390 BIVALVES. 



along the groove. Thus alternately extending 

 and contracting this muscular instrument, the 

 Mytilus contrives to creep to a convenient situ- 

 ation 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 crea- 

 ture is called the Pisum, or Pea Crab : it is sup- 

 posed 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, whom 

 it repays for a safe retreat, by going in search of 

 provender, and sharing it with him. 



MYTILUS Margaritiferus* 



PEARL-BEARING MUSCLE. 



Specific Character. Shell suborbicular, com- 

 pressed, the margin rounded, except on the hinge 

 side where it is strait and transverse ; surface 

 imbricated with transverse membranaceous scales, 

 having the laminae toothed in rays ; colour com- 

 monly greenish or pale chesnut, inside pearly and 

 iridescent ; the full grown shells are sometimes 

 ten or twelve inches long and rather less in 

 breadth : they are thick, and ponderous, but 

 young specimens are thin, brittle, and slightly 

 eared. 



* Plate X. figure 3. 



