Molluscs. 



31 



eight inches or even more. We usuahy come across empty 

 shells only, but the living creature may be found at 

 extremely low water buried deep in the sand ; being sought for 

 as bait, the fishermen catch them bv means of a barbed- wire 



riG. 31. TELLIMA BALTHICA (lEFt), DONAX VENUSTUS (RIGHT), 



AND MYTILUS EDULIS. 



thrust quickly into the small holes in the sand which betray 

 their presence. Preferring the open sea, the Razor-shell usually' 

 avoids estuaries. As in all burrowers, the foot is large and may 

 protrude considerabl)' from the gaping valves of the shell. 



The Bivalves mentioned above are able to move about, 

 but many among our British representatives of the group are, 

 except when quite young, permanently attached to rocks, 

 wrecked ships, breakwaters, wooden piles of piers, etc. So it 

 is with the Edible Mussel (Mytilus edulis) , of which such large 

 colonies are to be found on our coasts and in the brackish water 

 of the mouths of rivers (Fig. 31). As behoves its sedentary exist- 

 ence, the Mussel has the foot much reduced and unfit for locomo- 

 tion, but it secretes a bundle of long, tough threads, known as 

 the byssus, by means of which it attaches itself. As in the 

 cockle, there is a separation of the sexes. A larger and 

 rarer mussel, the Horse Mussel [M. modiolus), often twice 



