1. Simultaneous observations are recorded of some biological, physical, and chemical changes in the Clyde Sea Area in 1924, 1925, and 1926.

2. A close relation between diatom increases and changes in pH value, dissolved oxygen saturation and dissolved phosphate has been found.

3. It appears that the more numerous diatom increases in Loch Strivan as compared with the open sea are due to the more frequent mixing.

4. Confirmation of the fact that diatom increases begin at the surface and gradually spread into deeper water has been obtained.

5. The occurrence of Peridinians is apparently related to high temperature and low salinity.

6. The amount of sunshine in the early part of the year does not exert any apparent influence on the date of the spring diatom increase in the Clyde Sea Area.

7. The regeneration of phosphate in the deep water of Loch Strivan has been described.

In a former paper (Allen, Phil. Trans. Roy. Soc. B., Vol. 211, p. 131, 1921) it was shown that the Syllid Procerastea Halleziana Malaquin, addition to the ordinary mode of sexual reproduction which occurs in this group, reproduced asexually by a process of fragmentation, followed by the regeneration by each fragment of a new head and series of anterior segments and of a new pygidium and posterior segments. The fragments usually consisted of sections of two, three, or four segments each, in the region of the body behind the seventh setigerous segment. It was further shown that this breaking up of Procerastea, which could be produced artificially at any time by treating the worms with sea-water of low salinity, made by adding distilled water to natural sea-water, took place according to a definite law. The first section consisted normally of the head and setigerous segments 1 to 7, then followed three sections of two segments each (Segments 8 and 9; 10 and 11; 12 and 13), then three sections of three segments (14–16; 17–19; 20–22), followed by four or five sections of four segments each, and then a number of sections of three segments.

1. The characters of the species of “Portunus” are redescribed in detail and comparatively.

2. On the basis of the characters studied an attempt is made to determine the relationships of the constituent species, both within the genus and with allied genera, P. corrugatus and P. arcuatus, being regarded as the more primitive forms.

3. P. marmoreus and P. holsatus are separated by a large number of characters.

4. The status of various foreign, fossil and doubtful species is discussed.

5. An account is given of the present state of knowledge regarding the distribution of the species.

6. The synonymy of the generic name is considered at length.

The hermit-crab, Eupagurus Bernhardus, is common in the oyster beds in the upper part of the Fal Estuary, and experiments indicate, while observations and experiments on the mode of feeding virtually prove, that these hermit-crabs do not harm healthy small oysters one inch or more in length, and the larger oysters, but that spat less than one inch may occasionally be damaged or eaten by hermit-crabs picking with their claws indiscriminately at growths on shells and stones.

E. Bernhardus obtains food largely by its small claw and third maxillipedes, and may at certain periods of the year feed mainly on microorganisms, and at others on macroscopic, but mainly tiny living organisms which have been disturbed and captured in the deposits on the sea-bottom. Food can be obtained by the use of the third maxillipedes alone.

During the summer and autumn of 1926 frequent visits were made to the oyster beds in the upper Fal Estuary, and in view of the continued decline in the yield of oysters from these grounds, the opportunity was taken to record any facts which, besides being of value in the study of the bionomics of the oyster, might also be helpful in determining those steps most likely to lead towards an improvement of the general state of the fishery. A special investigation of the spawning in relation to tides and temperature was made, and will be compared in a separate publication with similar work carried out at about the same time on the River Blackwater beds in Essex.

The observations recorded form preliminary studies which it is hoped will be useful in special investigations on the physiology of shell-depositions in oysters. The distribution of chalky shell-deposits on the internal faces of the valves in young and old O. edulis and O. angulata has been studied, and its average distribution determined in shells of O. edulis of jnedium age from West Mersea and the Fal Estuary.

The distribution of this material in the shells of young O. edulis and in O. angulata is such that there can be little doubt that the function of the chalky deposit is to fill in rapidly grooves, hollows, and other spaces, which are inimical to efficient functioning in the changing needs of the individual.

In O. edulis of medium age large deposits occur regularly in the shell adjacent to the exhalent chamber and smaller ones on the border of the shell in the region of the inhalent chamber. The hypothesis is advanced that these regular deposits can be explained in the same way as those more easily understood by assuming that in a high percentage of shells of medium age, the contour of the shell in the region of the exhalent chamber and other parts is such that rapid extension and contraction of the mantle cannot be effected; deposition of chalky shell-material modifies this contour of the shell with certainty in some cases, but hypothetically in others, so that the mantle can be extended and retracted with a maximum of efficiency.

1. The ions present in sea-water are normally adsorbed by sand.

2. Percolation of sea-water through sand causes a slight abnormality in the concentrations of calcium and magnesium ions, due to the fact that sand is not an inert adsorbing agent, but is capable of discharging calcium ions into the water in exchange for magnesium and sodium.

3. The extent of adsorption and exchange is insufficient to cause hydrolysis and alteration in pH if the bulk of the sea-water is large.

The gonads of 1,121 oysters have been examined at various periods from a few hours to twelve months after the individuals had spawned as females; the material for the research was obtained by collection and experiment. In 702 individuals taken with young in the mantle cavity the gonad shows a progressive development of maleness in its primary sexual characters; within a mean period of 2¼ hours after spawning eggs, the gonad was found in 50 per cent of cases with only young spermmasses developing, followed at later periods by a progressive ripening of the sperm-masses, until in individuals carrying shelled and black-coloured larvae 77 per cent contained abundant ripe sperm-masses as well as advanced developing sperm-masses.

In 444 individuals examined at various periods after extruding their larvæ, the development of sperm was found to continue for about a month after egg-spawning, and to abate in about the second month. In from 2 to 3 months after egg-spawning sperm-development is completed and the male phase begins to wane, and a small percentage of individuals may become female or actively female-functioning again. In 12 months after the last egg-spawning a significant number of individuals become once more functional females.

The varieties of mixed sexes found in O. edulis are denned and their frequencies shown by an analysis of samples—from two widely separated beds—examined at the beginning of the breeding season in 1926. Experiments on the rate of growth of sperm-masses and on determining of age at different stages of development in embryos and larvæ are given.

In a previous paper in this Journal (Vol. XIV, p. 967) a theory of the control of sex in the oyster by rhythmical changes in the general metabolism has been outlined and discussed briefly with the information at present available. As the connexion between metabolism and sex has long been regarded as important, and by some, e.g. Geoffrey Smith (1913), as all-important, this relation may be considered briefly with regard to modern views on the physiology of sex.

1. The alteration in soluble phosphate content of untreated sea-water during storage is governed by the conditions of storage—a strong light, favourable to growth, produces a decrease, while darkness, unfavourable to growth, produces an increase.

2. Samples from the surface layers of the sea give greater increases on storage in the dark than samples from deeper water, the extent of the increase being conditioned by the number and quantity of planktonic organisms included in the water sample.