Experiments were made with apparatus, largely of glass, specially designed to study the vertical movements of plankton animals under as natural conditions as possible at various depths in the sea. The animals were introduced in darkness into the apparatus which was kept covered with a black clothuntil it was lowered below the surface so that from the time they were caught, in the opaque tow-net bucket, to the time they were returned to the sea in the apparatus, they were never subjected to above-surface illumination. The experiments were begun and ended at the required depth by the dispatch of messenger weights down the suspending wire from above. The copepod Calanus finmarchicus (Gunn.) was used, but a few experiments were also made with another copepod, Euchaeta norvegica Boeck.

The apparatus is described in detail; it takes two forms. No. 1 consists of two vertical parallel glass cylinders AB and αβ closed at the top by a glass plate and at the bottom by a metal one, and having trapdoors at the middle of each which divide them into upper and lower compartments, A and B in one, and α and β in the other. They were filled with sea water and the copepods were introduced injo only one compartment of each cylinder, in standard experiments into A on one side and j3 on the other. The apparatus was now lowered to the required depth and the trapdoors opened so that the copepods were free to move up or down the whole length of the cylinders. At the end of an experiment the trapdoors were closed again so that the percentage proportions that have moved up or down under different conditions could be estimated. In experiments other than standard, one side was left normal as a control and in the other the conditions were altered for the purpose of the experiment, e.g. darkened or the pH of the water increased or decreased. Apparatus No. 2 is a modification, a long tube of seven compartments each separated by a controlled trapdoor.

During the years 1940–5 plankton samples were examined regularly from inside the Plymouth Breakwater, with the exception of a few months. Although it was not possible to do any quantitative work, special attention was given to larval forms, especially of the decapods, molluscs and annelids. The presence of these larvae gave a good idea of the breeding seasons of the various species. As it is only recently that many of these larvae have been identified, this is the first time that detailed specific notes have been made of their presence; and it is thought to be worth while to publish these results. To help those who wish to identify the larvae, a reference to a description of each species is given whenever possible, in square brackets after its name. It should be clearly understood that the absence of any species from these notes does not necessarily imply that it might not have been there, although all the samples were carefully examined. Most of the Coelenterates, the Cirripedia, Copepoda, and young fishes, are omitted here. Of the remaining ingredients of the plankton, various records were kept; and the occurrence of the more noteworthy of these forms, according to months, is shown in Table I.

On 2 February 1943, after some very fierce south-westerly storms, a curious object was found in the inshore plankton which proved to be the ‘float’ of a very young Velella in the Rataria stage. In order to be certain of its identity some small complete Velellas were compared. These were kindly sent by Miss Delap of Valencia, to whom I am much indebted. They ranged from 1.5 to 2 mm. across and were very similar to the young forms described and figured by Huxley (1858), who identified them with the Rataria of Eschscholz (1829). In this stage the float is growing and may already have the crest, a soft, much higher structure, projecting above it. In order to see the float clearly one of the Irish specimens, 2 mm. across, was macerated in 10% caustic potash, the float separating out and proving to be exactly similar to the Plymouth specimen.

The peculiar structure of this float and the fact that the available figures of the Rataria of similar size show only the external features make a short note desirable, for in the existing figures one cannot be certain of the exact extent and position of the float. So difficult was it to decipher that, although it seemed very probable that we had the float of a Velella, it was not possible at first to be certain.

A statistical study was made of the fertility of Evadne nordmanni, Podon intermedius, P. leuckarti and P. polyphemoides in the Clyde Sea-Area and of the former two species off Plymouth. The mean fertility of parthenogenetic females varies with different species. This is not correlated with the size of the species.

Within the species, Evadne nordmanni and Podon intermedius, there exists, in general, a positive correlation between the size and the fertility of parthenogenetic females.

The fertility of Evadne nordmanni is subject to seasonal variation. This is not correlated with fluctuations in the abundance of diatoms.

