Chaetopterus variopedatus (Renier) is a tube-dwelling worm that has note worthy luminescent powers. Production of light is by an extracellular process resulting from the discharge of a luminescent slime into the sea water. Because of its luminescent ability, the epidermal glands of Chaetopterus have been repeatedly described in detail, but it is still not clear what elements produce the luminescent secretion. I have, therefore, re-examined the histology of the light-producing regions of Chaetopterus with a view to determining the character of the photocytes or light-producing glandular cells.

The work of F. S. Russell on plankton animals as indicators of different water masses, particularly the species Sagitta setosa J. Müller and S. elegans Verrill, and of Atkins, Harvey and Cooper on plant nutrients, have pointed the conclusion that the water in the English Channel, in the neighbourhood of Plymouth, was consistently richer in nutrients and plankton in the 5 or 6 years prior to about 1930 than it has been at any time since. (See Russell, 1935, 1936a, 1939; Cooper, 1938; Armstrong & Harvey, 1950; Harvey, 1950.) The comparative infertility of the sea in this area in the 1930's, and until the present time, has been shown not only by the low winter phosphate- maximum, and by the sparse plankton characterized by the presence of S. setosa, but also by the reduced numbers of young fishes surviving their early larval stages after the summer spawning (Russell, 1939).

Further study of the phenomenon of gregariousness during settlement, demonstrated in the oyster Ostrea edulis (Cole & Knight-Jones, 1949; Knight-Jones, 1951) and the barnacle Elminius modestus (Knight-Jones & Stephenson, 1950), seemed to require laboratory experiments, for which a viviparous animal with a brief, lecithotrophic, pelagic stage appeared to be needed.

The production or emission of light is under nervous control in many marine animals that have a differentiated nervous system. The nature of this control is a field of neuro-effector physiology that has been little explored, but one which could advance considerably our knowledge of the behaviour and functioning of luminescent marine animals. Nervous control of luminescence is achieved in diverse ways in different animals, namely, by squeezing forth a pre-formed secretion through muscular contraction, as appears to take place in Cypridina; by exposing a continuously luminescent organ through rotation or by movement of shutters, as in the teleosts Photoblepharon and Anomalops; by initiating cellular changes that lead to intracellular luminescence, for example in the photophores of the shrimp Acanthephyra debilis; and by directly activating light gland cells to secrete. The last method is a special example of nervous regulation of glandular secretion, and is the process occurring in the polychaete Chaetopterus (Dahlgren, 1916; Harvey, 1920, 1940).

A number of excursions to the Celtic Sea have been made, first, to obtain for Dr D. P. Wilson water as contrasted as possible with that in the English Channel near Plymouth, and secondly to obtain answers to hydrographical queries. This paper presents some of the chemical findings which may prove to be relevant to Wilson's investigations on the development of the young planktonic stages of invertebrates. Since not all sea water in the Celtic Sea was likely to be equally suitable for his work, some means was needed for quickly assessing waters on the spot. There are a number of objections to the use of phosphate or other chemical analyses on board so that examination of plankton hauls was chosen. Suitable water was selected on the basis of the bulk and variety of zooplankton and the presence of species known to indicate ‘elegans’ water. The 2 m. stramin ring-trawl, fished for the standard half-hour oblique haul, was used.

The copepod parasite Mytilicola intestinalis was first described by Steuer (1902) from the gut of Mytilus galloprovincialis (Lam.) in the Gulf of Trieste. Monod & Dollfus in 1932 recorded the same species from M. edulis from Marseilles. In 1939 the parasite was first recorded on the German North Sea coast by Caspers near Cuxhaven, and in 1947 by Ellenby from Blyth, Northumberland. It is now widespread along the English south coast in M. edulis, but the distribution still shows some irregularities that are discussed in this paper.

I am grateful to Prof. J. E. G. Raymont for facilities at University College, Southampton, and for the use of a research table at Plymouth; to Mr F. S. Russell, F.R.S., and the staff of the Plymouth Laboratory of the Marine Biological Association for their assistance; and to Dr D. P. Wilson for facilities to collect at Exmouth. Dr H. A. Cole and Mr J. N. R. Grainger have kindly given me information from their papers not yet published. Dr D. J. Crisp and Dr H. G. Stubbings have assisted by sending me several samples of mussels, and I am grateful also for information received from several other friends named in the text.

