Between the labial palpi is the mouth, which leads into the stomach by a short, wide tube, and then into a convoluted tube which finally passes through the heart, and terminates near the exhalent siphon as above described. The whole length of this tube may be followed by careful dissection, its direction being determined at short intervals by probing it with a bristle that has been tipped with a little melted sealing wax. It will be seen to wind through the base of the foot, surrounded through the greater part of its course by the digestive gland, from which a digestive fluid enters it through small ducts. The diagram on p. 194 shows the general internal anatomy of a lamellibranch, parts of which have been removed to reveal the underlying structures. The animal lies in its left valve, the right valve, the right mantle lobe, and the right set of gill-plates having been completely dissected away. The whole course of the digestive tube has also been exposed, and the positions of the three nerve ganglia, with their connecting nerve cords, constituting the central portion of the nervous system, are also indicated. It will be interesting, finally, to learn the direction taken by the water currents which supply the animal with air and food in their course through the system. Passing in through the inhalent siphon, the water immediately enters a large cavity between the mantle lobes. This cavity (the _branchial cavity_) contains gills, as we have already seen, and also extends to the mouth. The water, urged on by the motion of myriads of minute ciliated cells in the walls of the cavity, passes in part through the digestive tube, and in part around, between, and through the gill plates, which are perforated by numerous holes. After thus completely bathing the gills, and supplying the oxygen necessary for respiration, this latter current passes into a second cavity above the gills, and thence into the exhalent siphon, where it mingles with the fluid from the digestive tube as well as with other excretory matter. Lamellibranchs are, as a rule, exceedingly prolific, a single individual of some species discharging more than a million ova in one season. The larvæ swim freely in the water, and are provided with eyes that enable them to search for their food, but the eyes always disappear when the young settle down to a more sedentary life. It is true that adult bivalves sometimes possess visual organs, often in the form of conspicuous coloured spots on the edge of the mantle, these, however, are not the same that existed during the larval stage, but are of a more recent development.