The anatomy and histology of the food collecting and alimentary organs of the adult oyster are described.The anatomy of the stomach is investigated with the aid of gelatin casts and attention drawn to the food caecum, the ventral groove, and the two ducts of the digestive diverticula.Cilia and mucus glands are universal throughout the food collecting and alimentary organs.There is evidence that the gastric shield is composed of fused cilia.The histology of the style-sac resembles that described by Mackintosh for Crepidula. There is evidence that secretion of the style takes place in the groove.Phagocytes are everywhere numerous in the blood vessels, connective tissue and epithelia, and free in the gut and mantle cavity.The alimentary organs of the larva are described.The anatomy and histology of these organs in the spat isdescribed, the palps are relatively large and the gills asymmetrical. The style-sac is distinct from the mid-gut.The course of the ciliary currents on the gills and palps is described and the importance of the various selective mechanisms emphasized.Selection appears to be purely quantitative, large particles or mucus masses being rejected and smaller ones accepted.Muscular activity is of great importance in the functioning of both gills and palps. Reversal of cilia has never been seen.Rejected matter is removed from the mantle cavity.Material is sorted in the food caecum in the stomach, larger particles passing into the mid-gut and smaller ones towards the gastric shield and ducts of the digestive diverticula, within the tubules of which there is a constant circulation.The rotation of the style assists in the stirring of matter in the stomach.In the style-sac are cilia, which rotate the style and others which push it into the storuach.In the larva the velum acts as a food collecting organ ; the style lies in an extension of the stomach and rotates rapidly. Material passes freely into the digestive diverticula.In the spat rejective mechanisms are highly developed. The style revolves at a speed of between sixty and seventy revolutions per minute.The tubules of the digestive diverticula are the only place where soluble matter is absorbed, in adult, larvae, or spat.Fine particles are ingested and digested intracellularly in the tubules of the digestive diverticula, the products of digestion carried away by amoebocytes, and useless matter rejected into the lumen.Larger particles are ingested and digested by phagocytes in all parts, the products of digestion being carried to the vesicular connective tissue cells and there stored.Enzymes in the style digest starch and glycogen. The amylase, at pH 5.9, has an optimum temperature of 43'C, and is destroyed atThe optimum medium is pH 5-9. It is inactivated by purification with absolute alcohol or by dialysis, but action is restored on the addition of chlorides or bromides and to a less extent iodides, nitrates, and carbonates, but not with sulphates or fluorides.Sucroclastic enzymes in the digestive diverticula act on starch, glycogen, sucrose, raffinose, maltose, lactose, salicin, and amygdalin, but not on inulin, cellulose, or pentosans.The amylase, at pH 5-5, has an optimum temperature of 44-5, and is destroyed at between 64 and 67. It has an optimum pH of 5-5, and is inactivated after purification or dialysis, action being restored in the presence of chlorides or bromides.There is a weak lipase and protease, the latter has two optima at pH 3-7 and at or above 9-0 ; its action is very slow.The only enzymes free in the stomach are those from the style.There is no evidence of any enzymes free in the gill mucus.There is a powerful complete oxidase system in the style, and a catalase in the digestive diverticula and gonad, and traces in the palps, gills, and muscle.The style is the most acid substance in the gut and the cause of the acidity of the gut.The style is dissolved rapidly in fluid of pH 2-3 and above, but very slowly below that point. It is readily dissolved and reformed in the oyster, its presence depending on the maintenance of the balance between the rate of secretion and the rate of dissolution. Its condition is a valuable indication of the state of metabolism.Glycogen and fat are stored, particularly in the vesicular connective tissue cells, the former furnishing the principal reserve food material.The presence of abundant supplies of microscopic plant life rich in carbohydrates provides ideal food for the oyster, and represents optimum conditions for fattening and reproduction.

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The byssal apparatus appears during post-larval life when its secretion permits brief attachment during metamorphosis when the animal assumes the form which fits it for life in the adult habitat.The byssus persists for continued temporary attachment in animals which may lose it in adult life, e.g. Mya arenaria, or in which it becomes an organ of permanent fixation, e.g. Mytilus edulis, Pododesmus macroschisma and Tridacna crocea

