mid on the removal of portions of their Shells. 233 



are made with great rapidity. It is stated by Mr. Thompson, in 

 the paper before alluded to, that Teredo is known " to have de- 

 stroyed the keel of a vessel afloat in the short space of four or live 

 months.^^ And a piece of deal is said to have been riddled by 

 it in the course of forty days *. Saxicava and its allies, as well 

 as the Pholades, make their excavations of a pyramidal form, 

 increasing downwards as the animal enlarges. The boring must, 

 therefore, cease so soon as the shell has attained its full size, or 

 the excavations would be continued with the sides parallel. From 

 this it would appear that the burrows of these animals are the 

 work of a very limited period — that of growth. Neither must it 

 be forgotten, that the usual currents produced by cilia are so ex- 

 ceedingly feeble and limited as to require the aid of a lens to 

 exhibit them. But Mr. Garner supposes that their velocity, and 

 consequently that their strength, may be increased by being 

 drawn through the elongated body of the animal. Should the 

 currents be the ordinary siphonal currents, — and this seems to be 

 Mr. Garner's opinion, — it is difficult to conceive how they could 

 subserve their ordinary functions ; and how the delicate tissues 

 of the gills could escape injury, were the water hurried over their 

 surfaces in the manner supposed. 



The foot of Pholas is, undoubtedly, ciliated ; but the external 

 wall of the siphonal tubes, which we have seen enlarges the chan- 

 nel of the burrow, is not ; neither is the mantle, and it, as will 

 afterwards appear, is a very material agent in boring. The foot 

 of the Acephala is usually — perhaps always — clothed with vibra- 

 tile cilia ; there is therefore nothing extraordinary in their pre- 

 sence in this case. They are probably for the purpose of clean- 

 sing the surface, and may in this way be eminently useful to the 

 boring mollusks. Mr. Garner, however, appears to apply his 

 theory more particularly to Saxicava and its allies. But the 

 mantle of Saxicava rugosa is completely closed in front, and 

 consequently there is no outlet for the currents at the very place 

 where the greatest friction is required : further back there is a 

 small opening for the foot and byssus ; but wxre the currents to 

 pass by this orifice, the byssal attachment would be destroyed and 

 operations thereby stopped. Gastrochana also has no frontal 

 outlet, neither has Clavagella. There are no cilia on the anterior 

 portion of the animal of Saxicava — that portion which is the 

 excavating instrument. Towards the end of the foot, which is 

 small and narrow, there are certainly cilia as in the other Ace- 

 phala ; but it is impossible for a moment to entertain the opinion 

 that the currents produced by these could work out the excava- 

 tions of this species. 



* Penny Cyclopa3dia, vol. xxiv. p. 224. 



