PEARLY FRESH-WATER MUSSELS OF THE UNITED STATES. 
281 
running water. The adductor muscles allow the shell to open but a short distance, 
so that the teeth always lock, and the mantle cushion swells when tliey are open and 
prevents them from slipping. There can be no doubt but that the hinge teeth are 
developed in the river mussels to prevent the valves of the shell from twisting on 
each other, which they would be likely to do in swift currents or in time of tloods. 
The Anodontas, living in still water, have thinner shells and do not need any 
locking teeth. AVithin the mantle, tilling a large part of the shell and hung along the 
hinge line, is a sort of bag which contains the vital organs and is called the abdominal 
sac. This extends below and in front into a tough sort of hatchet or tongue shaped 
organ that is pushed out of the shell when the animal moves, and by expanding and 
contracting and moving forward and backward the whole is plowed along in the sandy 
or muddy bottom, leaving a little furrow. This is called the foot. Up under the 
forward adductor muscle is an opening into the abdominal sac called the mouth. It 
is carried through, as a tube, much folded and bent back on itself, the intestinal canal, 
and finally empties near the posterior adductor. Surrounding this canal, as it passes 
along the back of the animal, ^ 
is a sort of heart, which beats 
regularly. Fastened to the 
mantle and the top of the ab- 
dominal sac in some cases, 
hanging down between the two, 
and reaching from the hinder 
end of the shell well toward the 
front, are, on each side of this 
sac, two curtains or flaps of the 
most daintily beautiful and 
delicate texture, and these are 
organs of vital importance to 
the mussels. They are the 
gills or branchife, and answer 
to the gills of the fishes or the 
lungs of land animals. Just in front of them and near the mouth, on either side of 
the abdominal sac and under the mantle, is another pair of smaller flaps, somewhat 
triangular in our species, and extending behind, which are called the palpi, and when 
the animal is feeding these are constantly in motion. At the hinder part of the ani- 
mal the mantle, which is not fastened together here in our species, shows two small 
openings, one above the other, by having its edges pressed close together between 
and below these openings. One or both of these is fringed, and when tlie animal is 
feeding these fringes may be seen beautifully expanded between the hinder parts nf 
the shell. The upper is the anal and the lower the branchial opening. 
When the animal feeds, the front part of the shell is usually buried in the mud or 
sand, leaving the hinder part to lu'oject free into the water; the shell is opened, the 
branchial and anal openings are spread, the palpi forward begin a rapid flapping, 
which draws in a current of pure water through the branchial opening. This passes 
through the gills, aeratiug the blood, then into the mouth and along through the 
intestinal canal, carrying in confervie and microscopic forms of life which serve as 
food for the animal, and on out at the anal opening. 
Fio. 3 — Anatomy of female Lampsilix hiteolus. M., mantle folded back 
showing below; P., labial palpi; F., foot; I., inner gill; Mm., outer gill 
with the hinder part transformed into a marsnx>ium; B., branchial opening; 
Mo., position of mouth. 
