THE CABLE OF THE PINXA. 61 



Headless moUusks have, from their structure, little 

 communication with objects around them, and but very 

 limited powers of locomotion. Some, it is true, raising 

 themselves on the edge of their valves, urge them- 

 selves along by means of the foot, as we see in the 

 fresh-water mussel, and leave a furrow in the soft mud, 

 the index of their progress. Some, by this organ, bury 

 themselves in the mud, and others, by the alternately 

 opening and smartly closing their valves, can remove 

 themselves from one situation to another. 



The cable of the Pinna. 



Many, on the contrary, are fii-mly fixed to the rocks by 

 a calcareous exudation from their shells ; and clusters 

 are often thus cemented together, forming greater or 

 smaller masses. Others attach themselves to the rocks 

 by a cable, or byssus, as the common edible mussel, 

 and the pinna. This cable consists of threads, which 