An inverse relationship was observed in Evadne nordmanni between the reproductive capacity of parthenogenetic females and the intensity of sexual reproduction.

A brief account was given of the formation of the resting egg in Evadne nordmanni and Podon intermedius.

Inshore water collected near Plymouth during the late summer and autumn of 1945, after enrichment with nitrate, phosphate and iron, did not support continued growth of a species of Chlamydomonas, of Chlorella, of a Cryptomonad and of two species of Chrysomonads.

The addition of 0-5-2-0 mg. Mn" or Mnv/m.3 allowed vigorous growth and the production of heavy crops. The effect on Chlamydomonas of adding as little as o-i mg./m.3 was apparent.

The growth of Coscinodiscus excentricus varied in waters collected from inshore and from offshore which had been similarly enriched with N, P and Fe. The addition of manganese had only a minor effect.

The addition of other microelements to inshore waters enriched with N, P, Fe and Mn did not affect growth rate of Chlamydomonas. Manganese starvation led to the production of small Chlamydomonas cells.

Manganese starvation caused a resting condition or lag period in Chlorella. A supply of manganese alone was insufficient for recovery; a period of illumination was also required for internal changes to take place before logarithmic growth was resumed.

The addition of manganese was effective when either nitrate or ammonium was supplied as source of nitrogen, in dim, bright, continuous, or discontinuous illumination.

Metabolic products which leached out of the cells into the water during the growth of both Chlorella sp. and of Nitzschia closterium acted as a growth stimulant.

Manganese at great dilution is rapidly 'collected' from solution by Chlorella. During growth this alga abstracts a material proportion from water containing 5 mg. Mn/m.3

Using Chlamydomonas as 'analyst', it was found that added organic detritus, or the corpses of marine bacteria, adsorbed a material proportion of the manganese in sea water containing 1-2 mg. Mn/m.3

Estimations of sodium, potassium, calcium, magnesium, chloride and sulphate have been made on the blood and muscle of marine molluscs and of the freshwater clam, Anodonta.

On comparing marine blood with sea water it appears that the cephalopods show a regulatory power (i.e. difference between blood and sea water) with respect to all ions tested except sulphate. The gastropods have a regulatory power for calcium, magnesium and chloride; the pelecypods for calcium and magnesium.

Calcium is always higher in blood than in sea water, while magnesium is lower. Chloride, where it differs, is lower.

If muscle is considered as two phases, cells and intercellular blood space, then from whole muscle and blood analyses it is possible to calculate the spaces between the cells, which work out at 11 % for pelecypods and 18 % for the other two groups. Further calculation gives the constitution of the cells themselves, leading to the conclusion that, of the ions under consideration, only K is present in the Pelecypoda and Cephalopoda, while the Gastropoda may have some Ca and Mg as well as K.

As expected the fresh-water clam contains little inorganic material. In relative proportions its blood is characterized by more calcium and less magnesium and chloride than that of marine forms. In muscle cells potassium dominates but other ions are present as well.

This work was carried out at the Laboratory of the Marine Biological Association, Plymouth, in the summers of 1936 and 1937, and at the Oceanographic Institution, Woods Hole, in 1939. It is a pleasure to express our thanks to the Directors and Staffs of these establishments for accommodation,facilities and advice during the progress of the investigation.

Patina pellucida occurs in two facies with characteristics of habitat and structure, the latter' affecting mainly the shell. One variety, pellucida, which lives on the fronds of Laminaria, has a smooth, elpngated oval and low shell, which is transparent and brown in colour, with 2-8 blue rays radiating backwards from the summit which is placed at the anterior end; it is normally devoid of either epiphytic or epizoic growth. The soft parts are pigmented because of the translucency of the shell, the radular teeth only slightly worn, and the arrangement of the gut coils adapted to a long, narrow haemocoelic space. The second variety, laevis, lives in caves in the holdfasts of Laminaria, and has a rough, round and usually high shell, though the proportions are more variable than in the variety pellucida. The shell is opaque and brown in colour, with 2-46 blue rays and also red-brown ones more or less regularly alternating with them; it has a central summit and is frequently covered with growths of various sorts. The soft parts are not pigmented, the radular teeth are considerably worn, and the coiling of the gut modified to fit into the rounder haemocoel.

