THE PHOLADES. 



151 



of the foot which originally secreted them, and then to attach 

 themselves anew to some other object. If the byssus be ex- 

 amined under a powerful lens, before any of the filaments are 

 torn, it is easy to perceive 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. 



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

 burrow holes in limestone or sandstone rocks, though occa- 

 sionally they content themselves with houses of clay. How 

 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 dis- 

 covered that their short and truncated foot is the chief instru- 

 ment they use in their mining operations. For it is not only 

 extremely muscular, but provided at its base with a rough 

 layer of sharp crystals of flint, which when worn off are soon 

 replaced by others, and act as excellent files. Thus we find in 

 the same class of animals the same organ most variously modi- 

 fied in form and structure, now serving as a foot, now as a 

 spade, or as a spinning-machine or a rasp, and throughout all 

 these modifications admirably adapted in every case to the 

 mode of life of its possessor. The bur- 

 rowing molluscs must have been in per- 

 petual danger of suffocation if their branchiae 

 had not been carefully protected against the 

 ingress of mud or sand. To prevent this 

 danger, their respiration is generally effected 

 by means of a double siphon (a 6), one for 

 the entrance and the other for the exit of the 

 water, a perpetual change of which is abso- 

 lutely indispensable to the life of the animal. 

 The interior of these tubes is lined with in- 

 numerable delicate cilia, by the action of 

 which the surrounding water is drawn to- 

 wards the entering orifice, and conveyed 

 in a strong current through the tube over 

 the surface of the gills. Then, having been deprived of its 

 oxygen, it is poured through the other tube, and expelled 



