The Lucinacea (Eulamellibranchia) have been studied and particular attention has been paid to the feeding, respiratory and cleansing currents and their relation to the morphology and habits of the group. The Lucinacea comprise three families, the Ungulinidae, Thyasiridae and Lucinidae, and living specimens of thirteen species were studied; namely, Diplodonta rotundata, D. punctata, D. semi-aspera (Ungulinidae); Thyasira flexuosa (Thyasiridae); Lucina pennsylvanica, L. chrysostoma. Codakia orbicularis, C. orbiculata, C. costata, Loripes lucinalis, Phacoides borealis, Myrtea spinifera, Divaricella quadrisulcata (Lucinidae). These species occur on a variety of substrata ranging from coarse sand to fine mud particularly where the associated fauna is sparse. Shallow-water species are often found among the roots of marine grasses such as Zostera and Thalassia. The mantle is described, notably the extensive glandular tissue of the mantle margin, also the 'mantle gills' and the unique form of the exhalent siphon both of which are confined to the Lucinidae. The great development of the glandular tissue at the mantle margin is probably related to large quantities of sediment entering the mantle cavity. While many of the gland cells close to the epithelium show typical basiphilic reactions of mucus-secreting cells, those further from the epithelium show marked eosinophilic-staining reactions. The only visible difference between the cells appears to be a difference in cytoplasmic granule size, those to the inside possessing the larger granules. 'Mantle gills' consist of convoluted folds of the inner mantle epithelium close to the anterior adductor muscle. They occur in Codakia orbicularis and Lucina pennsylvanica. The folds are parallel to the direction of the ciliary currents. There are no gland cells, but there are many blood vessels below the folds and the latter are probably concerned with respiration. The exhalent siphon in the Lucinidae does not contract into a pallial sinus, but turns inside out and lies in the suprabranchial cavity, a method that appears to be unique. The anterior adductor muscle is elongate and the greater part of it lies ventral and posterior to the mouth. Its surface epithelium is ciliated and acts as a sorting area for particles entering by the anterior inhalent tube. The Ungulinidae are least specialized in this respect and the Lucinidae the most specialized. The variations of the ciliary currents on the body are described. The foot is highly specialized, it is used in burrowing and also forms the anterior inhalent tube. It is typically long and vermiform often with a distinct tip. In the Lucinidae the heel is well developed and may be distinct from the vermiform portion. The latter is chiefly concerned with tube formation and the heel with digging. In some species the foot can extend to more than ten times the length of the shell. The tip of the foot is profusely supplied with glands which supply material for tube formation. An account of this and of burrowing is given. The morphology and ciliary mechanisms of the gills are described. In the Ungulinidae and Thyasiridae the outer demibranch is reduced and in the Lucinidae this demibranch is absent. All the species studied have homorhabdic gills. In the Thyasiridae and Lucinidae development of the subfilamentar tissue gives a fleshy appearance to the gills. The palps show progressive reduction in size in the three families. The Ungulinidae and the Thyasiridae retain the typical ciliated folds. The palps of the Lucinidae are slight enlargements of the walls of the oral groove. Vestiges of not more than three folds remain which are still capable of a limited amount of sorting. The stomach and digestive diverticula are described; the sorting mechanisms are progressively reduced. They are least reduced in the Ungulinidae being essentially the same as those of other Eulamellibranchia. In the Thyasiridae and the Lucinidae there is reduction in the sorting areas of the stomach and the number of ducts of the digestive diverticula. The resulting simplification of the stomach emphasizes the importance of (1) the acceptance tract, (2) the typhlosoles, (3) the left caecum, (4) the dorsal groove, and (5) the tooth on the gastric shield. The digestive diverticula of the Ungulinidae are of the typical eulamellibranch pattern, those of the Lucinidae and Thyasiridae have much larger ducts and tubules, both in the size of the lumen and of the cells. Tubules only are present in the Thyasiridae. The excretory, circulatory and nervous systems do not differ significantly from those of other Eulamellibranchia. The evolution of the group is discussed particularly in relation to the anterior inhalent current and the correlated reduction in sorting mechanisms of the gills, palps and stomach, together with the development of the anterior adductor muscle, foot and exhalent siphon.