190 BIVALVES. 



the shell is drawn along the groove. Thus 

 alternately extending and contracting this mus- 

 cular instrument, the Mytilus contrives to creep 

 to a convenient situation 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 so have been placed 

 in the shell of the Mytilus and other Bivalves, 

 to assist by its sagacity, 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 straight and transverse ; 

 surface imbricated with transverse membra- 

 naceous scales, having the laminae toothed in 

 rays ; colour commonly greenish or pale ches- 

 nut, 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. 



