During a study of the fauna of the level sea-bottom off Plymouth on which a report has already appeared in this Journal (Ford, 1923), the samples taken in the Petersen bottom-sampler and in the conical dredge were preserved in alcohol and retained. A subsequent re-examination of the accumulated material has provided information on the growth of a number of lamellibranchs, which forms the subject matter of the present paper. As the samples were collected according to a programme arranged expressly to determine the animal communities represented, it was not to be expected that they would yield complete data on the growth of a particular species, but the results are interesting and useful when considered in their relation to the food supply of our food-fishes. I must again acknowledge my indebtedness to Mr. R. Winckworth of Brighton for his continued assistance in identification and measurement of specimens. Such expert confirmation of identification was naturally desirable’ in dealing with the growth of species from the earliest stages. Throughout the paper the term “length” is adopted, meaning the greatest antero-posterior dimension of the shell. This term was used by Weymouth (1923) who gave a figure on page 20 which shows clearly the measurement to which the term is applied. I have also used “length” as a measure of growth-rings on the surface of shells. In Fig. 7 on Plate III, I2 is the length of the second ring on a shell of Spisula solida. Thirty-three genera of lamellibranchs are represented in the material of which all but nine appear among the records compiled by Todd (1907) and Blegvad (1917) of the contents of the stomachs of fishes which include dab, flounder, long-rough dab, plaice, sole, cod, haddock, whiting, cat-fish, viviparous blenny, fresh-water eel, herring and mackerel.

1. The food of the bottom fauna of the Plymouth fishing-grounds, at an average depth of 27 fathoms, lias been investigated. This has been done by the examination of the stomach contents of animals, both fresh and preserved, and supplemented by observation of living animals in aquaria.

2. The sources of food have been discussed. Attention is drawn to the importance in these regions, not so much of organic detritus, which is considered by Petersen and Blegvad to be the only important ultimate supply in Danish waters, but of the contributions from the plankton itself and from the considerable microflora and microfauna inhabiting the bottom deposits.

3. Petersen and Boysen Jensen have stated that the detritus present in Danish waters can be traced almost entirely to the annual decay of the Zostera beds. The area occupied by Zostera in Plymouth waters is very smalLin.comparison with that in the Danish fjords and the detritus found here, though considerable in quantity, and in appearance much as described by Petersen, must originate largely from other sources. The possible1 sources of origin apart from land-sources are the coastal Algse and the plankton. A consideration of the annual production of each of these' suggests that the plankton is the more important factor.

4. Blegvad has classified marine animals according to their food into Herbivores, Carnivores, and Detritus-eaters, but this classification does not fit the conditions in the area here studied and is not therefore of general application. Herbivores are absent from these grounds, and few of the animals could be described strictly as detritus-eaters. The animals fall into the following natural groups, according to their food and the mode and location of their feeding:—

A. Carnivores—Animals which feed mainly upon other animals, either living or as carrion.

B. Suspension-feeders—Animals which feed by selecting from the surrounding water the suspended micro-organisms and detritus.

C. Deposit-feeders—Animals which feed upon the detritus deposited on the bottom, together with its associated micro-organisms.

5. Following the preceding classification the food of the common animals of the Plymouth fishing-grounds has been described briefly.

The Ciliates described in this paper were mostly found living in the tanks at the Plymouth Laboratory, which are supplied with a constant circulation of sea-water from two large storage reservoirs. Most of them came from a large shallow tank standing in front of a south window, which is referred to in the paper as “Drake's Island Tank.”

During the last three years further evidence has been obtained that water of relatively high salinity, from the Atlantic to the south-west of the entrance, enters the English Channel past Ushant (1).

It is of interest to summarize this evidence in view of the suggestion (2) that the English Channel is cut off from the Atlantic for considerable periods.

The surface samples of very high-salinity water, collected in the Channel by the Ministry of Agriculture and Fisheries, suggest the possibility of masses of oceanic water moving into the Channel past Ushant from time to time; and they further suggest that this oceanic water divides in the western end of the Channel, one part moving direct into the North Sea, through Dover Straits, and the other part turning and passing out northward between Land's End and the Scillies. This is discussed by Mr. J. R. Lumby (see p. 670 of the present number of this Journal).

The following short paper is based upon the results of very extensive experiments made with both surface-floating and bottom-trailing drift bottles, by the Ministry of Agriculture and Fisheries during 1920–21.

Comparison of the conceptions which have hitherto been held in regard to the hydrography of the English Channel with those which are offered as a result of the recent activities of the Atlantic Slope Committee, shows that a difference exists which, in the writer's opinion, lies in the interpretation of the material, rather than in the fundamental differences in the material itself. For example, it is stated that the physical character of the water in the English Channel is conditioned, especially in the summer months, by that of the North Sea water rather than by that of the Atlantic water. “En plein été, en août, les eaux chaudes de la mer du Nord affluent dans la Manche.” A similar regimen is suggested for the waters of the Irish Sea, which are said to be derived from the northward. Carruthers shows that the normal water movement in the eastern part of the English Channel is through Dover Straits into the Southern Bight, this movement appearing to be more persistent along the bottom than on the surface. Furthermore, one of the two months in which reversal of this direction appears least likely to occur is August.

