30 THE LIFE OF THE MOLLUSCA 
The muscular foot is a prominent feature in most 
Bivalves, lying in the middle line towards the front 
of the body (Plate XV., Figs. 3, 6-8, f). Towards 
the back of it is situated the gland for spinning the 
horny threads by which many Bivalves anchor them- 
selves to stones, etc. The Common Mussel and the 
Zebra Mussel are familiar examples, and the bunch 
of threads is known as the “byssus” (Plate XV., 
Fig. 9, 6). The animal generally has the power of 
rejecting its byssus at any time and spinning a 
fresh one. 
The mouth is situated a little behind and beneath 
the front adductor muscle; it is unarmed, neither 
jaw nor radula being present. It is flanked on either 
side by a pair of twin, triangular lobes, the “ labial 
palps” (Plate XV., Fig. 3, ¢), which are in a line 
with, and in front of, the gills. Their function is 
apparently to collect, and possibly to taste, the food 
before it passes into the mouth. 
Each valve of the shell is a hollow, irregular, 
shallow cone, the apex of which, termed the “ beak,” 
or “umbo,” is the point at which growth began, and 
is, in fact, the young shell, or ‘ prodissoconch”’: it 
generally differs in shape and markings from the 
later growth. 
The umbo is usually curved more or less to one 
side (Plate XVI., Figs. 1 and 2, u), and generally 
points towards the head, or anterior end, when the 
shell is said to be “ prosogyre,” in contradistinction 
