42 FIRST BOOK OF ZOOLOGY. 



forcing the valves apart. The way in which it works might 

 be illustrated by placing a piece of rubber inside the hinge 

 of a door : when the door is closed, the rubber is squeezed, 

 and the tendency would, of course, be for the rubber, in ex- 

 panding, to again push the door open. 



That it requires a continual effort for the clam to keep 

 the valves closed, is seen in the fact that when these 

 creatures are allowed to remain out of water for a while, as 

 they are when in the market, the muscles get tired, and, 

 relaxing, the shells partly open. If, now, the basket or 

 barrel which they are in be suddenly shaken, the clams 

 will as suddenly close, and a rustling sound is made, as the 

 water is forced out from the gill-cavity, the water often 

 squirting out in a stream from the siphonal openings. 



41. On the rocks between high and low water mark, and 

 adhering to the piles of the wharves, may be found clusters of 

 mussels which are attached to these places, and to each other, 

 by little brown threads which issue from between the valves 

 below. These threads are made at will by the creature, one 

 by one, and are fastened to the substances upon which they 

 rest. The threads are called lyssal-threads, and, combined, 

 form the byssus. 



If the pupils will collect a number of these salt-water 

 mussels, and place them in a large glass dish or bowl filled 

 with salt-water, they may watch the mussels as the creatures 

 attach themselves to the sides of the vessel. In the figure, 

 the byssus is seen like threads coming from between the 

 valves below, with their ends adhering to the stone. 



