688 MOLLUSCA [LAMELLIBRANCHIA. the mouth >, and the two left gill-plates are reflected so as to show the gill-plates of the right side (rr, rq) pro jecting behind the foot, the inner or median plate of each side being united by concrescence to its fellow of the opposite side along a continuous line (aa). The left inner gill-plate is also snipped so as to show the subjacent orifices of the left nephridium x, and of the genital gland (testis or ovary) y. The foot thus exposed in Anodon is a simple muscular tongue-like organ. It can be protruded between the flaps of the mantle (fig. 124, (1), (2)) so as to issue from the shell, and by its action the Anodon can slowly crawl, or burrow in soft mud or sand. It has been sup posed that water is taken into the blood-vessels of the Anodon through pores in the foot, and in spite of opposi tion this view is still maintained (Griesbach, 47). In fig. 124, (2) the letters ab, ac, ad, point to three pit-like depres sions, supposed by Griesbach to be pores leading into the blood-system. According to Carriere (48) these pits are nothing but irregularities of the surface ; in some cases they are the entrances to ramified glands. Other Lamelli- branchs may have a larger foot relatively than has Anodon. In Area it has a sole-like surface. In Area too and many others it carries a byssus-forming gland and a byssus- cementing gland. In the Cockles, in Cardium, and in Trigonia, it is capable of a sudden stroke, which causes the animal to jump when out of the water, in the latter and the posterior a continuation of the inner gill-plate. There is no embryological evidence to support this sug gested connexion, and, as will appear immediately, the history of the gill -plates in various forms of Lamelli- branchs does not directly favour it. Yet it is very prob able that the labial tentacles and gill -plates are modi fications of a double horseshoe -shaped area of ciliated filamentous processes which existed in ancestral Mollusca much as in Phoronis and the Polyzoa, and is to be com pared with the continuous prai- and post-oral ciliated band of the Echinid larva Pluteus and of Tornaria (49). The gill-plates have a structure very different from that of the labial tentacles, and one which in Anodon is singu larly complicated as compared with the condition presented by these organs in some other Lamellibranchs, and with what must have been their original condition in the ances tors of the whole series of living Lamellibranchia, The phenomenon of " concrescence " which we have already had to note as showing itself so importantly in regard to the free edges of the mantle-skirt and the formation of the siphons, is what, above all things, has complicated the structure of the Lamellibranch ctenidium. Our present knowledge of the interesting series of modifications through which the Lamellibranch gill-plates have developed to their most complicated form is due to R. Holman Peck (50) and to Mitsukuri (51). The Molluscan ctenidium is typi cally, as shown in fig. 2, a plume-like struc ture, consisting of a vascular axis, on each side of which is set a row of numerous la- melliform or filamentous processes. These processes are hollow, and receive the venous blood from, and return it again aerated into, the hollow axis, in which an afferent and an efferent blood-vessel may be differentiated. In the genus Nucula (fig. 134), one of the FIG 130. Psammobia florida, right side, showing expanded foot e, and g incurrent and g excurrent Arcacete we have an example of a Lamelli- siphons (from Owen). . . ,1-1 ,-T r e branch retaining this plume-like form of gill. In other Arcaceae (e.g., Area and Pectunculus) the lateral processes which are set on the axis of the ctenidium are not lamellae, but are slightly-flattened, very long tubes or hol low filaments. These fila ments are so fine and are set so closely together that they appear to form a continuous membrane until examined with a lens. The microscope shows that the neighbour ing filaments are held to gether by patches of cilia, called " ciliated junc tions," which interlock with one another just as two brushes may be made to do. .In fig. 133, A a portion of four filaments of a ctenidium of the Sea- Mussel (Mytilus) is repre sented, having precisely the same structure as those of Area. The fila ments of the gill (cteni dium) of Mytilus and Area thus form two closely set rows which depend from the axis of the gill like two parallel plates. Further, their structure is profoundly modified by the curious condition of the free ends of the depending filaments. These are actually reflected at a sharp angle genus to a height of four feet. In Mytilus the foot is reduced to little more than a tubercle carrying the aper tures of these glands. In the Oyster it is absent alto gether. The labial tentacles of Anodon (n, o in fig. 124, (3), (5) ) are highly vascular e f, flat processes richly supplied with nerves. The left anterior ten tacle (seen in the figure) is joined at its base in front of the mouth (w ) to the right anterior ten tacle, and similarly the left (o) and right posterior tentacles are joined behind the mouth. Those of Area (i, k in fig. 132) show this relation to the mouth (a). These organs are character istic of all Lamelli branchs ; they do not vary except in size, being sometimes drawn out to streamer-like dimen Their appear- sons. FIG. 131. Diagram of a view from the left side of the animal of Anodonta cygnina, from which the mantle-skirt, the labial tentacles, and the gill- filaments have been entirely removed so as to show the relations of the axis of the gin-plumes or ctenidia g, k. a, centro-dorsal area ; 6, ante rior adductor muscle ; c, posterior adductor muscle ; d, mouth ; e, anus ; /, foot ; g, free por tion of the axis of left ctenidium ; h, axis of right ctenidium ; k, portion of the axis of the left ctenidium which is fused with the base of the foot, the two dotted lines indicating the origins of the two rows of gill-filaments ; m, line of origin of the anterior labial tentacle ; n, ne- phridial aperture ; o, genital aperture ; r, line of origin of the posterior labial tentacle. (Ori ginal.) ance and position suggest that they are in some way related morphologically to the gill-plates, the anterior labial tentacle being a continuation of the outer gill-plate, 9 FIG. 132. View from the ventral (pedal) as pect of the animal of Area on", the mantle- flapand gill-filaments having been cut away. a, mouth ; b, anus ; c, free spirally turned extremity of the gill-axis or ctenidial axis of the right side ; d, do. of the left side ; e, /, anterior portions of these axes fused by concrescence to the wall of the body ; g, anterior adductor muscle ; A, posterior adductor ; i, anterior labial tentacle ; k, posterior labial tentacle ; /, base line of the foot ; m, sole of the foot ; n, callosity.
(Original.)