3 o 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,/). 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, b). 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, t), 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, «.), and generally 

 points towards the head, or anterior end, when the 

 shell is said to be " prosogyre," in contradistinction 



