114 CALIFORNIA FISH AND GAME. 
In structure the two species are very similar. Externally, there is 
the bivalve-shell, narrowed to the wmbo or ‘‘beak’’ at the anterior end, 
and hinged at the ‘‘back’’ or dorsum by an elastic piece of cartilage- 
like substance that tends to pull the shell open (see fig. 60). Internally, 
the shell is lined with a membrane, called the mantle, for the protection 
of the animal within; the mantle bears the shell-forming glands that 
serve both to add to the size of the shell and to repair breaks (see fig. 61). 
During most of the year the mantle is a very thin-looking tissue indeed, 
but in the spawning-season it becomes greatly thickened and swollen by 
the eggs or the milt. On each side, between the mantle and the ‘‘body”’ 
of the animal are the gills, two layers of extremely delicate tissue; they 
can be distinguished from the mantle by reason of the very fine longi- 
tudinal lines which mark them. 
Fig. 60. Exterior of California mussel... Photograph by W. C. Mathews. 
The ‘‘body’’ contains the digestive tract and bears the foot. In 
an opened mussel, one can see, at the anterior end, four leaf-like pieces 
of tissue growing out from around a slit-like opening; this opening is 
the mouth or Eo: and the pieces of tissue surrounding it are the 
labial palps, which by their motion help to direct the food into the 
mouth. The remainder of the digestive tract is rather difficult to trace ; 
its course will be merely outlined. From the mouth the food passes into 
the gullet, forward of the foot, and then into the stomach, which lies in 
a mass of dark-colored tissue, the so-called liver; the intestine passes 
back to the big posterior adductor muscle, around which it. bends, then 
passes forward to the heart and back again to the muscle already 
referred to. Like other molluses, the mussel has a closed circulatory 
system, with heart and blood vessels; it has also a very simple and 
primitive nervous system. The tongue-like foot is the organ of locomo- 
tion during the juvenile period; it contains the byssus gland, which 
will be referred to more fully farther on. 
