Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/721

LAMELLIBR ANCHI A. ] MOLLUSCA 693 mouth (fig. 144, B, a) and connected in front of it by a commissure, are the representatives of the cerebral, pleural, and visceral ganglia of the typical Mollusc, which are not here differentiated as they are in Gastropods (compare, however, fig. 67). A pair placed close together in the foot (fig. 144, B, b, and fig. 124, (6), ax) are the typ ical pedal ganglia ; they are joined to the cerebro- pleuro - visceral ganglia by connectives. Posteriorly beneath the posterior adductors, and covered only by a thin layer of elongated epidermal cells, are the olfactory ganglia, their epidermal clothing con stituting the- pair of os- phradia, vhich are thus seen in Lamellibranchs to occupy their typical position and to have the typical innervation, the norvp tn PI Mi nsnVirnrl Flo> 144. Nerve-ganglia and cords of three Lamellibranchs (from Gegenbaur) : A, of ium beinf given off by Teredo; B, of Anodonta ; C, ofPecten. a, i i v cerebral ganglion-pair ( = cerebro-pleuro- the Visceral ganglion visceral); b, pedal ganglion-pair; c, olfac- that is to say, by the tory (osphradkl) ganglfon-pair. undifferentiated cerebro-pleuro- visceral ganglion of its proper side. This identification of the posterior ganglion- pair of Lamellibranchs is due to Spengel (11). Other anatomists have considered this ganglion-pair as corre sponding to either the pleural or the visceral of Gastropoda, or to both, and very usually it is termed " the parieto- splanchnic " (Huxley). The sense-organs of Anodon other than the osphradia consist of a pair of otocysts attached to the pedal ganglia (fig. 124, (6), ay}. The otocysts of Cyclas are peculiarly favourable for study on account of the transparency of the small foot in which they lie, and may be taken as typical of those of Lamellibranchs generally. The structure of one is exhibited in fig. 146. A single otolith is present as in the veliger embryos of Opisthobranchia. In adult Gastropoda there are frequently a large number of rod-like otoliths instead of one. Anodon has no eyes of any sort, and the tentacles on the mantle edge are limited to its posterior border. This deficiency is very usual in the class; at the same time, many Lamellibranchs have tentacles on the edge of the mantle supplied by a pair of large well-developed nerves, which are given off from the cerebro-pleuro-visceral ganglion-pair, and very frequently some of these tentacles have undergone a special metamorphosis converting them into highly- organized eyes. Such eyes on the mantle-edge are found in Pecten, Spondylus, Lima, Ostrea (?), Pinna, Pectunculus, Modiola, Mytilus (?), Cardium, Tellina, Mactra, Venus, Solen, Pholas, and Ga- leomma. They are totally distinct from the cephalic eyes of typical Mollusca, and have a different structure and historical de velopment. They have not originated as pits but as tentacles. They agree with the !T"~" j i c r u-j- /T> i i. Flo> 146 -~ Otocyst dorsal eyes ot Oncludium (Pulmonata) in of cycias (from the curious fact that the optic nerve pene- ^psui^^hiated trates the capsule of the eye and passes in ceils lining the front of the retinal body (fig. 145), so that its fibres join the anterior faces of the nerve-end cells as in Vertebrates, instead of their posterior faces as in the cephalic eyes of Mollusca and Arthropoda ; moreover, the lens is not a cuticular product but a cellular structure, which, again, is a feature of agreement with the Vertebrate eye. It must, however, be distinctly borne in mind that there is a fundamental difference between the eye of Verte brates and of all other groups in the fact that in the Vertebrata the retinal body is itself a part of the central nervous system, and not a separate modification of the epidermis myelonic as opposed to epidermic. The struc ture of the reputed eyes of several of the above-named genera has not been carefully examined. In Pecten and Spondylus, however, they have been fully studied (see fig. 145, and explanation). The gonads of Anodon are placed in distinct male and female individuals. In some Lamellibranchs for instance, the European Oyster and the Pisidium jmsillum the sexes are united in the same individual; but here, as in most hermaphrodite animals, the two sexual elements are not ripe in the same individual at the same moment. It has been conclusively shown that the Ostrea edulis does not fertilize itself. The American Oyster (0. virginiana] and the Portuguese Oyster (0. angidata} have the sexes sepa rate, and fertilization is effected in the open water after the discharge of the ova and the spermatozoa from the females and males respectively. In the Ostrea edulis fertil ization of the eggs is effected at the moment of their escape from the uro-genital groove, or even before, by means of spermatozoa drawn into the sub-pallial chamber by the in- current ciliary stream, and the embryos pass through the early stages of development whilst entangled between the gill-lamellae of the female parent (fig. 6). In Anodon the eggs pass into the space between the two lamellae of the outer gill-plate, and are there fertilized, and advance whilst A - B p-ad FIG. 147. Two stages in the development of Anodonta (from Balfour). Both figures represent the glochidium stage. A, when free swimming, shows the two dentigerous valves widely open. B, a later stage, after fixture to the fin of a fish, sh, shell ; ad, adductor muscle ; s, teeth of the shell ; by, byssus ; a.ad, anterior adductor ; p.ad, posterior adductor ; mt, mantle-flap ; f, foot ; 6r, branchial filaments ; au.r, otocyst ; al, alimentary canal. still in this position to the glochidium phase of develop ment (fig. 147). They may be found here in thousands in the summer and autumn months. The gonads them selves are extremely simple arborescent glands which open

to the exterior by two simple ducts, one right and one