The oyster has also a heart and blood-vessels (Fig. 30) and blood; in some few bivalves and snails the blood is red like our own. The beating of the heart may be seen by careful examination of a freshly opened specimen. The oyster has also a “liver,” or digestive gland, and a kidney and a soft, branched, tubular structure embedded in the body, within which the egg-cells and sperm-cells grow by means of which the oyster propagates itself in the summer. Our north European oyster produces in the same individual both egg-cells, and the male fertilising sperm-cells or spermatozoa. The eggs are just visible to the unaided eye (Fig. 31), and as many as a million are produced in the warm breeding season by a single ripe oyster. About a fortnight after the eggs have been shed, the same tubular chambers in the oyster’s body which produced the eggs by growth from their inner walls, produce the spermatozoa, so that they are too late to fertilise the eggs of the same oyster. They pass out of the oyster into the sea water, and are carried within the shelter of the shells, and so on to the surface of the protected bodies of other neighbouring oysters by the currents created by the “ciliated” gill-plates of these neighbours. [Illustration: FIG. 31.—The eggs of the oyster—taken from a ripe individual—magnified 500 times linear.] The sperm particles or spermatozoa (Fig. 32) are produced by millions, and form a cloud finer than dust in the sea water. They are carried within the shells of both egg-producing and sperm-producing oysters, and are driven along into the openings of the tubular reproductive sacs, and into those sacs in the case of those oysters which are at the time producing eggs. There they fertilise the eggs. The minute eggs begin to develop whilst still within the parent’s body, and continue to do so whilst remaining within the shelter of the shell, adhering to the gill-plates (Fig. 33). In a day or two each fertilised egg has developed into a very minute creature, provided with a tiny circlet of cilia or vibratile hairs, the movements of which cause it to swim (Fig. 33F). The parent oyster is now said to be “white-sick.” In the course of a couple of days the young oyster still within its parent’s shell becomes dark in colour, and has formed on its surface a pair of symmetrical shells, not like those of an adult oyster, but convex (Fig. 34) like those of a clam or a cockle. The head region, with its circlet of vibrating cilia, can be projected between the open shells or withdrawn between them when the shells are shut. The mother oyster, laden with these little dark specks, is now said to be “black-sick.” [Illustration: FIG. 32.—The sperms or spermatozoa of a ripe oyster, as seen swimming in a drop of sea water: magnified 2000 times linear.]