To the naked eye the gills appear to be formed of radiating fibres of admirable structure; but the microscope shows that each leaf consists of a vast number of straight transparent and tubular filaments, arranged side by side so close that 1,500 of them might be contained in the length of an inch. These filaments, however, apparently so numerous, in fact consist of only one exceedingly long filament in each gill, bent upon itself again and again throughout its whole length, both at the fixed and free ends of the leaf. These long filaments are fringed on both sides by lines of cilia continually vibrating in contrary directions. By this action a current of water is perpetually made to flow up one side of the filaments and down the other, so that the blood which circulates in their interior is exposed throughout their long winding course to the action of oxygen in the water. The duration of these vibrations in the mollusca is marvellous. The cilia on a fragment of a gill put into water by Mr. Gosse fifteen hours after the death of the mollusk caused a wave to flow uniformly up one side of the filaments and down the other. Even twenty-hours after the death of the animal the ciliary motion was continued on such parts as were not corrupted, a remarkable instance of the inherent contractility of the animal tissues.
The refined mechanism of the gills of the common Mussel enables it to live when attached to rocks above high-water mark, so as only to be immersed at spring tides. By the movements of cilia, water is retained in the gill-chamber, which derives oxygen from the atmosphere, and animalcula supply the Mussel with food.
Fig. 165. Cardium or Cockle.
The mollusks that burrow in sand or mud have two tubes fringed with cilia, which they protrude into the water above them. The water which is drawn into one of these tubes by the action of the cilia passes in a strong current over the gills, aërates the blood, brings infusorial food for the animal, and is expelled in a jet from the other tube. The foot at the other extremity of the shell is the organ with which the mollusk makes its burrow in sand, clay, chalk, stone or wood.[[41]]
The common Cockle digs into the sand, and uses its foot both for digging and leaping; it is cylindrical, and when the Cockle is going to leap, it puts out its foot and bends it into an elbow; then having fixed the hooked point firmly in the sand, by a sudden contraction of the muscles it springs to a considerable height and distance, and leaps actively along the surface of the sand. The lowest part of [fig. 166] is a magnified section of the foot, showing the muscular system which gives the animal that power. It consists of many rows of longitudinal muscles, interlaced at regular distances by transverse fibres. When the foot is extended, the Cockle has the power of distending it by filling a network of capillary tubes with water till it is almost transparent. The water is also distributed through the body and into the gill-chamber, which opens and shuts every ten minutes or oftener, in order to maintain the supply; and it has egress through the pores in the mantle and foot, for some burrowing mollusks squirt it out through the foot when disturbed. This water-system is unconnected with the circulation of the blood.
Fig. 166. Foot of Cockle.
Each bivalve mollusk is both male and female; and the fertilized eggs pass into the gills of the parent, where they undergo a kind of incubation. At a certain time the yellow yolk of the egg is divided into a granular mass, which separates from the liquid albumen and produces cilia. The cilia cause the albumen to revolve round the interior of the egg; at last the granular mass revolves with it, while at the same time it rotates about its axis in a contrary direction at the rate of six or eight times in a minute. When still in the egg, all the organs of the little embryos are formed in succession, even the little valves of the shells are seen to open and shut, but the embryos are hatched before they leave the parent, and swim about in the cavity of the external gill.