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.

There are six species of Chaetognatha that are usually taken in Scottish plankton samples from the appropriate localities. These are

Sagitta setosa J. Müller, from lower salinity water such as that found in the southern and central North Sea and the Irish Sea.

S. elegans elegans Verrill, from mixed oceanic and coastal water.

S. serratodentata Krohn, from surface and subsurface warm oceanic water of fairly high salinity.

S. maxima (Conant), from cold deep water.

Eukrohnia hamata (Möbius), from cold deep water.

Sagitta lyra Krohn, from warm deep water.

All are valuable indicator species (Meek, 1928; Russell, 1935, 1939; Fraser, 1937, 1939). Of these, the first five have occurred in somewhat varying numbers and distribution according to hydrographic conditions every year since at least 1935, except possibly during the period 1940–45 when no investigations were made. S. lyra has been taken in most of these years, but was not found in 1935, 1936 or 1937. It was, perhaps, most abundant in 1948, and one specimen was taken in that year from the Moray Firth, though its presence there must be regarded as distinctly unusual.

Spadella cephaloptera (Busch) is widely distributed throughout the Scottish area as a bottom-living form and is therefore not included here as planktonic.

There are several other species which occur only occasionally in the Scottish plankton either as individual specimens or in small numbers in specialized localities: for convenience these may be termed unusual species. They are as follows.

Sagitta elegans arctica Aurivillius, which has occurred in several years in the Faroe-Shetland Channel associated with the community characterized by Eukrohnia hamata and Calanus hyperboreus.

The family Pyramidellidae contains a number of species of gastropod molluscs of similar structure and mode of life.

The shell is calcareous and spirally wound and may be closed by an operculum. The foot bears a transverse fold anteriorly, the mentum (Fig. 1, MT), separating the opening of the penial sheath (PO) below from the mouth (M) above. It has a lateral glandular streak on each side (Fig. 2), presumably sensory.

The head has ear-shaped tentacles (Fig. 1, T), richly innervated and with cilia setting up a strong water current, so that they constitute a powerful sensory mechanism. The eyes (E) lie between the tentacles.

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.

The nomenclature follows André (1946). No Acarina were listed in the Plymouth Marine Fauna (Mar. Biol. Assoc., 1931), but Hydrogamasus littoralis, Rhombbgnathus pascens, R. seahami, and Halacarus basteri have since been recorded from Wembury (Colman, 1940).