SynopsisThe Mollusca possess two growth axes associated with the body and the mantle/shell respectively. Evolution of the Lamellibranchia involved assumption by the mantle/shell of responsibility for growth and form; also the formation of anterior and posterior adductors. Change from this dimyarian to a heteromyarian and finally monomyarian condition involved changes in the relations of body and mantle/shell. These are considered with regard to their two major axes in the saggital plane. Although body form may be greatly influenced by changes in that of the mantle/shell, its proportions are only altered where the body is attached by byssus. Then the anterior half may be reduced and the anterior adductor finally lost. This occurs in many Anisomyaria, these monomyarians being divisible into five groups according to habit. Apart from the Limidæ, all are bilaterally asymmetrical. In those which have lost the foot, both axes of the body are effectively lost. Primary pallial attachment is lost but secondary attachment has been achieved in most cases. In the Tridacnidæ (Eulamellibranchia) the mantle/shell has rotated in the saggital plane in relation to the fixed body with resultant loss of the anterior adductor. There is hypertrophy and extension antero-dorsally of the originally posterior siphonal tissues in which zooxanthellæ are contained. Evolution may have proceeded by way of now extinct heteromyarians such as Lithocardium. In the freshwater Acostœa (Mulleria) the anterior adductor is lost during growth following cementation by one or other valve. The form of the body is little affected. Variation and natural selection account for the variety of form and habit exhibited by these diverse monomyarians.

Collections of some 22,000 female and 6000 male Crangon vulgaris were made throughout the year from the shrimp fisheries of the Severn Estuary and Bristol Channel. All animals were measured.The habits of the species are described; it can withstand a wide range of temperature but, though euryhaline, resembles other Decapoda in the inability to withstand low salinity combined with low temperature.Osmo-regulation is apparently largely inhibited at low temperatures and to a greater extent in the males than the females.Growth rate decreases with increasing age; in the female there is no increase in length when moulting from the ' neuter' to the egg-carrying intermoult. The duration of this intermoult, if spawning is successful, is about double that of the normal intermoult under the same temperature conditions. Growth almost ceases in the winter.Secondary sexual characters are described, especially the differences between the endopodites of the pleopods in the two sexes.Females become mature at a minimum length of 45 mm. in the Channel and seldom less than 50 mm. in the Estuary. The effect of the female sexual cycle on the size of the ovary and the form of the pleopods is described. The process of copulation is described; it can occur in the brackish waters of the estuary. Egg-laying always follows within two days of moulting into the egg-carrying condition but eggs are not retained if copulation has not occurred.The females lie on their sides during the act of spawning and the eggs are firmly attached within thirty minutes to the egg-carrying setae on the basipodites of the first to fourth pair of pleopods, then to those on the endopodite of the first pleopod, finally to those on the coxopodites of the last two pairs of pereiopods.

Spisula subtruncata is a shallow-burrowing member of the Mactridae which inhabits silty sand. Adaptations to such conditions include long frontal cilia on the gills (Atkins), and the presence of a siphonal membrane with associated mantle folds over-arching a waste canal, as postulated by Kellogg for allied species.The mantle edges are free ventrally, but are normally closely applied except for a short distance at the base of the inhalant siphon, an effective fourth pallial aperture being so formed. Through this opening material from the waste canal is ejected.Lutraria lutraria is a deep-burrowing species belonging to the same family. The general resemblance to Mya arenaria is due to convergence. Cleansing currents resemble those of Spisula subtruncata and there is a similar waste canal, but the siphonal membrane and the long frontal cilia associated with a silty environment are absent.

