It used to be thought (about twenty-five years ago) that the molluscs expand their bodies in this manner by taking water, through definite apertures provided with valves, into their blood, and that, having thus swelled themselves out, they could shrink and reduce themselves by pouring out again the in-taken water. The behaviour of some other marine animals, namely the sea-anemones, which really do act in this way, made this explanation of the swelling and shrinking of molluscs seem probable. It was also known that the star-fishes and sea-urchins actually do take in the sea-water into a system of vessels connected with their wonderful sucker-bearing tentacles. But it turned out on close examination that the molluscs do not take in or shed out water in this way. A hole, which was thought to let in water into the blood of sea-snails, was shown to be only the opening of a great slime-gland. In the case of some bivalves which have red-blood corpuscles, I showed that the blood is never made paler, nor are the red corpuscles shed during the great distensions and contractions of the body. Measurements were made to determine the removal of water from a glass jar by an expanding sea-snail, and it was found that none is removed or taken up; in fact, the whole of what is very often an astonishingly large and bulky distension of the foot, or of lobes of the body, and the subsequent rapid shrinking of the same parts, depend entirely on the blood being injected from the rest of the body into the swelling part, and squeezed from it into the depleted region when the swollen part shrinks again. The firm, opaque shell hides from view the change of shape of the concealed body, and we see only the distended foot or other lobes which project from the shell.