The life history may be summarized: the animals breed maximally in winter and spring, the planktonic larvae settling mainly in May as spat about 2 mm. in length. They grow so as to reach a length of about 5 mm. in the following autumn and 10 mm. after a year of sedentary life, becoming sexually mature at 5 mm. length. The majority of the animals die after a settled life of 12 months, but a few linger on into a second year: nearly all these belong to the variety laevis, living a sheltered life within the holdfast of the weed, not exposed like pellucida to the effects of wave action and storm.

The cercaria described here was found on 21 April 1936, in twelve out of sixteen specimens of Nucula nucleus (L.) from Cawsand Bay, Plymouth Sound. The parasite was found by Dr Marie V. Lebour, who kindly gave me the infested molluscs for examination. I have thought it advisable to publish these notes, incomplete as they are, as it may not be possible for me to obtain further specimens of infested Nucula for some time to come.

The trematode was found to parasitize the digestive gland and gonad. The cercariae develop in irregularly shaped parthenitae which each contain from, two to ten larvae in various stages of development. The parthenitae measure 0.9–2.0 mm. in length.

The cercaria is very large and has a long, forked tail (Fig. 1). The measurements on the left of Table I were made with an ocular micrometer when the living cercariae were slightly flattened under the pressure of a cover-slip.

The body is elongate in outline, with a slight constriction just behind the oral sucker, and reaches its greatest diameter opposite the ventral sucker. The pre-acetabular region of the body is highly muscular and capable of much. extension and contraction. Near the posterior border of the oral sucker there are two blunt processes, ventro-lateral in position. In section, the body is somewhat cylindrical, and consequently it is almost impossible to obtain true measurements from living cercariae because different individuals could not be subjected to the same pressure under a cover-glass. The measurements given on the right of Table I are of mature cercariae fixed in Bouin's fluid without pressure, and supplement those of living cercariae.

During the years 1930–9 a series of records was obtained on the abundance of pelagic young of teleostean fish occurring in standard half-hour oblique hauls made with the 2 m. stramin ringtrawl in the neighbourhood of the Eddystone. These results provided a record by which the composition of the young fish populations could be compared from year to year and any marked changes brought to notice. During the course of the observations there had been a striking reduction in abundance of young fish, and this had been correlated with the movements of water masses as indicated by certain plankton animals. The series of observations was brought to a conclusion in August 1939 on the outbreak of war. The earliest opportunity after the cessation of hostilities has been taken to continue these observations on young fish, and collecting was started in June 1946 when a research ship was once more available. There has thus been a gap of nearly seven years during which there has been no detailed information on the conditions in the waters off Plymouth. The 1946 collections were started too late to include the main period of abundance of young fish resulting from the spring spawners, but they afford evidence that as regards the summer spawners at any rate there is no significant change from the conditions existing in 1939. We cannot say what the conditions have been during the intervening years, but analyses of phosphorus content of the water during each of the winters in the period 1939–46 tend to show that conditions have remained much as they are at present, and that it is unlikely that there has been any large incursion of the rich water characterized by Sagitta elegans which supports a large population of young fish.

Two ascidians have been collected in the region of Plymouth that do not fit the descriptions of known species sufficiently well to be identified with them. They are species of Trididemnum and Distaplia.

