Continuous observations on the Upper Fal Estuary oyster beds during the years 1925, 1926, and 1927 have established the fact that 0. edulis exhibits on these beds two definite periods of shell-growth, i.e. increase in shell-area, in one year; one period of general shell-growth occurs in the spring, beginning early in April, and the other in autumn at the end of September or early in October. There is some evidence-but as yet incomplete-that internal shell-growth occurs solely and simultaneously with the deposition and thickening of the new shell-shoots in a period of one to two months.

These general bursts of shell-growth occur in the pre-breeding and post-breeding part of the season

Similar periods of shell-growth occur on beds in the River Blackwater (AVest Mersea), and also occur mainly in the pre-breeding and postbreeding part of the season; but shell-growth begins in the spring in April-May on these beds, a few weeks later than on the Upper Fal Estuary,and in the autumn in August-September a few weeks earlier than on the Upper Fal beds.

Spring and autumn periods of shell-growth are also known to occur on many other estuarine beds.

The vertical distribution of Calanus finmarchicus adults in the daylight in the Plymouth region as shown by stramin ring-trawl catches in described for the period April to September in 1926. The general results confirmed the indications given in 1925 that there is a, gradual descent of the region of maximum abundance from about 10 metres in April to-metres in June, with a definite rise towards the surface ia July, August, and September.

2. The rise towards the surface was evident on sunny days as well as dull, indicating that the suggestion that dull weather and low light intensity was its cause in 1925 may possibly have been incorrect.

Measurements of the total lengths of Calanus were made which showed a seasonal change, a brood of small adults occurring in July, August, and September, as opposed to a brood of large adults whichdominated in the spring.

It is suggested that possibly these two broods are physiologically different, and that the small type of adult prefers a higher light intensity and lives nearer the surface than the large type.

The males and females differed in their behaviour, the males being always slightly deeper in the water than the females.

Some indications are given of the course of the life-cycles of Calanus in the Plymouth area.

While from April to the end of June the abundance of the last copepodid stage (Stage V) rarely exceeded more than 20% of the total Calanus in any one catch, they became very much more abundant in July, August, and September, reaching even 60% and 70%.

Further measurements have been made of the penetration of daylight into the sea near Plymouth, photometers containing both vacuum and gas-filled photo-electric cells being used. The effect of obliquity of illumination on the sensitivity of these photometers was large, and has been allowed for as far as possible. The sensitivity of the vacuum cell used in the sea attained its maximum at a wave length of about that of the gas-filled cell at about 4,350. The sensitivity of both to wave lengths greater than 5,500 was small.

It has been shown by experiments formulated upon the conditioned response principle that the blenny, Blennius gattorugine, is able to perceive and to profit by very small changes in its environment. This fish is able to form conditioned motor responses using food as unconditioned stimulus, towards a momentary increase of 0-4C, or more, in the temperature of the surrounding water.

It is also able to form similar conditioned motor responses towards a momentary decrease in the salinity of the surrounding water, of as little as 3 parts per 1000, or towards a change of greater magnitude (up to 37 parts per 1000).

Conditioned responses have been established in the wrasses, Crenilabrus inelops and Labrus bergylta, towards visual stimuli of varying kinds. It appears that these fishes can discriminate after differential training between one or two sources of light, and between monochromatic red, green, yellow, or violet light, but not readily between even comparatively large differences in intensity of a luminous source. These results are those obtained upon dark-adapted fishes. More extensive experiments will be necessary before a final statement is made upon their capacity for colour discrimination.

The distribution and activities of animals and plants upon the sandy beach are largely influenced by the special physical factors operating in that environment. The paper is a contribution to the knowledge of these factors.

The tidal ebb and flow, and the resulting alternations of exposure, are the outstanding factors. The extent to which other factors may be operative upon the beach, during the period of ebb-tide, depends upon the vertical distance of the point in question above or below mean sea-level.

The presence of living organisms and their metabolic products introduce a further series of biochemical factors into the physical conditions of life on the sandy beach.

The gaseous exchanges of animals and plants lead to changes in the pH of the interstitial waters of the beach, but the calcareous matter associated with the sand acts as an alkali-reserve, preventing undue rise of acidity, and incidentally widening the potential range of carbonassimilation of the surface flora.

The developments of Polydora ciliata Johns, and Polydora hoplura Clap, are described from the egg to a very late planktonic stage in the case of the former, and to young metamorphosed individuals in the case of the latter. Only external characters are described.

In both species the eggs are laid in egg-sacs attached to the wall of the parent's burrow.

The larvae of P. ciliata are released at a stage with three chsetigerous segments and lead a long planktonic life.

The larvae of P. hoplura are provided with special food in the form of yolk-masses ; they undergo most of their development while in the protecting burrow of their parent; are released at a very late stage, and have only a short planktonic life.

