96 A NATURAL HISTORY OF THE SEAS 



In the Class Scaphopoda are included the Tooth Snails 

 (Dentalium) of our shores, which possess white shells like 

 miniature elephant tusks only an inch or so in length. 

 The animal digs itself into the sand by means of a foot 

 protruded from the shell's wide end. Our native species 

 abounds on the northern coast in the sand below three 

 fathoms, where it gathers in large companies, feeding 

 on Veraminiphera, a minute bivalve. A large tropical 

 species reaches about six inches in length and has a beauti- 

 fully-fluted shell of a jade green colour. 



The Lamellibranchia, or Scale-keeled Molluscs, include 

 the numerous company of bivalves, or shells, which 

 are in two separate but interlocking pieces — common 

 examples being the oyster, cockle, mussel, clam. They 

 abound in every sea and a large number are of the highest 

 economical importance to man. 



The typical bivalve has the shells united by a complex 

 interlocking hinge and tough ligament. The shells are 

 further held together on the inside by a stout " adductor ,: 

 muscle, either end being attached to one of the valves, 

 and which by contracting or expanding causes the two 

 shells to open or shut. The animal has the gills arranged 

 in a series of plates or leaves and possesses two syphon 

 pipes — the one an inhalent, the other an exhalent, by means 

 of which food is drawn into the shell and waste matter 

 afterwards expelled. There is frequently a fleshy foot, 

 which can be employed for creeping, leaping or climbing 

 and it also not infrequently weaves a tough fibrous bunch 

 of threads employed to anchor the shell securely to some 

 solid substance. 



Bivalves are usually divided into four main groups, 



