CALIFORNIA FISH AND GAME. as 
Finally, there are three sets of muscles which can be found more 
or less readily. First, there are the adductor muscles with which the 
animal closes his shell; one of these, the posterior adductor, is the large 
muscle which must be cut before the shell ean be opened; the other, the 
anterior adductor, is a small muscle at the anterior end. Then there 
are the muscles which protrude and retract the foot; these are fastened 
‘‘fore and aft,’’ some of them lying parallel to the hinge; the outlines 
of the posterior retractors are shown in the photograph (fig. 61). 
Lastly, the fine pallial muscles serve to attach the mantle edge to the 
shell. 
When the soft parts have been removed, one can see, on the inside 
of the shell, the ‘‘scar’’ where the posterior adductor muscle was 
attached, and the pallial line which marks the region of the edge of the 
mantle. 
SO te 
Fig. 61. Interior of California mussel showing body structure. Photograph by W. C. Mathews. 
It is not yet known when the Pacific coast mussels spawn. Dealers 
believe that spawning takes place as early as April or May, but at San 
Diego, in 1917, the mussels certainly were not in spawning condition 
before the latter part of July. However that may be, the resulting 
young mollusks are able to swim about within a very few hours. They 
continue to be free swimming (or, to express it more explicitly, they are 
carried about by the tital currents) for about four or five days. 
At any rate, this is what Dr. Field (1909) reports for the 
Atlantic coast mussels, and it is likely that curs behave in much the 
same fashion.* The young then grow a shell, begin crawling over solid 
objects by means of the foot, and at last attach themselves to something 
solid. The attaching threads, constituting the byssus, or ‘‘beard,’’ are 
produced by the byssus gland previously mentioned. These threads are 
*FKield, Irving A. 1909. Food value of sea mussels. Bull. U. S. Bureau of Fish- 
eries, vol. 29, pp. 85-128. 