The variability to be expected in replicate plankton hauls, using several methods of hauling and a number of different nets, has been examined by Winsor & Clarke (1940). Some of the variability is due to true sampling variations, inevitable when discrete entities in suspension are sampled, and some is due to inadequacies of sampling technique. Winsor & Walford (1936) found that the variability of their vertical net hauls could be explained on the basis of a random distribution of the population, the variations being ascribed to variations in the volume of water filtered in successive hauls. They fully realized that the alternative, namely a non-random distribution of the population, was not disproved; but they considered that since widely different organisms showed little difference in variability the evidence was strongly in favour of a random distribution. They did note, however, that agreement between these different organisms was less marked when the numbers caught were large.

Biological and physical evidence suggests that the area south-west of Ireland is one of loss from the Celtic Sea. In winter the shelf water there may cascade to 400 m. or even deeper (Cooper & Vaux, 1949). All the evidence suggests that new water enters the Celtic Sea and English Channel over the continental slope between Ushant and the Sole submarine promontory 250 miles to the west. No evidence exists that the Atlantic surface waters are ever rich enough to account for the enrichment of the English Channel observed in the nineteen-twenties. Although the existence then of sufficiently rich oceanic surface water cannot be excluded, it is much more probable that the enrichment was brought about by some form of upwelling. The cause of this is thought to be composite. Only a few times in a century may the component phenomena work together to produce the maximum effect. Each component needs separate study.

In the family Pyuridae the larval and post-larval stages are known only in Halocynthia pyriformis (Rathke) and Boltenia echinata (L.). Berrill (1948) has pointed out that this is the least specialized family of the Pleurogona and that its larvae are therefore of particular interest.

On 1 August 1950 specimens of Pyura squamulosa (Alder), up to 36 by 30 mm., were collected from boulders at low-water level in the Sound of Kerrera, Argyll. They were taken on the following day to the Marine Station, Millport, and there kept in running sea water until 4 August, when artificial fertilizations were carried out.

In September 1950 Dr H. W. Parker of the British Museum (Natural History) kindly sent me a specimen of an ascidian collected by the yacht, Manihine, for identification. This has proved to belong to an unnamed species of the genus Corella, here described as C. halli. The unique type specimen was obtained from dredgings on a shingle bottom at 35 fathoms, 10 miles WSW. of the Casquets in the English Channel, 30 August 1950 (Manihine, Station no. 81).

External features. The animal (Fig. IA) is enclosed in a transparent test, laterally flattened, circular, of 4 mm. diameter and fixed basally to pieces of sand; there is no stalk. The branchial aperture is terminal anteriorly; the atrial aperture is also anterior and dorsal to the branchial aperture. Both apertures are sessile.

In connexion with a study of plankton-benthos interrelationships off the coast of Northumberland, an apparatus was designed for the collection of plankton in the immediate vicinity of the sea-bottom, a layer usually missed by ordinary plankton sampling gear. The present account is a brief description of this apparatus, together with some notes on its handling at sea and the work carried out with it on board the Dove Marine Laboratory's boat, the Pandalus II.

I was fortunate in having the willing co-operation of numerous people who helped in various ways, and to whom I am very greatly indebted. These included my father, Mr E. Bossanyi; Mr T. E. F. Sanderson of Eastcote, Middlesex; Prof. A. C. Hardy, Oxford University; Dr C. E. Lucas, Fishery Laboratory, Aberdeen; Prof. A. D. Hobson, Director of the Dove Marine Laboratory; Dr H. O. Bull, Assistant Director; Messrs Ellis and Co., Engineering Works, Swalwell, County Durham; the Department of Mechanical Engineering of King's College, Newcastle upon Tyne; and the entire scientific, boat and technical staff as well as research students at the Dove Marine Laboratory. Without them, the design, construction and employment of the apparatus would not have been possible.

Knowledge of the hydrography of the English Channel and Celtic Sea is needed as a background for the life histories of the mackerel, pilchard and herring and, indeed, of every organism living within range of the Plymouth Laboratory. Such knowledge can never be fully attained until we know more of the exchanges with the deep Atlantic Ocean which take place 200–300 miles to the south-west and west over the continental slope. Moreover, at the slope we shall need to know not only what waters move in and out but what move up and down. Long ago Storrow (1925), and no doubt others, saw clearly some of the problems here to be discussed, but they were unable to bring factual evidence to bear. A critical reconstruction of the considerable but fragmentary observations in the neighbourhood of the continental slope of the Celtic Sea will be presented here and supplemented by observations made in 1950 with the generous co-operation of the vessels of the National Institute of Oceanography (Discovery Committee) and in 1948 by H.M. Surveying Ship Dalrymple. Though many of the conclusions remain tentative, it should now be easier to design observational work at sea to test specific hypotheses at the right time and place by the methods of experimental oceanography. The numerical results in 1950 will be published by the Conseil International pour l'Exploration de la Mer.

