Further tests on the settlement reactions of Ophelia bicornis larvae have shown that both Bullhill Bank and Salthouse Lake sands are less favourable to the larvae after sterilization than they are when fresh and untreated. The effect of a large variety of treatments of both sands has been investigated under conditions where the larvae were free to choose between two or more samples of sands presented together in the same dish. Results from these experiments, taken in conjunction with results obtained using conical vessels, show that sands may be classed as attractive, neutral or repellent with intermediate grading between these three main categories. It is concluded that organic material, living or dead, on the sand grains plays an important role in rendering a sand attractive or repellent to the larvae.

The presence of Trididemnum niveum (Giard) is reported from the Plymouth area. It is compared with the other two British species of Trididemnum, T. tenerum and T. alleni, with which it has in the past been somewhat confused, and all three are shown to be distinct species. A short diagnosis of the three species is given as an aid to identification.

The respiratory rates of three species of planktonic copepods, Acartia clausi, Centropages hamatus and Temora longicornis, were measured at four different temperatures.

The relationship between respiratory rate and temperature was found to be similar to that previously found for Calanus, although the slope of the curves differed in the different species.

The observations on Centropages at 13 and 170 C. can be divided into two groups and it is suggested that the differences are due to the use of copepods from two different generations.

The relationship between the respiratory rates and lengths of Acartia and Centropages agreed very well with that previously found for other species. That for Temora was rather different: the difference is probably due to the distinct difference in the shape of the body of Temora from those of the other species.

The application of these measurements to estimates of the food requirements of the copepods is discussed.

Records were made of the presence or absence of food in the guts of Calanus finmarchicus caught in three different depths, at intervals of 4 hr. through 24 hr. It was found that 80-100 % of the Calanus caught at the surface were full of food at all hours of the day.

The number of Calanus containing food in deeper water was distinctly less. This may be correlated with the abundance of its food close to the surface. Diurnal vertical migrations took place on some occasions when samples were taken, but not on all.

In the absence of vertical migration Calanus was abundant at the surface and feeding continuously at all hours of the 24 hr. Where vertical migration took place feeding was mostly at the surface and was restricted to the hours of darkness, i.e. in summer to a period distinctly less than 8 hr. round midnight. Laboratory observations confirm the absence of any feeding rhythm.

It has been observed by Chu (1946) that the marine diatom Nitzschia closterium, in bacteria-free culture, grows in the light with inositol hexaphosphate as phosphorus source, and with glycerophosphate.

Experiments have been made with the marine diatom Nitzschia closterium grown in artificial light with different nitrogen sources.

After the nitrogen source in the medium was exhausted, photosynthesis and cell division continued, the cells becoming nitrogen- and chlorophylldeficient.

On adding a nitrogen source to deficient cells and storing in darkness, synthesis of organic nitrogen, mostly water soluble, proceeded rapidly from ammonium, less rapidly from nitrate and very slowly from nitrite, with which it grows in light as rapidly as with nitrate or ammonium nitrogen.

Possible reasons why nitrate but not nitrite is reduced in the dark are discussed.

Chlorophyll and yellow pigments were synthesized in the dark by nitrogendeficient cells in quantity related to the organic nitrogen synthesized.

Chlorophyll was synthesized in small amount by non-deficient cells which had been growing rapidly before transfer to darkness. Cells contained less chlorophyll when grown in moderately bright than when grown in dim light.

The fifth paper of this series (1930) was the author's last, after which the work was handed over to L. H. N. Cooper. It was resumed by the author in 1948 to compare the early years with the post-war, since the determinations had lapsed. Measurements of the extinction coefficient of the water were also resumed, and it was desirable to obtain the chemical data at precisely the same time.

The phosphate results are, as before, expressed as P2O5 in mg./m.3, since for crop calculations this old unit is convenient; to convert to mg. P, which is more rational, multiply by 0437; to bring to milligram atoms multiply by 0–0141. The analyses have not been corrected for salt error, since the early results were not^ and the factor for this correction has not as yet been agreed upon.