The Tellinacea comprise four families of eulamellibranchs possessing common structural features all associated with the habit of feeding on bottom deposits. The habits and general structure of all available British species of the Tellinacea have been studied; in particular, Tellina tenuis (in greatest detail) and Macoma balthica (Tellinidae); Abra alba and Scrobicularia plana (Semelidae); Donax vittatus (Donacidae); Gari tellinella , G. Solecurtus chamasolen and S. scopula (Asaphidae). The most significant structures are the very mobile separate siphons formed by fusion of the inner (muscular) lobes of the mantle edge, while the middle (sensory) lobe and the periostracal groove continue around the posterior margin of the shell bordering a deep siphonal space from the base of which the siphons arise. The siphonal apertures are fringed only by six blunt lobes (pinnate and with intermediate lobes in Donax ). They are also very insensitive. In the Tellinidae and Semelidae the siphons are very long and the inhalant siphon gropes widely over the surface, actively drawing in bottom deposits. The siphons are shorter and wider in the Donacidae and Asaphidae, but the inhalant siphon opens widely for the passive intake of much material lying on or just above the bottom. The siphonal retractors are highly developed, while the siphons possess a characteristic arrangement of concentric layers of circular and longitudinal muscles with radiating fibres dividing the major layers of longitudinal muscles into a series of bundles. Six nerves, evenly spaced, run down each siphon, always in the same relative position, terminating in the lobes bounding the aperture. In association with the mode of life, the foot is very large and active. It is very much compressed laterally in the Tellinidae and Semelidae, more rotund and pointed anteriorly in Donax , where there is an additional elevator pedis muscle possibly correlated with the habit of burrowing in firm sand, and rounded in Solecurtus (and the Novaculininae), where alone the pedal gape is reduced. Ventral attachment of the siphons in all but the last is reduced to the minimum and takes the characteristic form of the cruciform muscle. This muscle consists of crossed bundles of fibres providing, it is here suggested, an ideal tie for the siphons. The anterior attachments will take the strain when the siphons are extended and the posterior ones when they are withdrawn. There is a sense organ near each of the two posterior attachments consisting of an intramuscular slit opening into a ciliated pit with an associated ganglion and usually communicating with the exterior. Theories concerning the function of the sense organ are discussed and the new view expressed that it is a proprioceptor giving indication of the degree to which the siphons are extended or withdrawn. Need for such information may reside in the presence both of large siphons and of a large foot in the Tellinacea; escape from danger involving first withdrawal of the siphons and then extension of the foot, i.e. unlike the Solenidae or deep burrowers such as Mya in which, respectively, only the foot and the siphons react to danger. In Solecurtus the inner mantle lobes are extensively fused both anteriorly and posteriorly. The cruciform muscle is displaced anteriorly and the pedal gape reduced (hence the superficial resemblance to the Solenidae) and the mantle cavity extended posterior to the shell when the animal is expanded. Broad marginal areas covered with periostracum protect the foot and siphons when these are withdrawn. Conditions in the subfamily Novaculininae are discussed and evidence produced for its inclusion in the Tellinacea in association with the family Asaphidae. The relationships are discussed between Tagelus (with cruciform muscle and no ventral mantle fusion), Solecurtus (with cruciform muscle and extensive ventral mantle fusion) and the Novaculininae (with no cruciform muscle but great anterior ventral mantle fusion). The observations of Atkins (here confirmed) on the ctenidia are discussed. The greatest modifications are in the Tellinidae and Semelidae where the outer demibranch is upturned, consisting of a single surface, and the margin of the inner demibranch has no food groove. These features are associated with the great intake of material. Need to convey this material rapidly forward explains the presence of powerful cirrus-like cilia on the frontal surface of the posterior region of the outer face of the inner demibranchs. These cilia are absent where forward currents exist on the gill axis. In the Donacidae and Asaphidae the gill is less modified, the outer demibranch being reflected although with a supra-axial extension, while there is a food groove on the margin of the inner demibranch; also on the outer demibranch in Tagelus . In general the palps tend to be large when the gills are small, typically in mud dwellers. Pseudofaeces are removed in the normal manner through the inhalant siphons; they are extremely abundant in the Tellinidae and Semelidae in which mantle folds (in all species examined) protect them from being swept forward by the inhalant current, concentrated in the narrow inhalant siphon, while there is an additional ventral channel in Abra and Scrobicularia . In general the gut is longest in species that take much mud into the mantle cavity; faecal pellets of characteristic form are moulded. The initial region of the mid-gut is associated with the style-sac in all but Donax . The stomach is highly characteristic of the group; it can be readily dissected free from surrounding tissues. It is most highly modified in the Tellinidae and the Semelidae. The proximal stomach is elongated vertically so that the style is not bent within it as in suspension-feeding eulamellibranchs. The gastric shield is exceptionally large and strong and carries two massive teeth which bear against the side of the style near its head. The style has been observed to rotate in Tellina tenuis . There is evidence of trituration of large particles presumably between the style and the teeth of the gastric shield. In association with this, a new structure, here termed the postero-dorsal caecum, has been developed to act as a safety valve within which the excess accumulations of large particles are temporarily stored before trituration. The ciliary currents within the stomach are essentially similar to those in other eulamellibranchs, and there is the usual dorsal pouch and food-sorting caecum (this associated with the openings of the ducts into the digestive diverticula). But in general these are less highly developed than in the suspension-feeding eulamellibranchs, but more so in the Donacidae and the Asaphidae than in the two more specialized families. The appearance and probable mode of action of the intact stomach are described and comparisons made with conditions in suspension feeders such as Mya . The only other eulamellibranchs with similarly formed siphons are equally specialized, so that it is necessary to derive the Tellinacea direct from the Filibranchia. The possible course of evolution within the group is discussed. The conditions in Gari are least specialized, those in the Tellinidae and Semelidae, on the one hand, and in Tagelus-Solecurtus -Novaculininae on the other, are the most specialized. Different families, genera and species are adapted for life in all types of bottom material, and are there able to exploit as nutriment the contained organic detritus. The nature of this is briefly discussed, but its importance is best indicated by the variety and number of bottom-living invertebrates that would appear to depend upon it. The Tellinacea are a highly successful group of lamellibranchs which have evolved along characteristic lines with a success revealed by their vast abundance in suitable substrata within both the littoral and sublittoral zones.

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