THE MOLLUSCA 113 



The Pectens, or Scallops, in their early days dart through 

 the water by rapidly opening and shutting their valves, but as 

 they grow older they become more sedate and spend most of 

 their time resting on one spot, much as oysters do. The Pecten 

 has a small but distinct foot, which is not used as a means of loco- 

 motion, but is employed by some species to spin a small bunch 

 of threads called the byssus, or beard, by which the mollusc attaches 

 itself occasionally to its resting-place. The eyes of the Pecten 

 are very remarkable. They number from 80 to 120, and lie in 

 two rows on the borders of the mantle. When the valves of the 

 shell are open these eyes may be seen shining like jewels through 

 the folds of the mantle, which depends like a misty veil from the 

 edges of the shell. 



The Limas (Limidte) are very like the pectens, but their shells 

 are always white and the fringe of the mantle is very long and 

 beautifully coloured. These molluscs are more active than the 

 pectens, and dart through the water with quick, jerky movements, 

 flapping the valves of their shells like a pair of wings. The Limas 

 are sometimes called " Sea- butterflies " a name, by the way, which 

 is given indiscriminately to a great many different sea-creatures. 

 They are remarkable for making a kind of nest with pieces of 

 coral, stones, and shells, bound together with threads spun from 

 the byssus gland, and in this several Limas will live contentedly 

 together. 



In the Mussels the byssus gland is very highly developed, and 

 the molluscs habitually anchor themselves by their strong threads 

 to their resting-place. They form huge beds of tens of thousands, 

 connected together by their byssus, and it is a common sight to 

 see great masses of the shells clinging together on the rocks and 

 breakwaters down by low-water mark, with small stones, grains 

 of sand, fragments of shells, etc., all mixed up in the network 

 of threads. The attachment, however, is not entirely permanent, 

 for a mussel will sometimes cast off the byssus and move away 

 to another place. 



The Edible Mussel (Mytilus edulis) is found near the coast in 

 most parts of the world, from the Arctic to the Antarctic Ocean ; 

 it is used very largely as bait as well as food, but is often very 

 unwholesome and even poisonous to many people. 



The characteristic foot of a bivalve is axe-shaped ; it is often 

 I 