The silicate results are given as mg./m.3, of SiO2, but though the standard used in Plymouth was checked for me by Dr E. J. King in Canada and agreed with his (King & Lucas, 1928), yet a different value had to be assigned to the silica factor of the picric acid solution. All values have accordingly been multiplied by I.44.

The phosphate results for E1, surface, from 11 February 1948 to 1 February 1949 are shown in Fig. plotted against the dates.

In previous papers (1932, 1933) the author showed that sea water taken from the English Channel contained about 10 mg./m.3 of copper, and obtained agreement between electro-deposition with subsequent colorimetric analysis and colorimetric analysis carried out on sea water concentrated to one-fifth of its volume. For extraction of the copper complex, carbon tetrachloride was found to be the most convenient solvent. The changes throughout the year have, however, only recently been determined. It is of interest to see that water from Station E1, surface, taken 23 March 1931, gave 10 mg./m.3 by direct colorimetry and 9·7 with electro-deposition, whereas the extraction method curves for 1948 and 1949, read off for the same date, showed respectively 9·4 and 11·4 mg./m.3.

Chlorophyll from the phytoplankton at station Ei was examined from September 1951 till August 1952, from o to 50 m. The minimum was in June, i-8 mg./m.3, and the maximum 342 in March, both surface samples. The maximum 50 m. sample contained 18-4 mg./m.3 in April. The winter minima were 4-6 and 43, surface and bottom, in November. Converted to water column (70 m.) values, over iog./m.2 is obtained for the wet weight of phytoplankton for September and March, the April maximum and the June minimum being 132 and 015 g./m.2. A comparison with the production obtained from phosphate analyses suggests that the phytoplankton crop each month is rapidly devoured.

Exact proportionality was found between concentration and absorption in a chlorophyll band up to 40 mg./l. and a moderate error up to 80 mg./l. The spectral absorption curves have been given for pure cultures of phytoplankton and for cells filtered out of sea water. The chlorophyll in such extracts, made with 80% acetone, is stable when kept in total darkness.

The collodion disks containing suspended clay arid the algal cells may show a surprising variation in their colour intensity, from dark grey to a very faint tint, even though obtained from sea water about 20 miles from land and over 70 m. in depth. The surface may be far darker than 5, 10 or 15 m. samples.

The botanical composition of the water was studied by allowing the algae to multiply in diffuse light after enriching the water chemically. Eight species of Chlorophyceae, and one species each of the Chrysophyceae and Dinophyceae were recorded. Of the Cryptophyceae one winter sample from 50 m. gave a nearly pure growth of Hemiselmis rufescens Parke. The diatoms Melostra borreri, Nitzschia closterium and Navicula sp. occurred commonly.

A trade waterproofing of sisal rope had still some effect after 3 days in sea water, the untreated being four-fifths saturated in 1 hr.

Treating manila, sisal and coir rope with Cuprinol or tar increases the wet weight at the most only slightly as less water is taken up than without preservative.

Water equilibrium is not attained till long after the B.S.I, specification test period for waterproofing sisal rope, but some effect of waterproofing may be distinguished up to perhaps 3 days.

On cotton netting copper naphthenate preservatives were most durable in a clean sea-water basin, but Dr Olie's method with cutch and ammoniacal copper sulphate ran them close and is cheaper. The best lasted 9 months as against untreated 5½ months.

On trawl twines the British Columbia grade of Cuprinol, a proprietary mixture containing copper naphthenate, was the best of those tried, and the twines lasted 16 months as against almost 9 for untreated manila.

On thin sisal rope six copper preparations are still under test.

In the course of a study of the British Nuculidae the specific characters of a large number of specimens of N. turgida and N. moorei were re-examined. It was found that no distinction could be made between the two.

