304 



TJBCiS INHABITANTS OF THE SEA. 



which, sadly irritating the nerves of the poor creature, generally 

 brings it to the surface. He must, however, be very quick in 

 grasping it firmly, for should he fail, the animal speedily sinks 

 again into the sand and will remain there, being either in- 

 sensible to the additional irritation or its instinct of self- 

 preservation teaching it to remain beneath. 



The pholades, which have very delicate milk-white valves, 

 burrow holes in limestone or sandstone rocks, though occa- 

 sionally they content themselves with houses of clay or turf. 

 How creatures invested with shells as thin as paper and as 

 brittle as glass are able to work their way through hard stone 

 has long been a puzzle to naturalists, some of whom asserted 

 that they attained their object by means of an acid solvent, 

 others that they bored like an auger by revolving ; but recent 

 investigations have discovered that their short and truncated 

 foot is the chief instrument they use in their mining operations, 

 being provided at its base with a rough layer of sharp crystals 

 of flint, which, when worn off, are soon replaced by others, and 

 K*! act as excellent files. 



In several of the sedentary genera the rudi- 

 mentary foot, though incapable of locomotion, makes 

 itself useful by spinning a bundle of silken threads, 

 called byssus, or beard, which serve to anchor the 

 animal to any solid submarine object as firmly as a 

 ship in harbour. Generally the connection is per- 

 manent, but some species, among others the edible 

 mussel, are able to detach the filaments from the 

 glandular pedicle situated at the inferior base of the 

 foot which originally secreted them, and then to seek 

 another point of attachment. 



If the byssus be examined under a powerful lens, 

 before any of the filaments are torn, it is easy to per- 

 ceive that these are fixed to submarine bodies by 

 means of a small disc-like expansion of their ex- 

 tremities of various extent, according to the genus 

 and species. Certain genera are celebrated for the 

 abundance and fineness of their byssus ; that of the 

 Pinnae, or Wing-Shells, among others, which are very common 

 in some parts of the Mediterranean, and attain a considerable 

 size, is so long and firm that in Naples it is sometimes manu- 



