8 
gans. Still farther back, on the middle of the posterior face 
of the adductor muscle, is the anus, a long vertical slit, open- 
ing into the space between the lobes of the mantle and above 
the posterior ends of the gills. 
In front of the gills, that is between them and the hinge, 
there are four fleshy flaps— the lips — two on each side of the 
body. They are much like the gills in appearance, and they 
are connected with each other by two ridges which run 
across the middle of the body close to the anterior end, and 
between these folds is the large oval mouth, which is thus 
seen to be situated, not at the open end of the shell, but as 
far away from it as possible. As the oyster is immovably 
fixed upon the bottom, and has no arms or other structures 
for seizing food and carrying it to the mouth, the question 
how it obtains its food at once suggests itself. If a frag- 
ment of one of the gills is examined with a microscope, it 
will be found to be covered with very small hairs, or cilia^ ar- 
ranged in rows. Each of these cilia is constantly swinging 
back and forth, with a motion something like that of an oar 
in rowing. The motion is quick and strong in one direction 
and slower in the other. As all the cilia of a row swing to- 
gether, they act like a line of oars, only they are fastened 
to the gill, and as this is immovable, they do not move for- 
wards through the water, but produce a current of water in 
the opposite direction. This action is not directed by the an- 
imal, for it can be observed for hours in a fragment cut out 
of the gill, and if such a fragment be supplied with fresh sea 
water, the motion will continue until it begins to decay. 
"While the oyster lies undisturbed on the bottom, with its 
muscle relaxed and its shell open, the sea water is drawn on to 
the gills by the action of the cilia, for although each cilium is 
too small to be seen without a microscope, they cover the 
gills in such great numbers that their united action produces 
quite a vigorous stream of water, which is drawn through the 
shell and is then forced through very small openings on the 
surfaces of the gills into the water tubes , inside the gills, and 
through these tubes into the mantle cavity, and so out of the 
