1. A brief survey is given of the methods for the estimation of mercury by means of dithizone.

2. The precautions necessary in mercury estimations are outlined and the stability of dithizone in chloroform solutions shown to be satisfactory.

3. Data are presented on the rate of extraction of copper, from sea-water solutions, at low pH values with chloroform and carbon tetrachloride solutions of the reagent. Using the former solvent the rate of extraction of copper is shown to be slow, while mercury is rapidly extracted under the same conditions.

4. The use of potassium cobalticyanide in combination with a chloroform solution of dithizone has enabled a method to be developed which eliminates interference of copper in mercury estimations; with the procedure given, mercury in concentrations of 1 to 55µg/l. may be determined in the presence of copper concentrations up to 69 mg./l.

5. Copper may be determined in the same solution subsequent to the mercury estimation by the use of sodium diethyldithiocarbamate, after the solution has been made alkaline with ammonium hydroxide.

A fungus has been found causing a fatal disease among mackerel in British waters, and a very high rate of infection has been maintained for over three years. It is spread throughout the viscera by the circulation, being particularly common in the kidney and spleen: it is very rare in the muscles and has never been found in the gonads or central nervous system of these fish.

Growth of the aseptate, sparsely branched hyphae is usually radial, from one of three types of central body. The length and thickness of the hyphae varies according to the density of the infected tissue and the degree of resistance which is set up. Connective tissue capsules are laid down round spores and hyphal bodies, occluding the small vessels or capillaries in which they become lodged. Growth of the parasite stretches the capsule, which may become a tubular gallery containing many growth stages. This may effectively seal the parasites which then degenerate; or, more usually, the strong proteolytic enzymes secreted by the advancing hyphae perforate it, and proceed to convert the whole organ into a necrotic mass in which hyphae grow to great lengths. There is always a tendency for hyphal walls to collapse behind the outgrowing hyphae, giving the appearance of hair-like threads with swollen ends.

Any of the following developments may occur, apparently on any hypha, in tissue undergoing necrosis: (1) Bunches of large elongated chlamydospores, with a very hard exospore and a thick (?) gelatinous endospore—a germination pore is present; (2) Single dome-shaped conidia (like those of Entomophthoraceae); (3) Hyphal bodies of various shapes, either growing at once where they become lodged or becoming rounded and encysted—these may subdivide irregularly, each spherule having a separate cyst; (4) Branched conidiophores with rounded tips which contain endo-conidia—liberated as minute amoeboid bodies into the blood vessels; (5) Simple clavate sporangia from the ends of unbranched hyphae, also containing endo-conidia; (6) Hyphal fusions of numerous kinds, immediately proceeding to further outgrowths of similar hyphae; (7) ‘Spores produced by hyphal fusion’: fusion of two hyphae followed by the participation of neighbouring hyphae which also contribute to form the outer, resistant, rugose wall.

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.

The prosobranchiate gastropods are divided into three orders, the Archaeogastropoda, the Mesogastropoda and the Stenoglossa. In the first of these groups the gonad is connected with the right kidney by a duct which is developed from the ovary, and the sex cells escape to the posterior end of the mantle cavity through the right ureter. The eggs are usually shed singly into the sea where fertilization occurs. In the Mesogastropoda and Stenoglossa the right kidney is not developed and the ovarian duct leads to a short and narrow section of the genital duct which is typically ciliated, and may communicate with the pericardium by a gonopericardial duct and passes anteriorly to a long glandular tract running forwards to the mouth of the mantle cavity. This glandular section is incipient in the archaeogastropod Calliostoma zizyphinum. It is probably formed, as Thiele (1935) suggests, by an ectodermal intucking and will be referred to as the pallial oviduct, for it lies anterior to the opening of the original right kidney, and must be derived from the ectoderm of the mantle (Bourne, 1908; Giese, 1915). The short duct which links it with the ovarian duct will be termed the renal oviduct: in Paludina (=Viviparus), in which its development has been investigated (Drummond, 1903), it is formed from the vestige of the right kidney, and even in the highly specialized Stenoglossa it retains the connexion with the pericardium which is characteristic of the right and left kidneys of the archaeogastropods. Similarly, in the male system of the Mesogastropoda and Stenoglossa there can be distinguished a testis duct, which is connected to the posterior end of the mantle cavity by a renal vas deferens, and this in turn is followed by a pallial vas deferens.

