280 
BULLETIN OF THE UNITED STATES FISFI COMMISSION. 
L. — Diagram of interior of TJnio. B.,beak; 
• adductor scars: P. L., pallial line; A. E., ; 
L., ligament; H.H., hinge; A., ante- 
nterior end; P. E., posterior end. 
tions, one on each valve, and opposite; these are called the beaks or uniboes, ana are 
tlie oldest part of the shell. The rounded end of the shell is the front or anterior part, 
as this is always ahead when the animal is moving, and the pointed end opposite is 
the hinder or iiosterior part. In most of the heavier species there are developed inter- 
locking or hinge teeth 
along the upper inner 
edge of the shell. 
If a living mussel is 
taken and a tliin knife is 
inserted at the front and 
hinder ends and a cut 
made toward the hinge, 
it will gape and the ani- 
mal may be examined. 
A thin veil of soft ani- 
mal tissue, called the 
mantle, covers the entire 
inside of lioth valves, 
reaching out to their 
edges, and Joined together at tlie upper i^art of the shell. It is fastened to the shell 
near the border in a slightly indented furrow called the pallial line, and by muscles 
at the upper part of it. The edge and outside, of this mantle throw out a sort of 
milky liquid, containing carbonate of lime and animal matter, which builds up the 
shell, thickening it 
from the inside and 
adding to it at the 
edge; the brilliant 
nacre next to it, 
a layer of vertical 
prismatic cells out- 
side this, and over- 
all the protecting 
epidermis. The 
mantle is carried 
between the lock- 
ing hinge teeth, fill- 
ing the space be- 
tween them like a 
cushion. At each 
end of the shell 
inside, near the 
back, will be found 
a mass of tough, 
wldte, mu.scular libers runrdng from valve to valve, which have been cut into by the 
knife, and these are the powerful adductor muscles by which the animal closes the shell. 
Some of the N'aiarlcs have thin shells and no hinge teeth, the Anodonfas, for example, 
and these almost invariably live in stagnant or slow-moving water. The Unios, 
the thicker- shelled forms, which have well-developed teeth, live for the most part in 
Fig. 2. — Diagram slioTving position of Tnio plowing 
its M-ay throngli the bottom of .a river. S.,s1ip11; 
F.,foot; Hr., brancbial 0 ])ening; A., anal oxtening. 
Arrows indicate direction of tbo animal and tlie 
currents. 