These genera are represented in waters around the British Isles only by Trididemnum tenerum (Verrill) and Distaplia rosea Delia Valle. If Trididemnum niveum should be a valid species and not a synonym for T. tenerum as considered by Hartmeyer (1924), it also is probably present. Accordingly, any species that cannot be identified with these forms must either be new, or must represent geographical extensions of species not previously recorded. In this last respect, the possibility of the Mediterranean Distaplia magnilarva reaching the western Channel must be considered. As the following discussions indicate, the conclusions are that the two forms described here cannot be properly identified with any of the above and are to be treated as new species.

Trididemnum alleni n.sp. is named after the late Dr E. J. Allen, so long the inspiring and humane director of the Plymouth Laboratory, while Distaplia garstangi n.sp. is so named as a small tribute to Professor Walter Garstang for his pioneering interest in the ascidians of Plymouth waters.

Trididemnum Alleni sp.nov.

Colonies are usually small, less than a centimetre in greatest length, and about 3 mm. thick. They are brilliantly white, an appearance retained even in preserved specimens of long standing. The surface is uneven. They are commonly attached to Eunicella and larger hydroids such as Antennularia at depths of from 10 to 30 m., and have been recorded with certainty only from the Plymouth area of the English Channel.

Ciona intestinalis (L.) is probably the most cosmopolitan species of ascidians and has long been of general interest. The adult morphology has been well described in monographic form by Roule (1884), the physiology of the heart and circulation by Heine (1902), Enriques (1904) and Wolf (1932), of the nervous system by Magnus (1902), Hecht (1918, 1926), Cate (1928), Haffner (1933), and Bacq & Florkin (1935), and of the digestive system by Yonge (1925). Developmental studies include that of the early embryology by Conklin (1905), problems of fertilization by Morgan (1945) and Damas (1899,1900). In no work, however, has there been a presentation of the entire Ciona organism from the tadpole stage through the critical post-larval stages to the young cionid ascidian. The present account portrays this period of development, together with a discussion of some significant but relatively obscure aspects of adult structure.

Eggs and theRearing ofCiona intestinalis

While ascidians in general are difficult to rear to maturity under laboratory conditions, Ciona is relatively easy, and together with Botryllus schlosseri (Pallas) and Diplosoma gelatinosum (M.-Edw.) is liable to appear more or less spontaneously in large aquaria into which tadpoles may have been brought. Artificial fertilization is readily accomplished, and at almost any time of the year, since Ciona is sexually mature above a certain size and reproduction is seasonal only to the extent of the rhythm of the growth cycle. Normally eggs are set free spontaneously at dawn, although individuals kept in the laboratory may accumulate eggs and the oviduct become swollen.

Collections of some 22,000 female and 6000 male Crangon vulgaris were made throughout the year from the shrimp fisheries of the Severn Estuary and Bristol Channel. All animals were measured.

The habits of the species are described; it can withstand a wide range of temperature but, though euryhaline, resembles other Decapoda in the inability to withstand low salinity combined with low temperature.

Osmo-regulation is apparently largely inhibited at low temperatures and to a greater extent in the males than the females.

Growth rate decreases with increasing age; in the female there is no increase in length when moulting from the ' neuter' to the egg-carrying intermoult. The duration of this intermoult, if spawning is successful, is about double that of the normal intermoult under the same temperature conditions. Growth almost ceases in the winter.

Secondary sexual characters are described, especially the differences between the endopodites of the pleopods in the two sexes.

Females become mature at a minimum length of 45 mm. in the Channel and seldom less than 50 mm. in the Estuary. The effect of the female sexual cycle on the size of the ovary and the form of the pleopods is described. The process of copulation is described; it can occur in the brackish waters of the estuary. Egg-laying always follows within two days of moulting into the egg-carrying condition but eggs are not retained if copulation has not occurred.

The females lie on their sides during the act of spawning and the eggs are firmly attached within thirty minutes to the egg-carrying setae on the basipodites of the first to fourth pair of pleopods, then to those on the endopodite of the first pleopod, finally to those on the coxopodites of the last two pairs of pereiopods.