328 Mr. A. Hancock on the Excavating Powers of Sponges, 



the branches bifurcate ; — in short the young Cliona has now 

 assumed the character of the mature sponge (fig. 2). Thus we 

 can trace Cliona from its earHest stage of growth, — not larger 

 than the gemmule of Halichonclria and resembUng it in form, — 

 up to its perfect development, step by step, excavating its compli- 

 cated habitation in sound shell, within which it lies closely im- 

 bedded, but unobscured by its pearly envelope which is perfectly 

 free from decay and is untouched by worms : to neither of which 

 by any constrained imagination can the chambers in this instance 

 be attributed ; — Cliona makes them for itself. And now having, 

 I trust, established this fact, we shall endeavour to ascertain the 

 nature of the apparatus by which these sponges work out their 

 abode, — a subject of much difficulty. 



The mollusks being furnished with a shell, the investigation 

 into the nature of their excavating instrument is much compli- 

 cated. The burrowing sponges, however, having no such hard 

 covering, we have in them only the animal to look to for an ex- 

 planation. The excavations of Cliona and Thoosa can only be 

 effected by the surface of the sponge, aided either by some 

 minute mechanical instruments in connexion with it, or by a 

 solvent : unless, indeed, the water-currents of the papillae, as 

 hinted by Professor Grant, be thought equal to perform the 

 task. But were these currents of sufficient magnitude to 

 penetrate rapidly into shell or hard limestone, it is difficult to 

 see how they could be brought into effective operation. The 

 papillae are closely adherent to the sides of the orifices through 

 which they protrude; and here the water could have no effect; and 

 yet these orifices are at first small, and are afterwards consider- 

 ably increased in size. And at those points where the water is 

 drawn into the sponge, the currents, of course, cannot be sup- 

 posed to act in the way proposed. To show, however, how in- 

 adequate these minute currents are to work out the chambers of 

 Cliona, which we have seen are form^ed very rapidly, we have 

 only to reflect on the comparatively slow action of the enormous 

 currents of the sea, — of the tidal currents, and of those resulting 

 from the lashing of the waves. The puncturing of the sides of 

 the chambers also seems unfavourable to such an hypothesis. We 

 shall not therefore stop to discuss this branch of the subject 

 further, but at once inquire how far a solvent is likely to be the 

 agent. 



The extreme simplicity of the organic structure of these beings 

 forbids a belief in the existence of a special secreting apparatus. 

 If therefore a solvent fluid be the agent, it must be supposed to 

 exude from the entire surface of this humble animal. The 

 character of the excavations would also lead to the same con- 

 clusion ; for it is evident that the form of the sponge is influen- 



