THE MANTLE- CO VERED ANIMALS. i r i 
beat roughly, and to be safe it is necessary to with- 
stand them. But the mussels (M, Fig. 42) do not, 
like oysters, cement themselves down for life. They 
have a different stratagem which enables them to get 
free if they wish. They have below their body a 
muscular flap, which goes by the name of a "foot," 
and is made up of layers of muscle crossing and 
Fig. 42. 
Group of headless Mollusca. 
C, Cockle * springing. M, Mussel, t S, Scallop. J R, Razor-fish. 
f, Foot ; t, anchoring threads ; si, breathing siphons ; e, eyes of scallop. 
recrossing each other. In this foot there is a deep 
groove, out of which they force a milky fluid which 
hardens into threads (/) and anchors them to the 
rock. Any one who has tried to wrench mussels 
from their bed, knows how strongly these threads 
hold ; and if you remove the mussels carefully and 
put them in an aquarium, you may see them anchor 
* Cardium. t Mytilus. Pecten. Solen. 
