57'i THE MUSCLES. 



upon land, and likewise of swimming on the surface 

 of the water. When this animal happens to be 

 deserted by the tide 5 it opens its shell to the full ex- 

 tent, then shuts it with a sudden jerk, by which it 

 often rises five or six inches from the ground. In 

 this manner it tumbles forward till it regains the 

 water. When the sea is calm, troops or little fleets 

 of Scallops are often observed swimming on the 

 surface. They raise one valve of their shell above 

 the surface, which becomes a kind of sail, while the 

 other remains on the water, and answers the pur- 

 pose of a keel, by steadying the animal, and thus 

 preventing its being overset. When an enemy ap- 

 proaches, they instantly shut their shells, plunge to 

 the bottom, and the whole fleet disappears By 

 what means they are enabled to regain the surface 

 we are still ignorant. 



THE MUSCLES. 



The Muscle tribe is distinguished by the shell 

 being bivalve, without any tooth in the hinge, but 

 in having the hinge marked with a longitudinal hol- 

 low line ; and by the animal's being generally fixed 

 to some substance by a byssus or silky beard. 



Some of the Muscles penetrate into the interior of 

 calcareous rocks, where they reside out of the reach 

 of danger. Others adhere by their beard to the exte- 

 rior of rocks or stones j and so very tenacious is their 

 hold that, in the larger species, they cannot be 