The larvae of both species have a complicated vestibule surrounding the mouth, and are provided with special sensory cilia on the head.

The larvae are provided with a special kind of cilia, situated at the ends of every nototroch, which are used to take hold of the long provisional bristles in swimming. This suggests that one of the functions of the long bristles is that, in conjunction with these grasping-cilia, they increase the rigidity of the swimming larva, and hence the efficiency of its driving cilia.

Previous references to Polydora larvae are briefly discussed.

It has been shown that there is a lunar periodicity in reproduction of P. opercularis near Plymouth in 1927-28. The ripening of sexual products corresponds with the full phase of the moon.

The condition and shape of the gonad changes with the development of the tissues.

The colour, too, of the hermaphrodite organ changes with development.

Black pigmentation has been observed on the gonad in thin-shelled P. opercularis, a phenomenon apparently due to actinic rays penetrating the thin shell.

The breeding season is from January to June inclusive.

The gut contents seem to show no appreciable difference either in quantity or in the natureof the food during a lunar month.

The moonlight has apparently no effect on the lunar periodicity in reproduction.

It is suggested that in the animal there is a physiological rhythm that causes the development of the gonad to coincide with the full moon of each lunar month

Normal healthy oysters with openings drilled in both inhalent and exhalent chambers, remove considerable amounts of glucose from solution. The average diminution in 5 experiments, each consisting of two such animals kept in 4 litres of sea-water containing about 0-2% of glucose, was 8-17% at the end of 36 hours.

In similar experiments, using the same oysters after their mouths had been plugged with wax and plasticine, there was only an average diminution of 1-4:6% in the glucose at the end of 36 hours. In two of the experiments there was no diminution, and in the three others there was evidence that one of the two oysters in each had been incompletely plugged and glucose had passed into the alimentary canal.

Oysters which had been plugged for 8 days so that they were bleeding profusely, the mantle cavity containing vast numbers of leucocytes, were used for a third set of experiments in which the average diminution was 7-4% at the end of 36 hours.

There was no evidence of bacterial decomposition of glucose within the experimental period.

These results, taken with those of previous investigations, show that the ciliated epithelia of Lamellibranchs cannot absorb (nor ingest phagocytically). Absorption takes place in the tubules of the digestive diverticula within the alimentary canal, and, in the mantle cavity, only through the agency of phagocytes which are extruded in great numbers when Lamellibranchs bleed as a result of bad conditions.

The opinion of Ranson that Marennin and other soluble matter is absorbed directly by the ciliated epithelia in the mantle cavity cannot be upheld, this material being deposited in the cells by the phagocytes which either absorb it directly from the mantle cavity and gut, or transport it from the digestive diverticula.

The following two sections represent two preliminary communications to the study of relative growth in a small Crustacean admirably adapted for use as a laboratory animal in all studies involving growth and development.

The present paper deals with the results of investigations into certain phases in the life-history of the bivalve mollusc Tellina tenuis carried out during the autumn of 1926 and 1927.

Collections in the intertidal zone and below L.W.M., were made in a number of bays in the Cumbrae and neighbourhood, Kames Bay, Millport, being selected for intensive study.

In Kames Bay Tellina tenuis ranges from a little below H.W.M. to depths of about 4 fm. The maximum concentration of about 1000 per sq. m. is found at L.W.M. springs and the numbers decrease to zero at H.W.M. and 4 fm.

In Kames Bay the density of Tellina tenuis falls in a progressive manner from L.W.M. to H.W.M.

The size-frequency distribution shows a regular gradation from the lower to the higher levels. At L.W.M. and below individuals of small size predominate, while at H.W.M. they are proportionately few.

At higher levels growth is more rapid than at the lower levels.

The size-frequency curves and density of Tellina tenuis in the other bays at L.W.M. correspond with those of half-tide in Kames Bay.

The rate of growth may therefore be influenced by the density of population as well as by the habitat.

In all areas the 1926 brood predominates.

The Tellina tenuis population in Kames Bay seems to be composed of four year-groups, one of which is almost unrepresented. Collections from neighbouring bays indicate that older groups may be present.

The amount of young brood on the bottom seems to vary considerably from year to year, being large and small in alternate years.

The two closely related species, T. tenuis and T. fabula, are both plentiful in Kames Bay, but their range is not coincident.

Eipe sperms were found from May onwards and ova were rounding off in June.

Young Tellina tenuis, passing the 2-mm. sieve but retained by the 1-mm. sieve, were found in Kames Bay in August, chiefly below L.W.M., but in October plentifully distributed up to half-tide and in lesser numbers higher up the shore.

The food usually consists of vegetable detritus, but during the spring increase diatoms appear almost exclusively in the gut.