EXCAVATORS. 



345 



harbours and docks, in the course of two or three 

 seasons." l 



Another mollusk excavates not only wood but 

 stone (fig. 67). It is the P kolas, of which there are 

 several species. The pair of shells are very thin and 

 delicate, gaping at both ends, and when applied to 

 the animal, with the syphons ex- 

 tended, has an elongated conical 

 form, which is doubtless very 

 useful in its boring operations. 

 Although living usually in stone 

 or wood, they may be detached, 

 and examined " all alive," with 

 the pair of syphons extended at 

 the narrow end. The water enters 

 by the larger syphon, and is ex- 

 pelled again from the smaller. 

 The lining membrane is coated 

 with vibratile cilia, which, by their 

 incessant action, keep up the 

 current of the water. A great 

 deal has been written on the subject of how its 

 perforating operations are performed, which resolves 



FIG. 67 (Pholas 

 dactyl its}. 



1 Quatrefages, "Rambles of a Naturalist" (London, 1857), 

 vol. ii. p. 231. The author states that one pound of corrosive 

 sublimate or two pounds of " sugar of lead " would destroy all 

 the organic corpuscles in more than twenty thousand cubic 

 yards of water. 



