INCLUSA. PHOLAS. 415 



of the animals composing it; which have the power of forming their 

 habitations, not only in sand and mud, but by excavating wood 

 and rock. No general description can be given of the shells of 

 this group ; for although they often bear some resemblance to 

 those of other bivalves, they frequently depart from them so 

 completely, as to leave their character in doubt, until the struc- 

 ture of the animal producing them has been examined. The 

 tubes cannot in any instance be drawn within the valves ; and 

 these frequently cover but a small part of the whole surface. 

 We then often find a kind of supplementary shell, formed by a 

 calcareous exudation, lining the hollow which the animal has 

 pierced ; and this sometimes involves the original shell, so that 

 the latter is entirely lost in it. 



959. As connecting this Order with the preceding, and as 

 departing least widely from the general type of Bivalves, we 

 may first mention the P kolas ; this has a shell formed of two 

 principal valves, which leave a considerable space between them 

 at each end when they are closed j and of supernumerary 

 pieces, the number and position of which vary considerably. 

 The foot issues at the opening through the larger end ; and the 

 respiratory siphons, which are very long and extensible, pass out 

 by the other. Some Pholades form their cells in mud or clay ; 

 but many in rocks, and others in wood. It is evident that a 

 fleshy foot can be of little use in the excavation of a stony mass ; 

 and the organ here appears to serve a different purpose. The 

 boring operation seems to be performed (in many cases at least) 

 by the shell itself, which has a rasp-like surface, and which is 

 renewed by vital action as fast as it is worn down. In order to 

 make the valves rotate backwards and forwards (like a surgeon's 

 trephine), the foot is affixed to the bottom or end of the hole, 

 and becomes a fixed point from which the muscles can act. 

 They seem to commence this operation almost as soon as they 

 quit the egg ; the young beginning to bore the rock on which 

 they are cast, and enlarging their cell, which they never volun- 

 tarily quit, in accordance with their own increase in size. They 

 possess a very curious means of freeing the tube from the rasp- 

 ings of the rock produced by their penetration. The siphon 



*FF2 



