332 NATURAL HISTORY. 
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 Veneraceze. The foot of these, of the Cockles, and 
of the Pholadacez, the group to which the Teredo be- 
longs, is used mostly for burrowing. 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 answering 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- 
tion of dikes by this little Mollusk. 
572. One of 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- 
cae ward in the sand. Now it gives it 
the shape of a bell-clapper, as at 0, 
and the end furnishing it a hold in 
the sand, it moves its body forward 
a: b 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- 
