In a previous paper (Steven, 1948), brief accounts are given of three distinct mackerel fisheries that at one time existed in the south-west of England—an inshore fishery in the English Channel carried on from Plymouth, a deep-sea fishery from Newlyn, Cornwall, and an inshore fishery from Newlyn and some other Cornish ports. One of these fisheries, the Plymouth one, no longer exists. After a few years of considerable activity following the termination of the 1914–18 war, this fishery collapsed in 1924 (Table I) and came entirely to an end a few years later. The small quantities of mackerel landed at Plymouth in subsequent years have been incidental catches by vessels fishing for herrings or pilchards.

There are few adequate descriptions of British members of the ascidian family Didemnidae and little attention has been given to the larvae. In the autumn of 1947 a didemnid was collected from the intertidal zone in the Menai Strait, North Wales. This ascidian agrees superficially with Leptoclinum maculosum Edwards, as figured by Milne Edwards (1842) and by Alder & Hancock (1905–1912), although differing somewhat from the diagnosis of Didemnum maculosum (Edwards) given by Harant & Vernières (1933). It is closer anatomically to the D.fulgens (Edwards) of Harant & Vernières, but differs in colour. In most features the colony and zooid closely resemble the descriptions given by Michaelsen (1923) for his species D. helgolandicum Michaelsen, and it is probable that this is the species to which it should be assigned. Nevertheless, the name D. maculosum has been retained since it is under that name that it has probably appeared in most faunistic lists.

Until recently all accounts of didemnid larvae have described three anterior adhesive organs. This was the number given by Giard (1872) for Diplosoma gelatinosum Edwards, and by Lahille (1890) for Trididemnum tenerum Verrill and Polysyncraton lacazei Giard. Berrill (1947), however, has described a new species of didemnid, Trididemnum alleni Berrill, the larva of which has only two anterior adhesive organs, a condition not previously found in any ascidian larva.

The larva (Fig. IA) of the colonies found in the Menai Strait resembles that of T. alleni in many features. The trunk is about 0.35 mm. long from the tip of the adhesive organs to the base of the tail.

Certain marine algae were treated with sea water containing carrier-free radioactive strontium (mixture of Sr89 and Sr90), to test whether these ions were taken up by the weeds.

At the same time the behaviour of radioactive yttrium (Y90), which occurred as a short-lived daughter-product of Sr90, was investigated by the expedient of studying the ‘decay’ characteristics of specimens of water and weed.

Promising possibilities of this technique, by which an isotope is made available for study by applying its parent substance, are pointed out.

Radioactive strontium is extracted from sea water by the brown sea-weeds, in particular by Fucus serratus.

It is held that this effect is simply a result of ionic exchange, and that the algae regularly contain many times as much strontium in their cell fluids as exists in sea water.

It has been shown that the main factor which influences the local distribution of the lugworm at Whitstable is the depth of the muddy sand overlying the clay stratum below.

In places this clay is furrowed by straight channels which are revealed only by the arrangement of the lugworm castings. These are more numerous along the channels.

Migration of adult lugworms has been shown to occur, most freely in the spring.

Larvae with three and four chaetigerous segments and also an early post-larval stage of Arenicola marina are described with figures. These are all bottomdwelling stages enclosed in mucous tubes and were collected from the pebble and Fucus zone at Whitstable.

The rinding of these larvae furnishes additional evidence for the view that, except when migrating in the water to new habitats, the lugworm possesses no pelagic larval stage in its life history.

Experiments are described which suggest that Calliactis behaves as a closed animal in its movements, although its stomodaeum appears to be widely open to the exterior.

Observations of the movements of the animal are correlated with variations of the internal pressure. It is shown that expansion is not merely ‘passive relaxation’, but is generally accompanied by a rise in pressure which is, however, much lower than that which accompanies rapid closing.

It is concluded that the fluid-muscle system of Calliactis behaves in a similar way to that of other soft-bodied invertebrates, but that the system works at a much lower pressure.

The accuracy of volume estimation in filling a marked tank by means of a plankton pump has been determined. The method even in its simplest form is satisfactory under good conditions. Further refinements, in which the plankton was filtered off, after filling a modified tank (for example with an overflow device) would no doubt increase the accuracy.

A device is described by which, using an ex-N.F.S. pump, a constant (but variable by selection) volume of water can be taken without the use of large tanks.

A small number of sets of samples taken with adequate volume control indicate the same order of variability as net hauls. Comparison of the distribution of x2 for paired samples with nets and pumps also indicates variation of a similar type.

These results suggest that the variability of net hauls is not entirely due to the variations in the volume of water filtered. Further work is in hand to elucidate this question.

