and on the removal of portions of their Shells. 243 



Pholas and Teredo little more than the mere contraction of the 

 cutting surface is sufficient ; each portion of the foot and mantle, 

 which together nearly fill up the bottom of the excavation, acting 

 immediately on the substance with which it is in contact. The 

 same thing takes place in Patella, which evidently does not 

 rotate, for the burrows are elliptical like the animal, and fit with 

 great accuracy the marginal indentures of the shell. But the 

 cutting disc of Saxicava and Gastrochcena being narrower than 

 the burrows, these species must, at intervals, move a little from 

 side to side, anchoring themselves afresh by the byssus whenever 

 they shift their position. In all, however, the same vermicular 

 contraction of the parts observed by Sir Everard Home in the 

 foot or " proboscis " of Teredo will be required to remove the 

 substances into which these animals bore. 



Thus this perplexing subject is simplified ; and judging from 

 analogy, there can be little doubt that all the boring mollusks 

 excavate in the same manner : none by the rasping or cutting of 

 their valves, — none by a solvent, — none by ciliary currents*. We 

 should therefore be inclined to doubt that any of the Acephala 

 bore, which are not provided with either the broad adhesive foot 

 or with the thickened mantle united in front. Venerupis per- 

 forans may be, perhaps, cited as an exception to this rule ; but 

 it is doubtful whether it ever bores. On the coast of Northum- 

 berland, where there is abundance of soft shale and a great variety 

 of rocks, it certainly never does so : but it frequently takes up its 



* The stone-boring Annelides will probably be found to work out their 

 excavations in a similar manner. The Cliona celata of Grant would also 

 appear to make the chambers it inhabits in the shells of bivalves and in 

 other substances by mechanical agency. This curious production was sup- 

 posed by its describer to take up its abode in excavations made by marine 

 worms ; but after carefully examining the cavities occupied by the Cliona, 

 there can be no doubt that they are the work of this creature, most pro- 

 bably aided by its siliceous spicula, which penetrating the surface of the ani- 

 mal give to every portion of it the character of rasping-paper. In this case 

 such an apparatus, at first sight, might be thought inefficacious, as the 

 sponges are not contractile ; but the Cliona is an exception to this general 

 law. Dr. Grant states that it is " distinctly irritable," and describes the 

 papillae as being highly contractile ; and it is worthy of remark, that this, the 

 only boring species, is the only one that possesses this power. Dr. Grant 

 also states that it adheres so closely to the smooth parietes of the cavities 

 that it cannot be removed without tearing — a fact corroborative of the 

 theory now advanced, but not at all favourable to that of an acid solvent or of 

 ciliaiy currents. Were either of these agents employed, it is not likely that 

 laceration would attend the removal of the animal, as no very intimate 

 connexion could exist between it and the walls of the bore. But from the 

 nature of the apparatus, as just described, it is evident that the creature 

 would be held in its place by the siliceous points on its surface, and thereby 

 removal rendered difficult. 



17* 



