The labial tentacles are vascular, richly supplied with nerves, and ciliated. Their opposed surfaces are covered with fine parallel ridges. The upper tentacles appear to rise in part from the mantle, the lower from the sides of the visceral mass, but the two tentacles of the same side undergo partial concrescence of their surfaces, and the furrow between them does not extend back between the mantle and visceral mass. In some instances they are of very large size. Embryology does not favour an homology with gills. Professor Ray Lankester however has suggested that they with the gills are homologues of the prae- and post-oral ciliated bands of the Echinoid and Ophiurid larva, Pluteus, or the Tornaria larva of Balanoglossus.
The gills require careful examination. In Anodonta they appear to be profoundly modified from their original structure. Each gill consists of two lamellae, an outer and an inner. The inner lamella of the outer gill and the outer lamella of the inner gill arise close together and along the line between them run the afferent and efferent blood-vessels. This line represents the original (ctenidial) axis of the gill. Posteriorly the axis is free for a short distance, but anteriorly it is part of the under side of the organ of Bojanus and the side of the body. The space between the two lamellae, i. e. outer and inner, of each gill, is the 'interlamellar' space, and examination shows that it is crossed by numerous 'interlamellar' junctions. If the surfaces of the lamellae be regarded attentively, they are seen to be perforated by many series of apertures. Hence the lamellae resemble a fenestrated membrane, but may be regarded as composed of a number of parallel vertical filaments united from place to place by 'interfilamentar' junctions. The latter is the view justified both by a comparison with certain other forms as well as by embryology. Nucula and Yoldia (Arcacea) have each ctenidial axis bearing two series, an outer and inner, of gill-filaments which are lamellate in shape.
In Mytilus and other bivalves they are filamentous, and the filaments of the outer series are folded on themselves, the folded part being external, while the inner filaments are similarly folded, but the folded part is internal. The filaments are also united laterally in Myti/us, Arca, etc, by peculiar long cilia into a lamella, the component filaments of which are easily separated. Solid interlamellar junctions are sparingly developed. Further steps in complication are, the development of tubular interfilamentar junctions, e. g. in Dreissena, the union of the reflected portions of the filaments to neighbouring parts and inter se, and the great development of interlamellar junctions. The individual filaments, originally tubular, become nearly solid at the same time by the development of rods of a condensed gelatinous tissue in their interior. The vascular channels then run chiefly in the junctions. These changes are carried to a great extent in Anodonta. The surfaces of the filaments are clothed with cilia. The ciliated cells are distinguishable into three sets: frontal, with medium-sized cilia; lateral-frontal, forming a single row of large cells with very long cilia; and lateral with much the shortest cilia.
The vascular channels in the gills have been stated by Kollmann to posses an endothelium. This is certainly the case in many forms, e. g. Arca; but Bonnet has failed to prove the fact in Anodonta. The vascular channels have a very complex arrangement in Anodonta, and have been fully described with figures by the last named author. The tissue of the branchiae in the Unionacea (Unto, Anodonta, &c), contains a very large amount of lime carbonate. Hence the brittleness of these structures.
From the account given above it is clear that the two gills so-called of each side in the Lamellibranch are really parts of a single gill, a highly complex and modified ctenidium.
The cilia covering the surfaces of the gills cause the currents of water to flow from the outer free surfaces into the interlamellar spaces and thence outwards through the superior siphonal notch. The cilia at the margins of the lamellae are said however to cause currents towards the mouth and thus subserve alimentation.
Lamellibranchiata, Bronn, Klass. und Ordn. des Thierreichs, iii. 1. 1862; Do. with Anodonta as a type, Ray Lankester, 'Mollusca,' Encyclopaedia Britannica (ed. ix), xvi. 1883; cf. Haren Noman, Niederland. Archiv fur Zool. i. Suppl. 1881-82.
Anodonta, Howes, Atlas of Practical Elementary Anatomy, London, 1885.
Keber's organ.Grobben, Arb. Zool. Inst. Wien. v. 1883, p. 40; cf. Griesbach, A. N. 43, 1877.
Absorption of water, etc. Schiemenz, Mitth. Zool. Stat., Naples, v. 1884 (with lit.); cf. Schuler, A. M. A. xxv. 1885. Movements of foot in Lamellibranchiata, Fleischmann, Z. W. Z. xlii. 1885; for valve of vein from foot, see p. 419, and Fig. 5, p. 420.
Byssus gland.Carriere, Arb. Zool. Zoot. Inst., Wurzburg. v. 1882. Mode of attachment. Cattie, Tijdschr. Nederl. Dierk. Vereen, vi. 1882-85.
Gills. Posner, A. M. A. xi. 1875 j Peck, Q.J. M. xvii. 1877 j Mitsukuri, Q. J. M. xxi. 1881; circulation and endothelium of, Kollmann, Z. W. Z. xxvi. 1876; A. M. A. xiii. 1877, and Festschrift zur Feier des 300-jahrigen Bestehens der Universitat zu Wurzburg, 1882, p. 42 (with lit.); Bonnet, M. J. iii. 1877. Rods of gills. Bonnet, op. cit. p. 321; cf. Kollmann, SB. Bayer. Akad. 1876, p. 163. Development of gills, De Lacaze-Duthiers, A. Sc. N. (4) v. 1856.
Ciliary-currents, Sharpey, Encyclopaedia of Anat. and Physiol. i. 1835-36, p. 621.