It is often desired to grow animals and plants of the intertidal zone in the laboratory, under conditions as far as possible simulating those in nature and with the least trouble and expense. This need was felt by the writer during an ecological study of the diatoms inhabiting the mud-flats at Whitstable, Kent. It was found necessary to maintain such diatoms in the laboratory in London in a healthy condition for several days so as to study the effect of light and tides on their movements. The apparatus described in this paper was therefore devised. It has an advantage over other tidal systems (Bracher, 1919; Martin & Reid, 1935) in that the substratum could be automatically alternately flooded and drained at regular intervals by a much simpler mechanism. Besides, it is more handy and can be operated with only a small supply of sea water, a matter of some importance in localities remote from the sea.

It consists of a tidal chamber, T, and a reservoir, R (Fig. I). The former is an inverted bell-jar fitted with a rubber bung through which passes a long glass tube, g, which extends to the base of the reservoir. At the upper end of the tube is a small funnel by which sea water enters the apparatus. A ⋯tube, t, serves to connect the tidal chamber and reservoir, its horizontal arm being connected to the siphon S.

The wide-mouthed bottle constituting the reservoir is fitted with a stopcock, v, which is useful for expelling air when setting up the apparatus.

The first larva of Typton spongicola was described in 1925 from the eggs carried by the parent (Lebour, 1925). No later stages are known; therefore it was interesting to find a last larva in the plankton from 2 miles north-east of the Eddystone (21 August 1946), which changed to the post-larva in the laboratory. There seems to be no doubt that the identification is correct although, surprisingly, the antennal scale, rudimentary in the adult, is still large. The agreement in colour of the last larva and post-larva with the newly hatched first larva, and the great similarity in the structure, in most ways, of post-larva and adult, makes it practically certain that we are dealing with T. spongicola.

Monthly samples of the Laminariaceae, L. saccharina and L. cloustoni, from three localities on the Argyllshire coast have been taken from March 1948 to March 1949, and analysed for dry matter, ash, mannitol, laminarin, crude proteins, inorganic nitrogen and alginic acid, and the seasonal variation in these constituents correlated with the changes in composition of the sea water.

The results show that a correlation does exist and that a period of rapid photosynthesis occurs from March to June/July, but is restricted in July/August when nitrate is undetectable in the water and phosphate is as low as 0·16–0–20 mg. atom/m.3. The replenishment of the photosynthetic layer with nutrients is retarded in July/August, apparently due to the warming of the inshore waters, which may set up a thermocline restricting vertical mixing.

The autumn cooling of the uppermost waters facilitates vertical mixing, regenerating the nutrients in the photosynthetic region, and a second burst of photosynthesis at a reduced rate from the spring ‘outburst’ occurs in October/November.

The results of phosphate determinations in sea water by the Deniges method, using red light, in an absorptiometer, may be in error if the stannous chloride reagent has low HC1 concentration, and contains stannic tin.

The error is in the reagent blank determination, and seems to be caused by colloidal stannic compounds which increase the opacity of the solution. In sea water the effect may be smaller or immeasurable so that results are over-corrected.

When the stannous chloride solution contains about 5% hydrochloric acid, reagent blank figures are low, and independent of stannic tin concentration. This acidity is recommended.

Records of the young fish and plankton of Plymouth off-shore waters taken in ½ hr. oblique hauls of the 2 m. stramin ring-trawl were continued in 1948. They show an even lower production of young fish than in 1947 and a continued poverty of plankton organisms other than young fish. The dates on which collections were made are given in Table I.

The method described below was evolved as a substitute for counting, which is unsatisfactory for epiphytic marine diatoms. It depends upon the estimation of the amount of diatom-silica associated with unit dry weight of the sub-stratum. The sea-weed samples are first freed from sand particles and are then treated with strong nitric acid in the usual way. In order to convert the insoluble silica of the diatoms into a soluble form suitable for chemical estimation, the material is fused in a platinum crucible with a mixture of K2CO3 and Na2CO3 and the fused mass is dissolved in (silica-free) distilled water and filtered. The amount of silica in the filtrate is determined colorimetrically according to the method of Dienért and Wandenbulcke (cf. Atkins, 1926). The method is simple in practice and saves considerable time and labour. The full procedure is illustrated by reference to a study of the periodicity of Grammatophora marina (Lyngb.) Kütz. at Swanage, Dorset.

This species (PL. I) is found all the year round on Cladophora rupestris in a pit in one of the reefs at Peverel Point, Swanage. In summer it is the only epiphytic diatom on the Cladophora, while in spring and autumn small numbers of other epiphytes (Cocconeis, Opephora, Rhoicosphenia) are present. In order to obtain a fairly representative picture of the abundance of Grammatophora at different times, samples of the Cladophora were collected monthly from a definite area in the pit, each including 3–4 cm. of the apices of five to ten Cladophora tufts cut at random.

