I58 BULLETIN OF THE BUREAU OF FISHERIES. 



are present as a single layer of small bundles. The circular layer surrounds the foot with 

 a thin sheath of fibrils which lies just beneath the surface layer of pigmented, ciliated 

 epithelium. Near the tip of the foot on the right and left sides the circular muscles 

 give rise to numerous oblique muscles which run in various directions in such manner as 

 to form a coarse network. 



The purpose of this system of arrangement is obvious when the function of the foot 

 is known. By contraction of the circular muscles the foot is thrust out as a long slender 

 organ which may be directed in its course by the longitudinal and oblique muscles; 

 contraction of the powerful longitudinal muscles with the synchronous relaxation of the 

 circular muscles serves to draw the foot into a short, thick organ. The importance of 

 these movements will be discussed later when the formation of the byssus and the move- 

 ment of the mussel are described. 



The anterior retractors of the byssus and foot (fig. 141, A Ret, opp. p. 158) arise 

 from the base of the byssus as a pair of cylindrical muscles which run forward on the 

 ventral surface of the body (fig. 140, ARet, opp. p. 1 58) slightly diverging in the form* of a 

 letter V. They are inserted in elliptical impressions which lie on the dorsal anterior 

 end of the shell parallel with the ligament. These impressions are about three times as 

 long as they are broad, but are not symmetrical with each other, one usually being longer 

 proportionately than the other. Although the name infers that these muscles are re- 

 lated to the foot, they really are not in the adult, for all the fibers terminate at the base 

 of the byssus or are interwoven with the fibers of the posterior retractors of the bvssus. 



The posterior retractors of the foot and byssus may be described together since 

 they are fused together in such close relation (fig. 140, PRet; fig. 141, PRB). Whereas 

 the anterior retractors consist of a single cylindrical pair of muscles, those of the posterior 

 retractors consist of several paired bundles which may vary in number from three to six 

 or even more. They arise from the base of the foot and byssus as a single powerful 

 muscle which divides up into separate bundles that spread out on either side in a fanlike 

 manner and terminate in the impression of the valve which runs forward from that of 

 the posterior adductor muscles parallel with the dorsal edge of the shell. The most pos- 

 terior bundle runs directly over and in contact with the posterior adductor. The most 

 anterior bundle arises from the base of the foot itself and properly constitutes the 

 retractor of the foot (fig. 141, PRB and PRF). 



The pallial muscles, or those of the mantle edge, are present on the ventral, poste- 

 rior, and dorsal border of the mantle. They are composed of numerous small bundles 

 of fibers which are separated a short distance from each other and run perpendicularly 

 to the outer edge except in the region dorsal and anterior to the posterior adductor 

 muscle where they slope backwards obliquely to the outer edge of the mantle (fig. 133, 

 Pal, p. 149). The muscles are most strongly developed in the posterior region where the 

 inner mantle fold is thicker and in the area about the anal syphon. 



The anal muscles (fig. 117, An, opp. p. 138) are merely modified pallial muscles 

 which arise from the wall of the anal syphon and are inserted in the shell impression 

 which forms the triangular area on the posterior ventral edge of the impression made by 

 the posterior adductor muscle. 



With such a muscular system the sea mussel is wonderfully adapted for living in an 

 environment where it is subjected to strong currents, the surge of the ocean, and other 

 forces which exert great strains upon it. The pallial muscles firmly bind the edges of 