Turbonilla elegantissima (Montagu), though one of the larger and more familiar members of the family Pyramidellidae, is known only by the shell and the appearance of the animal when crawling. Forbes & Hanley (1853) and Jeffreys (1867) state that it is common around the British Isles, and that its foreign distribution is from Tromsö to the Mediterranean. In the fauna list of the Marine Biological Association (1931) it is recorded as T. lactea (L.) ‘occasionally under stones, particularly where there is a certain amount of silt’ from low water to a depth of 10 fathoms, and, in the Salcombe estuary, ‘very common on the Zostera south of Pilworthy Pt., (Allen & Todd, 1900)’. Actually this opisthobranch has its own extremely specialized habitat, as do, apparently, all other members of its family (Fretter & Graham, 1949): like them it is an ectoparasite, restricted to the vicinity of its host. Specimens have been collected from the laminarian zone along the south coast of Devon and Cornwall, the largest number from Church Reef, Wembury, where silt and sand around boulders and between shale ledges provide suitable niches for such sedentary worms as Audouinia tentaculata and Amphitrite gracilis. When covered by the tide the tentacles of these polychaetes extend through the silt and sand and spread over the surface of the rocks for a considerable distance, and it is in the vicinity of these tentacles that Turbonilla lurks to suck liquids from them by means of a long proboscis (Fig. 1). Audouinia and Amphitrite were also recorded at Pilworthy Pt., but their association with Turbonilla was then unknown.

The 1949 production of young fish and plankton in Plymouth off-shore waters continued to remain at the low level characteristic of the last few years. Collections were made by half-hour oblique hauls of the 2 m. stramin ring-trawl; the dates are given in Table I. The fortnightly averages of all young fish, excluding clupeids, are shown in Table II with comparative figures for 1948 and for the period 1930–34. The highest value, in the second half of June, may have been unduly weighted, since only one collection was made in the fortnight. The monthly total catches of young fish are given in Table III. The sum of the monthly averages for all young fish (459) was slightly higher than the 1948 value (3111), due to larger catches of clupeids; young fish other than clupeids continued to decrease. A few post-larval herring were present in January, and for the third successive year young Mugil spp. occurred; two in October and one in December. No young plaice were caught, although a very small number of plaice eggs were present in January and February. Plankton, other than young fish, was very sparse throughout the year, Calanus being even scarcer than in 1948. Sagitta setosa was the dominant Sagitta species, although never abundant, and as in recent years the numbers were particularly low from February to June. A few S. elegans occurred in November and December (maximum, 66 specimens). The abundance and percentages of the Sagitta species are shown in Fig. 1.

In the deep water of the eastern North Atlantic below 2000 m. the variations with depth of salinity, temperature, density, oxygen, phosphorus compounds and nitrate are quite small. By contrast the silicate content is doubled in a descent from 2000 to 4000 m.

The distinctive behaviour of silicate is revealed by diagrams (Fig. I) relating it to salinity, temperature, density and total phosphorus at station 2659 worked by R.R.S. Discovery II on 12 May 1950 (Armstrong, 1951; Cooper 1952, Table IV). The temperature-salinity diagram (Cooper, 1952, fig. 15, to 1500 m. only) suggests that between 1200 and 2000 m. we have to deal with simple mixing of the mean Gulf of Gibraltar and North Atlantic Deep waters. If silicate concentration were subject only to mixing processes the curves in Fig. I between these depths would be straight lines. They are not—consequently it would seem that solution of either particulate silica or of aluminosilicates may be occurring. As yet, clear interpretation is not possible. At least five hypotheses may be erected to explain, in whole or in. part, the observed distribution: (i) solution of bottom deposits; (ii) solution of ‘clay’ and of silica in suspension; (iii) concentration by vertical partition; (iv) tundra drainage; (v) sinking of surface water. These are examined in turn.

Comparatively little is known about the factors affecting egg-laying in Calanus. Deductions from work in the field have been made by Nicholls (1933a, b) and Marshall, Nicholls & Orr (1934), and experimental work in the laboratory has been done by Raymont & Gross (1942).

