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.