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.
Lamellibranchs are classified in various ways by different authorities, the arrangement being based principally on the number and position of the adductor muscles, or on the nature of the gills. For our present purpose we shall look upon them as consisting of two main divisions—the Asiphonida and the Siphonida, the former including those species which do not possess true tubular siphons, the inhalent and exhalent openings being formed merely by the touching of the mantle lobes; and the latter those in which the mantle lobes are more or less united and tubular siphons formed. Each of these divisions contains a number of families, most of which have representatives that inhabit the sea; and we shall now note the principal characteristics by which the more important families are distinguished, and take a few examples of each, starting with the Siphonida.
Examining the rocks that are left exposed at low tide we frequently find them drilled with holes that run vertically from the surface, seldom communicating with each other within, and varying in diameter from less than a quarter of an inch to half an inch or more. Some of these holes are the empty burrows of a boring mollusc, while others still contain the living animal in situ.
The molluscs in question belong to the family Pholadidæ, which contains a number of species that exhibit very remarkable features both as regards structure and habit. The shell is very thin and fragile, but yet composed of hard material, and its surface is relieved by a series of prominent concentric ridges that bear a number of little rasp-like teeth. It gapes at both ends, has neither true hinge nor ligament, and is often strengthened externally by two or more extra or accessory valves. The hinge-plate is a very peculiar structure, for it is reflected over the exterior of the umbones, above which they are supported by about ten thin shelly plates, the whole thus forming a series of chambers. The accessory valves are supported by these bridged structures, and a long, straight, calcareous plate also fills the space along the dorsal side of the shell in some species. The muscular scars and the pallial line are distinctly seen on the inner surface, and a peculiar curved shelly plate projects from under the umbo of each valve.
Fig. 132.—Pholas dactylus