Mr. A. Hancock on the Excavating Sponges » 235 



of Dr. Bowerbank^s experience. Many worms and other ma- 

 rine animals conceal themselves in any hole or crevice they can 

 find, and numerous worms or annelids perforate both shells 

 and limestone and other hard calcareous bodies. Living worms 

 occupying their own burrows in these substances are frequently 

 met with ; and it is not uncommon to find such burrows in 

 shells perforated by Cliona, and mingling in the most intri- 

 cate manner with the excavations of the latter. But there is 

 never any difiiculty in determining which was made by the 

 worm and which by the sponge. And if the instances men- 

 tioned in the ' Monograph of the British Spongiadse' are genuine 

 worm -burrows, neither can there be, in these cases, any uncer- 

 tainty as to the fact of their being so. 



Worm-burrows are always linear, usually cylindrical, and are 

 more or less tortuous ; they never assume a dendritic form, are 

 sometimes double, or as it were bent upon themselves, and a little 

 flattened ; the surface is invariably smooth, never punctured 

 or shagreened, as it is in the burrows of Cliona, the excava- 

 tions of which, on the contrary, are always dendritic, dividing 

 dichotomously, anastomosing, usually constricted at intervals 

 by perforated septa, so as to form a congeries of small chambers, 

 and having the surface constantly punctured or shagreened, 

 and generally giving oif on every side numerous delicate csecal 

 tubes. 



To account "for the vast number of perforated shells, and 

 the comparative rareness of the Annelids,^' it is suggested, in 

 the work on the British Spongiadse before quoted*, that the 

 worms assumed to have made these perforations obtain their 

 nutriment by passing the excavated substance, " abounding in 

 animal matter,'^ through the digestive organs, the analogy of 

 the earthworm being relied on. Unfortunately, however, for 

 the advocacy of such an idea, the excavations inhabited by Cliona 

 are of the same character and equally extensive in limestone. 

 Whatever 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, how- 

 ever much they might obtain from shell. This fact, indeed, 

 sufficiently disproves the nutritive theoi-y; 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 obstacle in the 

 way of attributing these excavations to worms, and which ap- 

 pears to be perfectly insurmountable. In all the excavations 

 * Vol. ii. p. 220. 



