332 NATtTEAL HISTOBT. 



dian and Australian waters. There is , a pair of these 

 shells in the Church of St, Sulpice, in Paris, used as re- 

 ceptacles of holy water, which weigh over five hundred 

 pounds. The common Clam belongs to a different group, 

 the Veneracese. The foot of these, of the Cockles, and 

 of the Pholadacese, the group to which the Teredo be- 

 longs, is used mostly for bun-owing. Most of them bur- 

 row in sand or mud, some in rocks, and some in wood. 

 Those that burrow or bore in hard substances can not 

 do this with the foot. It is done with a sort of rasping 

 operation of the edges of the shell, the foot answeiing in 

 this case only as a means of holding on while the animal 

 bores. The Teredo, by this boring operation, is largely 

 destructive to ship bottoms, piles, etc. Holland has been 

 sometimes threatened with an inundation by the destruc- 

 tigiLof dikes by this little Mollusk. 



' 572. Oneof the most interesting of the Mollusks which 

 burrow in sand is the Razor-shell, so called from its length. 

 It can burrow very rapidly, and therefore it is quite dif- 

 ficult to catch it. It bores in the sand with its foot, 

 which it can elongate so as to make it quite pointed. Its 

 burrow is recognized by the little jet of water which it 

 throws out when it is alarmed. If a little salt be thrown 

 upon its hole it will make its appearance, but one must 

 be quick in his movements to catch it before it can get 

 out of sight again. Its mode of burrowing is very curi- 

 ous. It puts its foot into a dagger-shape, as represented 

 at a in Fig. 263, and thrusts it down- 

 ward in the sand. Now it gives it 

 the shape of a bell-clapper, as at 5, 

 and the end furnishing it a hold in 

 the sand, it moves its body forward 

 J / \ by shortening the foot. By repeated 

 Fig. 263. movements of this kind it gets along 



quite rapidly in the loose sand. 



573. What is stated in the previous paragraph exem- 

 plifies one of the many modes in which the foot of Mol- 



