[Illustration: FIG. 35.—Young of the pond-mussel after escaping from the maternal gill-pouch: A, as it escapes, swimming by opening and shutting the shells; _sh_, shell of one side; _al_, shell-muscle; _t_, teeth of the shell’s edge; _by_, adhesive filament. B, after it has fixed to a fish; _mt_, mantle; _f_, muscular foot; _br_, gill processes; _pad_, _aad_, _al_, muscles; _auv_, heart. (From drawings by the late Frank Balfour.)] A beautiful little bivalve common in weedy streams in England is known as _Cyclas_ (it has no English name); it has a pair of shells shaped like those of a cockle, but smooth, and only half as big as one’s little finger-nail. The nursing of the young in the gill-sacs is carried to a much further point by _Cyclas_ than by the pond- or river-mussel. Before they are ejected by the parent they are quite large—like their parent in appearance, and half as big as a hemp-seed. Necessarily there are not many produced in a season—there is not room for more than twenty or thirty young in the gill-sacs. XVI THE HEART’S BEAT The beat of the heart is one of those great and elemental features of man’s life which, in spite of our familiarity with it and its momentary recurrence, never loses its quality of mystery and isolation. The ceaseless accompaniment to our lives which the heart is always beating, like the inexorable stroke of an unseen pendulum, fills even the stoutest and bravest at times with a sense of awe. It seems now and then as though an independent living thing were in our breasts, and when it quickens and struggles, as it were, with its work, or languishes and hesitates in its efforts we have a sense of helpless domination by an existence—a living thing—over whose vagaries we have no control. The heart of man is no special endowment of the human race, nor even of the higher animals. As I mentioned a few pages back, the oyster and other shell-fish have a heart which keeps time and beats the seconds for their uneventful lives, as does that of man for his more varied career. Not only the molluscs, but the insects, the spiders, the crabs, lobsters, and shrimps, and even the worms, have each a rhythmically beating heart. In all of them the significance of this heart and its beat are the same—it is driving the nourishing, oxygen-carrying blood through the great vessels (arteries), which branch from it like a tree into the living tissues of the body, whence it returns by other vessels (the veins) back to the heart.