Examination of variation in shell colour showed that it was not possible to differentiate into two distinct types. There was a continuous graded seiies from a dark purple-grey in which the basic yellow colour was almost completely masked, through a series with decreasing numbers of purple-grey radiations, to shells in which the markings could not be differentiated as rays, and finally to shells without purple coloration. The purple-grey colour is not necessarily laid down continuously, and it does not develop until the end of the second year. As the shells get older an increasing percentage of them become coloured.

Although the two types of faecal pellets described by Moore were found they could not be associated with the radiate and non-radiate shell types. The five grooved faeces that he claimed were typical of the radiate variety, were found in only three of the fifty-six specimens examined. Only one of them was from a definitely radiate shell.

Shell measurements and growth rates were studied, and no feature was found on which two forms could be separated. Samples show that reproduction takes place at the same time in the rayed and unrayed shells.

As the name N. nitida Sowerby is preoccupied it is suggested that the name N. turgida Marshall should now be used with N. moorei a synonym of it.

The black body-wall pigment of Holothuria forskali shows the characteristics of melanin.

From histological evidence it appears that the pigment is formed in association with the amoebocytes of the coelomic fluid, which eliminate the pigment in the body wall.

The amoebocytes contain a phenolase system, distinct from the cytochromecytochrome oxidase system, with the properties of tyrosinase.

The relation of these findings to those of a preceding and more complete investigation into melanogenesis in Diadema is discussed.

The previous work on spontaneous activity in ascidians is briefly reviewed: there is clear evidence for the existence of spontaneous contractions of the siphons of certain monascidians but no function has been ascribed to this activity. Phallnsia mammillata exhibits spontaneous contractions of its siphons at intervals of 6-9 min. over long periods in Plymouth tank sea water. The siphon contractions are synchronous. The frequency of the contractions may shift fairly quickly under constant conditions to a higher or lower rate which is usually twice or one half the normal rate.

A method of recording the volume of water propelled by spontaneous activity is described. The method also records the ciliary through-current at the same time as the squirts.

The frequency of the squirting is increased during a period of partial or complete starvation. It is reduced again by adding food cultures to the water.

The physiological mechanisms involved in the spontaneous activity are investigated and comparisons are made with the spontaneous activity exhibited by Metridium senile and Arenicola marina. It is finally suggested that the spontaneous squirting of certain ascidians is an integral part of their feeding mechanism.

In the pericardial cavity of the decapod Crustacea the nerve fibres end in characteristic neuropile-like networks which surround bundles of thicker fibres or spread over the pericardium wall and the heart ligaments. It is proposed to designate the whole system of these elements as 'pericardial organs'.

The arrangement of the elements constituting the pericardial organs varies in different groups of the Decapoda. In the Brachyura the bundles of fibres surrounded by neuropiles form conspicuous trunks linked with each other; the main parts of these trunks are situated at the openings of the branchiocardiac veins into the pericardial cavity. In Eupagurus bernhardus two main trunks flank the heart giving off branches which spread in various directions and form at some places plexuses on the pericardium wall. In Leander serratus the pericardial organs are represented by several trunks running near the lateral wall of the pericardium in the dorsal direction and uniting by archshaped branches. In Homarus vulgaris there are four comparatively much shorter trunks situated at the lateral wall of the pericardial cavity, but plexuses of neuropile-like nerve fibres spread over large areas.

The trunks of the pericardial organs are suspended in the pericardial cavity in such a position that they may be bathed on all sides by the blood passing from the gills. The plexuses extending over the membranes are situated in places also exposed to the blood stream. There is evidence indicating that the pericardial organs produce substances influencing the heart-rhythm. It is assumed that these substances are liberated into the blood by a process of neurosecretion occurring at the terminations of the nerves.

A new longifurcous, pharyngeate, distome cercaria has been found to develop in the marine bivalves Hiatella arctica (L.) and H. striata Fleuriau. This larva was present in 12 of 200 specimens of H. arctica collected from Drake's Island during the summer months (May-August) of 1952,.while the same parasite was found in 15 of 100 specimens of the limestone borer, H. striata, dredged from Plymouth Sound during June and July of the same year.