150 INTRODUCTION TO ZOOLOGY CHAP. 



or any pieces of wood submerged in the sea. The narrow 

 shells have two equal valves, are from one to three inches 

 long, and of a beautiful blue colour. Sea-mussels can move 

 freely on their long, narrow, fleshy foot, but can also fix 

 themselves by a number of threads which form the organ 

 of attachment known as the byssus (cp. Fig. 92). Each 

 thread is secreted, from a gland in the foot, as a very fine 

 viscid string which hardens in the water. 

 Dreissena Closely allied to this Sea-mussel is the little 

 " Chambered " or " Zebra " Mussel (Dreissena poly- 

 morpha), which, although it is not truly indigenous to Britain, 

 is now found widely distributed in our docks and rivers, as 

 well as in brackish and salt waters. 



It has a two-valved shell very similar to Mytitus, but 

 there is no inner pearly lining. Also there is a little 

 shell-plate that stretches across from side to side of the shell 

 near the umbo, giving the creature the name of " chambered 

 mussel." It attaches itself to a support by byssal threads 

 in the same way as Mytilus. 



There are many other well-known marine bivalves which 

 can only be shortly referred to here. 



The Common Edible Cockle (Cardium edule) 

 Th^Cockle ^ ves buried in the sand or mud, often above low 

 water mark, with its two short respiratory siphons 

 projecting just out of the sand into the water. Although 

 it lives, on the whole, such a retired life, the cockle can 

 leap freely over the surface by means of its large curved 

 foot. 



Scallops are perhaps even better known than 

 TheScallop coc kles on account of their ornamental shells, 

 which are fluted and prettily coloured, red and 

 yellow. The lower valve of the shell, on which the creature 

 rests when adult, is more deeply concave within than the 

 upper valve. The young scallops can swim freely by the 

 alternate sudden opening and shutting of their shells. The 

 Common Scallop (Pecten maximus) is often to be seen in the 

 fishmonger's shop, where the bright colouring of the body, 

 especially the bright orange of the foot, makes it a con- 

 spicuous object. Round the double margins of the mantle 

 black eyes are situated. 