Emgg-LayingInCalanus

The anatomy of the reproductive system has been described in detail by Lowe (1935). In the female there is a pair of diverticula extending from the proximal end of the oviduct into the head region, and into this the developing eggs pass. Each diverticulum has a dorsal and a ventral channel, and the eggs pass from the dorsal into the ventral channel and then into the oviduct proper. The oviduct rises from the forward end of the ovary and crosses the body laterally and ventrally so that a short part of it can be clearly seen.

The chain-forming prosobranchiate gastropod Crepidula fornicata is not indigenous to British waters, but has been introduced during the last 70 or 80 years, probably from North America, upon imported oysters, and, during this period, has spread to most of the oyster beds on the south-east and south coasts of this country (see Orton, 1950b, for a recent discussion of its present distribution). In spite of the interest shown in this species because of its detrimental effect upon oyster culture, and because of its well-known change of sex from male to female, little is known in the literature of its breeding habits here, or in North America. Conklin (1897) states that the spawning period in New England lasts from early summer to about mid-August, no larvae being found in late August. Murie (1911), in discussing the introduction of this species, elaborates upon Conklin's observations, and gives evidence that females, in samples of Crepidula taken from the River Colne in May and September 1898, contained spawn beneath their shells. Orton (1912a, p. 438) states that he is ‘informed by Professor Conklin that American Crepidulae begin to spawn in May and possibly in April, whilst English Crepidulae begin to spawn in early March’. In a footnote, he adds ‘Crepidula spawned in tanks at Plymouth in early February’.

Since the estimation of ammonia in sea water by existing methods either presents difficulties or requires much time, an alternative based on a different principle has been explored.

The first serious attempt to determine the age and growth rate of the common mackerel (Scomber scombrus L.) appears to have been made by Captain Atwood in 1856 (quoted by Brown Goode, 1884, p. 116) in the Massachusetts Bay area of northern North America. Small fish caught by Atwood in October of that year measuring 6½–7 in. in length (16.5–17.5 cm.) he believed to be the young of the year (i.e. they belonged to the O-group). Mackerel belonging to this group he calls ‘spikes’. ‘Blinks', ‘tinkers’ and ‘second size’ fish he assigns to the I-, II- and III-year age groups respectively, but unfortunately gives no data as to the sizes of those categories, merely stating that everyone well acquainted with mackerel makes the same groupings ‘as there seems to be a line of demarkation between the different kinds which stands out prominently’. Sixteen years later, on 27 July 1872, Malm (1877, p. 409) observed large numbers of small mackerel close inshore in the Gullmarfjord near Christineberg. Several tons of those mackerel were enclosed in a seine, but only ten specimens were retained as all the others escaped through the meshes. These ten fish ranged in length from 67 to 100 mm. and Malm surmised their age to be 13 months. Collett (1880, p. 18) stated that on the coast of Norway I-year-old mackerel are ‘fingerlang’. To fish of 20 cm., taken at the end of August, he ascribed (without supporting data) an age of 2 years, with sexual maturity supervening at 3 years at an unspecified length.

The Common Dragonet, Callionymus lyra L., is one of the commonest fishes in the Plymouth area, and is widely distributed in European seas. Recent reports indicate that it occurs also off the coast of West Africa (Fowler, 1936; Poll, 1949). This fish, like others of the genus, attracts attention because, although it has very little economic importance, it is strikingly coloured and the sexes are markedly different. Work has been done on the breeding by Holt (1897, 1898), and by Holt & Scott (1898); on ova and larvae by M'Intosh (1885), M'Intosh & Prince (1889), Cunningham (1891), Holt (1897), Ehrenbaum (1905–9), Fage (1918), Mielck (1925), Duncker, Ehrenbaum, Kyle, Mohr & Schnakenbeck (1929); on seasonal abundance and distribution of post-larvae off Plymouth by Russell (1930–47) and Corbin (1948); and on the skeleton by Günther (1861) and Ford (1937). The mature males are provided with remarkable secondary sexual characters both in coloration and in relative lengths of snout and of median fins, which render them so different from the females that they were originally regarded as different species and known as the Gemmeous Dragonet (male C. lyra L.) and the Sordid Dragonet (female C. lyra L.=C. dracunculus L.) respectively (Donovan, 1808; Yarrell, 1859; Couch, 1863). The sexual dimorphism and seasonal variation of this species has been much studied by Holt (1898), Smitt (1892–95), Gallien (1934), Letaconnoux (1949) and Desbrosses (1949). Very little information has so far been provided about its age and growth, with which the present paper deals.