88 
bulletin of the bureau of fisheries. 
with the horse mussel, Modiola modiola, which it most closely resembles. Close obser- 
vation, however, will show that the umbo or beak of the horse mussel is not at the 
extreme end of the shell, but a short distance back near one margin, and that the 
epidermis is brown instead of blue. 
Internally, the most conspicuous part of the body is the mantle (pi. xix, fig. 3 and 4), 
which is made up of two lobes, each attached to and filling one of the two valves of the 
shell. Just before breeding, the mantles are thick and fleshy and assume a characteristic 
color by means of which it is possible, in a general way, to distinguish the two sexes. 
The males are white or pink, while the females vary from an orange to a brick red 
color. Another means of distinguishing the sexes is to note the surface character of 
the mantles, which in males shows closely aggregated follicles filled with spermatozoa 
(pi. xix, fig. 1); in the females it presents a uniform granular appearance containing 
scattered groups of pigment cells (pi. xix, fig. 2). During the quiescent period the 
mantles are thin and almost transparent. 
The foot (pi. xix, fig. 3), so well marked in the fresh-water mussel, is a muscular 
organ of small size in the sea mussel, tongue-like in form, with a longitudinal groove 
on the underside. Its hinder portion contains the byssus gland, which secretes the 
byssus or “beard” for the attachment of the mollusk (pi. xix, fig. 3). 
There are three important sets of muscles in addition to those in the foot. (1) The 
adductors (pi. xix, fig. 3) are two in number. They extend across from one valve to the 
other and serve for closing the shell. The posterior adductor is the large muscle which 
it is necessary to cut before the shell is opened. The anterior adductor is inconspicuous 
and located, as its name implies, at the front end of the shell. (2) The retractors (pi. xix, 
fig. 3), which are two in number and serve for withdrawing the foot, are long, narrow, 
paired muscles attached to the foot, from which one pair extends forward and the other 
backward to attach to the shell. (3) The pallial muscles (pi. xix, fig. 3) are a row of 
delicate structures along the border of the mantle which serve to attach it to the shell. 
The digestive tract has a complicated arrangement. It consists of a large mouth 
(pi. xix, fig. 3) located at the anterior end just in front of the foot, a short gullet opening 
into a stomach which is surrounded by a large, dark-colored digestive gland, sometimes 
called the liver (pi. xix, fig. 4). From the posterior end of the stomach the intestine 
passes backward to the posterior adductor muscle, where it turns forward in an oblique 
manner to the left side of the stomach. At this point it turns back again and passes 
through the ventricle of the heart and over the posterior adductor muscle to the anus, 
which is a short distance behind this muscle. The labial palps (pi. xix, fig. 3), two pairs 
of loose flaps which lie just inside the edge of the mantle attached to the lower lip of 
the mouth, may be considered as accessory structures of the digestive system. They 
are covered with cilia and serve to direct food to the mouth. 
The gills (pi. xix, fig. 3) are a pair of filamentous structures extending along each 
side of the body from between the inner and outer palps to the posterior end of the 
animal. In cross section they present the form of a narrow W attached by the central 
part of the letter; the outer and inner arms remain free at their upper ends. 
