54 THE LIFE OF THE MOLLUSCA 
under surfaces of big stones. The Mussels, as well 
known, attach themselves to the rocks by their stout 
byssus threads, their clustered masses affording 
secure shelter to many lesser animals; while in 
mangrove swamps the Oysters will attach them- 
selves to the branches of the trees that dip in the 
water at high tide—a fact which was observed and 
recorded by W. Smith at Sierra Leone in 1726. 
Periwinkles, Top Shells (Trochidz and Turbinide), 
and other holostomes, haunt the tangled masses of 
seaweeds ; while among the siphonostomes are the 
Dog-Periwinkles (Purpura), Dog-Whelks (Nassa), etc., 
all stout-shelled forms capable of withstanding con- 
siderable buffeting amid the waves. The Piddock, 
or Pholas, Saxicava, Lithodomus, and other boring 
molluscs, excavate burrows in various rocks, in coral, 
and even the shells of their bigger confréres. Others, 
like Tapes and Coralliophaga, too lazy to make their 
own retreats, take possession of the deserted burrows 
of others. The latter generally selects the crypt of 
a dead Lithodomus, which it closely resembles in 
shape (Plate XXXII., Figs. 17 and 18), and the 
shells of successive generations of Coralliophaga, 
packed one within the other, will be found lying 
between the valves of ‘the original architect of the 
home. In the cracks and crevices of the rocks the 
Octopods hide. The majority of the Pelecypods, 
such’as the Cockles, Ark-Shells, 7, apes, etc., remain 
more or less buried in the sand or silt, the Razor-fish 