In the Celtic Sea, to the south of Ireland, water in some winters becomes sufficiently cooled and heavy to flow to the edge of the continental shelf and to run down the continental slope to a depth of several hundred metres. A theory of the phenomenon, termed ‘cascading’, has been developed. Three winters have been examined in detail.

In February 1927 much water, heavy enough to cascade, was present in the Celtic Sea and also in the English Channel. A probable course and speed of the cascading water over the shelf has been established. Since there were few observations of salinity and temperature over or beyond the slope, and none of oxygen anywhere, the theory cannot be completely established on the basis of the 1927 observations, full though they were.

Vertical extinction coefficients can be determined in water using selenium rectifier cells and colour filters. The cells are connected positive to negative with a simple 0-50 microammeter across the circuit, as in the Campbell-Freeth method. There is no deflexion when the two points of contact are brought to the same potential. This is done by lowering one cell into the sea to balance the rather less sensitive deck cell. The light reaching the latter is then reduced by the successive addition of opalized plates, to approximately 60, 30, 20 and 10 %, and the sea cell is lowered further to balance at each stage. The extinctions can then be calculated by the usual formula from the known percentage transmissions and the observed depths of balance. For each plate or combination a factor may thus be obtained which when divided by the appropriate depth gives the extinction. The method is rapid in operation and the drift error can be rendered negligible. The results agree with those found by the standard potentiometer method, but the applications of the new method are necessarily more limited.

During a cruise in the southern North Sea and English Channel in 1948 a series of measurements on light penetration was secured.

The apparatus used has been described, and methods of handling it discussed. A description of the experiments for determining the transmission curve of the niters used in the photometers, has been included.

Of the systematic and random errors to which the photometer measurements were liable the most serious lay in not knowing the precise depth of the sea cell.

Extinction coefficients have been determined from the gradients of logarithmic transmission-ratio/depth graphs, and the results tabulated for each station.

Temperature gradients were never large, usually of the order of 0·02 degrees per metre, the temperature usually dropping with increased depth, frequently there was no gradient at all.

The mysid fauna of eight night hauls made near the Eddystone Lighthouse is analysed.

Keys are given for the identification of Mysidacea which have been recorded from, or are likely to occur in, the Plymouth area.

During the past eight years records have been kept of the contents of the stomachs of all dogfish dissected in the Zoology Department of Reading University. The specimens, numbering about 450 altogether, came from Swansea Docks, and were caught off the west coasts of the British Isles. Students were asked to spread out the contents of the stomach of the fish they were dissecting in a dish, and to make notes of those animals they could identify. I then, as far as was possible, checked the identifications, and removed for more detailed investigation any specimens of particular interest. In the circumstances it was impossible to make accurate numerical records of each species, or to work out fragments from partially digested material, but the rare specimens were recorded by numbers. Sometimes a record of one means one perfect specimen.

On 13 November 1947 a specimen of the sea hare, Aplysia depilans L., which had been trawled in Babbacombe Bay, was sent to the Plymouth Laboratory. When it was realized that the animal was not the common A. punctata Cuv., collecting trips to likely places were undertaken in the hope of finding more. No others were found, but on one of the expeditions Dr D. P. Wilson picked up a specimen of A. limacina L.

Both A. depilans and A. limacina are found in the Mediterranean and on the west coast of Europe: A. depilans has been found in British seas before, but so far as is known A. limacina has not.

These occurrences provide the main reason for publishing this study. The paper also includes an account of the distribution of aplysiids in British waters and a review of the controversy over the identity of large specimens. As the animals are not usually described in natural history books, notes on the field characters are added.

I would like to thank the Director of the Plymouth Laboratory for affording me laboratory and collecting facilities and for his interest in the work. I am most grateful to Dr G. Bacci, who went to much trouble to send me specimens from Naples; to Dr W. J. Rees, who arranged for me to have access to the British Museum collection; to Dr D. P. Wilson, who has provided the photographs of A. punctata (Pl. I) and A. limacina (Pl. II); and to D. J. Slinn, who prepared the map.

The following records were obtained during a fortnight at the beginning of April 1949. The localities investigated were rocky shorepools below the Laboratory, and a small area at the upper tidal limit of Tamerton Lake. In addition to imagoes collected on the wing, larvae were reared from a mud sample taken from the stream bed just below the upper limit of tidal penetration. None of the Tamerton species are marine in the strict sense adopted by Edwards (1926), but those bred out are tolerant of brackish water and belong to the estuarine fauna.