MYTILUS COMMON MUSCLE. 185 



This shell is the most common of all the British 

 species, and is found in large beds, attached by 

 a byssus. The mollusca have a tongue-shaped 

 muscular foot, capable of great powers of elon- 

 gation and contraction ; with this instrument 

 they are able to effect a progressive motion. 

 Forming a furrow in the sand, and placing their 

 shell erect, they stretch out the foot, which be- 

 ing rather viscid, adheres to the ground, and an 

 effort being made to contract it, the shell is drawn 

 along the groove. Thus alternately extending 

 and contracting this muscular instrument, the 

 Mytilus contrives to creep to a convenient situa- 

 tion for anchorage, and by means of this same 

 foot it then forms a coarse byssus, which fixes it 

 to the chosen spot. Often towards the end of 

 autumn, a little crab is found sheltering itself 

 within the valves of the muscle. This little 

 creature is called the Pisum, or Pea Crab : it is 

 supposed to have been placed in the shell of the 

 Mytilus and other Bivalves, to assist by its saga- 

 city, the more limited powers of its host, and to 

 repay him for a safe retreat, by going in search 

 of provender, and sharing it with him. 



MYTILUS Margaritiferus.* 

 PEARL-BEARING MUSCLE. 



Specific character. Shell sub orbicular, com- 

 pressed, the margin rounded, except on the hinge 



* Plate X. figure 3. 



16* 



