and also of finding 

 "living annelids in deeply-seated portions of the perforations in 



* " Monograph of the British Spongiadse." Vol. II, p. 217 and 221. 



