LAMELLIBRANCHIA.] MOLLUSCA 687 that the Monomya have been developed from Isomya- like ancestors, and have lost by atrophy their anterior adductor. The single adductor muscle of the Monomya is separated by a difference of fibre iy -- into two portions, but neither of these can be regarded as possibly represent ing the anterior adductor of the other Lamelli branchs. One of these portions is more ligamentous, and serves to keep the two shells con stantly attached to One another whilst Fio. 127. Left valve of the same shell from the inner , , a , face. (Figs. 125, 126, 127 from Owen.) the more fleshy por tion serves to close the shell rapidly when it has been gaping. In removing the valves of the shell from an Anodon, it is necessary not only to cut through the muscular attach ments of the body-wall to the shell but to sever also a strong elastic ligament, or spring resembling india-rubber, joining the two shells about the umbonal area. The shell of Anodon does not present these parts in the most strongly marked condition, and accordingly our figures (figs. 125, 126, 127) represent the valves of the Sinupalliate genus Cytherea. The corresponding parts are recognizable in Anodon. Referring to the figures (125, 126) for an ex planation of terms applicable to the parts of the valve and the markings on its inner surface corresponding to the muscular area which we have already noted on the surface of the animal s body we must specially note here the posi tion of that denticulated thickening of the dorsal margin of the valve which is called the hinge (fig. 127). By this hinge one valve is closely fitted to the other. Below this hinge each shell becomes concave, above it each shell rises a little to form the umbo, and it is into this ridge-like upgrowth of each valve that the elastic ligament or spring is fixed (fig. 127). As shown in the diagram (fig. 127*) representing a transverse sec tion of the two valves of a Lamelli branch, the two shells form a double lever, of which the toothed-hinged is the fulcrum. The adductor muscles placed in the concavity of the shells act upon the long arms of the lever at a mechanical advantage ; their con traction keeps the shells shut, and stretches the ligament or spring h. On the other hand, the ligament h acts upon the short arm formed by the umbonal ridge of the shells ; when ever the adductors relax, the elastic substance of the ligament contracts, and the shells gape. It is on this account that the valves of a dead La mellibranch always gape ; the elastic ligament is no longer counteracted by the effort of the adductors. The state of closure of the valves of the shell is not, therefore, one of rest ; when it is at rest that is, when there is no muscular effort the valves of a Lamellibranch are slightly gaping, and are closed by the action of the adductors when the animal is disturbed. The ligament is simple in Anodon ; in many Lamellibranchs it is separated into two layers, an outer and an inner (thicker and denser). That the condition shells, ligament, and ad ductor muscle, a, b, right and left valves of the shell ; c, d, the umbones or short ai ins of the lever ; e, f, the long arms of the lever ; g, the hinge ; h, the ligament ; t, the adductor muscle. of gaping of the shell-valves is essential to the life of the Lamellibranch appears from the fact that food to nourish it, water to aerate its blood, and spermatozoa to fertilize its eggs, are all introduced into this gaping chamber by currents of water, which are set going by the highly- developed ctenidia. The current of water enters into the sub-pallial space at the spot marked e in fig. 124, (1), and, after passing as far forward as the mouth w in fig. 124, (5), takes an outward course and leaves the sub-pallial space by the upper notch d. These notches are known in Anodon as the afferent and efferent siphonal notches respectively, and correspond to the long tube-like afferent inferior and efferent superior " siphons " formed by the mantle in many other Lamellibranchs (fig. 130). Whilst the valves of the shell are equal in Anodon we find in many Lamellibranchs (Ostraea, Chama, Corbula, &c.) one valve larger, and the other smaller and sometimes flat, whilst the larger shell may be fixed to rock or to stones (Ostraa, &c.}. A further variation consists in the develop ment of additional shelly plates upon the dorsal line be tween the two large valves (Pholadidae). In Pholas dadylus we find a pair of umbonal plates, a dors-umbonal plate and a dorsal plate. It is to be remembered that the whole of the cuticular hard product produced on the dorsal surface and on the mantle-flaps is to be regarded as the " shell," of which a median band-like area, the ligament, usually remains uncalcified, so as to result in the production of two valves united by the elastic ligament. But the shelly substance does not always in boring forms adhere to this form after its first growth. In Aspergillum the whole of the tubular man tle area secretes a continuous shelly tube, although in the young condition two- valves were present. These are seen (fig. 129) set in the firm substance of the adult tubular shell, which has even replaced the ligament, so that the tube is complete. In Teredo a similar tube is formed as the animal elongates (boring in wood), the original shell- valves not adhering to it but remaining mov able and provided with a special muscular apparatus in place of a ligament. Let us now examine the organs which lie beneath the mantle-skirt of Anodon, and are bathed by the current of water which cir- Fig. 128. Fi- 129- Fio. 128. Shell of Aspergillum vaginiferum (from Owen). Fio 129 Shell of Aspergillum vaginiferum to show the original valves a, now embedded in a continuous calcification of tubular form (from Owen). culates through it. This can be done by lifting up and throwing back the left half of the mantle-skirt as is re presented in fig. 124, (3). We thus expose the plough- like foot (/), the two left labial tentacles, and the two- left gill-plates or left ctenidium. In fig. 124, (5), one of
the labial tentacles n is also thrown back so as to show