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 feeding mechanism of the nudibranch Calma glaucoides (Alder & Hancock) has been described by Evans (1922) and that of the crustacean Nebaliopsis typica (Sars) by Cannon (1931) and myself (1943). Calma is seasonally relatively abundant in the coastal waters of parts of Europe where it is found attached to the eggs of certain fishes. It is known to feed exclusively on these eggs and young embryos. Nebaliopsis is a deep pelagic form from the Antarctic Ocean. It has been obtained only rarely and cannot be observed alive and in its natural environment. Two specimens were available for examination. Thus complete certainty about the diet is impossible. The external features and mouth parts were described by Cannon and the structure of these suggested that Nebaliopsis, like other known Nebaliacea, might be a filter feeder. Examination of the morphology of the gut and its contents indicates that this is very improbable. There is no mechanism for the sorting out of particles of different sizes such as is found in other filter feeders. The food passes into a blind sac and there is no apparent means by which the large quantities of indigestible material taken in when particles are filtered from the water indiscriminately might be evacuated. In addition to this negative evidence the food mass found in the large storage chamber of one specimen was completely homogeneous and structureless, its appearance in section suggesting coagulated yolk. It has therefore been suggested that Nebaliopsis feeds on fish eggs found floating in the deeper water layers.

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.

1. Aloidis (Corbula) gibba is a eulamellibranch specialized for life in muddy gravel substrata to depths of up to about 80 fathoms.

2. The shell is asymmetrical, the margin of the smaller, left valve being uncalcified and so fitting within the marginal region of the right valve. A possible manner in which this asymmetry is produced by the differential secretory activities of the two mantle edges is discussed.

3. The marginal periostracum of the left valve has strengthening calcined regions posteriorly, probably to protect the siphons when extruded.

4. The external ligament is reduced and the resilium condensed, possibly permitting some antero-posterior rocking of the shell valves when the adductors contract.

5. The process of burrowing is described; on its completion the animal is anchored by a single byssus thread.

6. The siphons are very short, the tentacles of the siphonal sheath lying on the surface of the substratum. The inhalant siphon is wide and relatively insensitive; it draws in much bottom material. The exhalant siphon is tubular and very sensitive. It is controlled by two paired bands of muscle.

7. The great quantities of pseudo-faeces which accumulate are expelled by periodical contractions of the 'quick' portions of the adductor muscles, the asymmetry of the shell valves causing great reduction in the size of the inhalant chamber. The foot may also assist in clearing the chamber. 8. The large ctenidia create a very powerful current; they are adapted for dealing with large amounts of sediment by means of specialized terminal, guarding and cirrus-like cilia. Control of 'pumping' is primarily by means of the exhalant siphon.

9. The stomach is large in correlation with the great amounts of inorganic material carried in with the food.

Tumtella communis is a highly specialized burrower in gravelly mud. Once buried, it moves little in correlation with its apparently exclusively ciliary feeding habit. An inhalant depression in the mud is made by lateral movements of the foot. There is a unique exhalant siphon constituted by two overlapping folds, and through this water and faecal pellets are expelled without disturbing the surrounding mud.

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.

In the many references to Teredo from Plymouth early workers assumed that the common form was T. navalis L. Orton (1913) refers to this species, but he has recently told me that he did not try to identify the species precisely. It is almost certain that he was dealing with T. norvegica Spengler, which, it is now known, is the common large species in these waters. Harington (1922) ascribes the species with which he worked at Plymouth to T. norvegica, with a query, and no doubt he identified the species correctly. Yonge (1926) states that the Teredo with which he worked from the experimental raft near Plymouth Breakwater were probably all T. norvegica. Later he identified them as that species. Unlike T. navalis it does not carry its young in a brood pouch, but sends its eggs directly into the sea.

Three species have been found in recent years from the Plymouth district, two from the experimental raft moored near the Breakwater and from other fixed wood, and one from driftwood. The two commonly occurring from the raft are (1) T. norvegica and (2) a much smaller species carrying its young in a brood pouch, closely related to, if not identical with, T. navalis. The third species, occurring occasionally in driftwood, is T. megotara Forbes & Hanley. Purchon (1941), who worked at the last species from specimens in a tank at the Plymouth Laboratory, saw no sign of a brood pouch; and in those collected later from Wembury and examined by myself it was found that the eggs, shed directly into the sea, were very numerous.

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.