686 MOLLUSCA [LAMELLIBKAXCHIA. other Lamellibrancliia. Tlie Swan Mussel lias superficially a perfectly-developed bilateral symmetry. The left side of the animal is seen as when removed from its shell in fig. 124 (1). The valves of the shell have been removed by severing their adhesions to the muscular arese h, i, 1-, /, m, u. U) (6) y FIG. 124. Diagrams of the external form and anatomy of Anodonta cygnea, the Pond-Mussel ; in all the figures the animal is seen from the left side, the centro-dorsal region uppermost, as in the drawings of fig. 75, which compare. (1) Animal removed from its shell, a probe g passed into the sub-pallial chamber through the excurrent siphonal notch. (2) View from the ventral surface of an Anodon with its foot expanded and issuing from between the gaping shells. (3) The left mantle-flap reflected upwards so as to expose the sides of the body. (4) Diagrammatic section of Anodon to show the course of the alimentary canal. (5) The two gill-plates of the left side reflected upwards so as to expose the fissure between foot and gill where the probe g passes. (6) Diagram to show the positions of the nerve-ganglia, heart, and nephridia. Letters in all the figures as follows : a, centro-dorsal area ; b, margin of the left mantle-flap ; c, margin of the right mantle-flap ; d, excurrent siphonal notch of the mantle margin ; e, incurrent siphonal notch of the mantle margin ; / foot ; g, probe passed into the superior division of the sub-pallial chamber through the excurrent siphonal notch, and issuing by the side of the foot into the inferior division of the sub-pallial chamber ; h, anterior (pallial) adductor muscle of the shells ; i, anterior retractor muscle of the foot ; fc, protractor muscle of the foot ; I, posterior (pedal) adductor muscle of the shells ; m, posterior retractor muscle of the foot ; n, anterior labial tentacle ; o, posterior labial tentacle ; p, base-line of origin of the reflected mantle-flap from the side of the body ; q, left external gill-plate ; r, left in ternal gill-plate ; rr, inner lamella of the right inner gill-plate ; rg, right outer gill-plate ; s, line of concrescence of the outer lamella of the left outer gill- plate with the left mantle-flap ; t, pallial tentacles ; u, the thickened mus cular pallial margin which adheres to the shell and forms the pallial line of the left side ; v, that of the right side ; w, the mouth ; x, aperture of the left organ of Bojanus (nephridium) exposed by cutting the attachment of the inner lamella of the inner gill-plate ; y, aperture of the genital duct ; z, fissure between the free edge of the inner lamella of the inner gill-plate and the side of the foot, through which the probe g passes into the upper division of the sub-pallial space ; aa, line of concrescence of the inner lamella of the right inner gill-plate with the inner lamella of the left inner gill-plate ; ab, ac, ad, three pit-like depressions in the median line of the foot supposed by some writers to be pores admitting water into the vascular system ; ae, left shell valve ; a/, space occupied by liver ; ag, space occupied by gonad ; ah, muscular substance of the foot ; ai, duct of the liver on the wall of the stomach ; ak, stomach ; o , rectum traversing the ventricle of the heart ; am, pericardium ; an, glandular portion of the left nephridium ; ap, ventricle of the heart ; uij, aperture by which the left auricle joined the ventricle ; or, non-glandular por tion of the left nephridium ; as, anus ; at, pore leading from the pericardium into the glandular sac of the left nephridium ; OK, pore leading from the glandular into the non-glandular portion of the left nephridium ; av, internal pore lead ing from the non-glandular portion of the left nephridium to the external pore x aw, left cerebro-pleuro- visceral ganglion ; ax, left pedal ganglion ; ay, left otocyst ; a.*, left olfactory ganglion (parieto-splanchnic) ; Jib, floor of the pericardium separating that space from the non-glandular portion of the nephridia. The free edge of the left half of the mantle-skirt b is repre sented as a little contracted in order to show the exactly simi lar free edge of the right half of the mantle-skirt c. These edges are not attached to, although they touch, one another ; each flap (right or left) can be freely thrown back in the way which has been carried out in fig. 124, (3) for that of the left side. This is not always the case with Lamellibranchs ; there is in the group a tendency for the corresponding edges of the mantle-skirt to fuse together by concrescence, and so to form a more or less completely closed bag, as in the Scaphopoda (Dentalium). In this way the notches d, e of the hinder part of the mantle-skirt of Anodon are in the Siphonate forms converted into two separate holes, the edges of the mantle being elsewhere fused together along this hinder margin. Further than this, the part of the mantle-skirt bounding the two holes is frequently drawn out so as to form a pair of tubes which project from the shell (figs. 130, 141). In such Lamellibranchs as the oysters, scallops, and many others which have the edges of the mantle-skirt quite free, there are numerous tentacles upon those edges. In Anodon these pallial tentacles are confined to a small area surrounding the inferior siphonal notch (fig. 124, (3), t). The centro-dorsal point a of the animal of Anodonta (fig. 124, (1)) is called the umbonal area ; the great anterior muscular surface h is that of the anterior adductor muscle, the posterior similar surface i is that of the posterior adductor muscle ; the long line of attachment u is the simple " pallial muscle," a thickened ridge which is seen to run parallel to the margin of the mantle-skirt in this Lamellibranch. In some of the Siphonate Isomya, which are hence termed " Sinupallia," the pallial muscle is not simple but deeply incurved at the posterior region so as to allow of the large pallial siphons being retracted within the shell or expanded at will (fig. 127, and figs. 140, 141). It is the approximate equality in the size of the anterior and posterior adductor muscles which has led to the name Isoyma for the group to which Anodon be longs. The hinder adductor muscle may be considered as re presenting morphologically the transverse fibres of the root of the foot of Nautilus by which it adheres to its shell (fig. 91, &), the annular muscular area of Patella (fig.27,c), andthecolumella muscle of the Gastropods generally. It is always large in Lamellibranchs, but the - anterior adductor may be very small (Heteromya), or FlG , J . /, r o absent altogether (Monomya). The anterior adductor muscle is in front of the mouth and alimentary tract altogether, and must be regarded as a special and peculiar deve lopment of the median anterior part of the mantle -flap lunale ligament n w valves . f the shell of Cytherea (one of the Sinupalliate Isomya), from the dorsal aspect. FIG. 120. Bight valve of the same shell from the outer face. in Heteromya and Isomya. Amongst those Lamelli branchs which have only a posterior adductor (Monomya), it is remarkable that the oyster has been found (by Huxley) to possess, when the young shells and muscles first develop, a wejl-marked anterior adductor as well as a
posterior one. Accordingly there is ground for supposing