While experimenting on the vertical distribution of plankton in comparatively shallow waters I found it necessary to devise a releasing apparatus for use with horizontally towed plankton nets. Under existing working conditions there were four circumstances for which the instrument had to be adapted:

Since April, 1921, observations of temperature and salinity have been made at depths from top to bottom of the sea at Station El, 20 miles to the south-west of Plymouth, where the depth is 70 metres. These observations already throw some light on the causes of different sea temperatures during the same month in different years. As these temperature differences from year to year play an all-important part in the life of cold-blooded marine animals (1), it is of particular interest not only to follow their seasonal changes, but to arrive as far as possible at the causes which govern them.

Changes in salinity and the temperature conditions during 1921 showed a marked inflow of warm highly saline Atlantic water into the English Channel, and a very marked inflow into the El area during September, October and November of that year (2). From that time until the end of 1923 there has been an irregular fall in salinity of the water at El.

In a previous paper (1924) the writer considered the factors which bring about the mixing of surface water, warmed by the sun, with the cooler water below. Observations were cited to show that in the English Channel, between May and September, a warm surface layer may exist, which differs from the deeper water in being more alkaline and poorer in phosphates; both these differences are due to the activity of the phyto-plankton. The reduction in phosphate proceeds in the upper well-illuminated layer till none is left. No further increase in phytoplankton is then possible till phosphate is set free by decomposition or supplied by mixing with the deeper water. The phenomena of thermal stratification are consequently of great importance in the study of the phyto-plankton.

1. The seasonal changes in the sea water of the English Channel and. of Plymouth Sound have now been followed for almost two years, and the second year's results closely resemble those of the first, and again confirm the earlier surface results obtained by Matthews. For 1923 the average consumption at Station El, surface to bottom, 70 metres, was 29·6 mg. of P2O5 per cubic metre, leaving a balance of 7·4 mg. out of the original 37 mg. For 1924 the corresponding figures were, used up 28·3 mg., balance 8·7 mg., winter concentration 37 mg. The minimum average value was obtained in June and July in 1924, in July only in 1923. During June, July, and August, 1924, the surface water was almost entirely denuded of phosphate, values from 1·5 to 2·5 mg. per cubic metre being obtained. These figures would be milligrams per metric ton if fresh water were being considered, as it is they are milligrams per 1027 kg. for. water of S35·3% at 11°C, so it may be appreciated that very little phosphate remains unutilized by the phytoplankton.

2. Owing probably to the high sunshine record for March the year 1924 was over a month ahead of 1923 in the spring as regards phosphate consumption, and, therefore, it may be presumed in the multiplication of the phytoplankton. At the same time, in the attainment of definite temperatures 1924 lagged about a month behind. 1923 in spring. Since temperature has such a marked effect upon the rate of development of the plankton as a whole it appears that the zooplankton must have been retarded in 1924 as compared with 1923.

3. Samples obtained from the tropics show that even in winter phosphate may be much diminished, for the light is bright. The periodic alterations in phosphate content are, therefore, suppressed or much reduced as compared with the temperature zones. It is indicated that in arctic latitudes the sea becomes even richer in phosphate during winter than it does here, accordingly the summer development of phytoplankton is all the. more abundant.

It is only very rarely that the spawn of the Angler Lophius piscatorius is captured near Plymouth, although it has been recorded (Cunningham, 1896). It is well known that it consists of a gelatinous ribbon several yards long and a yard or more wide, with the eggs (over a million) in one layer divided from one another by roundish capsules. It is probable that the Angler spawns well out to sea, for when these ribbons of eggs have been captured anywhere near the coast the larvae have nearly always already hatched and are lying loose in the capsules (Bowman, 1919). Moreover, most of the spawn caught inshore consists of portions only of the ribbon which have probably drifted towards the coast.

(a) Vertical Migration

The material for this paper was obtained in two voyages on the Milford steam trawler Macaw, and I owe all thanks to Capt: Rumble, and to the crew, not only for much practical help, but for a pleasant time. Capt. Rumble's knowledge of the area worked, based on 30 years' experience, can only be compared with a farmer's knowledge of his own acres, and he has been responsible for determining the position of each haul.

While carrying out a general examination of rock crevices and their fauna, making the best advantage of a specially low tide on March 24th, 1924, at Wembury, near Plymouth, the writer was very pleased to find a single specimen of Obisium maritimum Leach. Several fissures in rocks between high and low tide marks were split open with a hammer and chisel, revealing large numbers of Anurida maritima, together with a Coleopteron, Aepus sp., Acari, and a few Collembola. It was after splitting one such fissure that a single specimen of Obisium maritimum Leach was found in company with Anurida maritima in large numbers, and a few Acari and the Coleopteron. Although a thorough search was made both at the same time and at a later date, no further specimens were found. On splitting the rock the O. maritimum, rushed backwards and forwards in an attempt to conceal itself in some crevice. The particular rock was covered with weed which it was necessary to lift up in order to reveal the crevices, and was situated a few yards from extreme spring low tide mark.

The seasonal changes in the heleoplankton of two fresh-water ponds have been compared with alterations in the solutes, and it has been shown that in each there was a vernal rise in pH followed by a period of stagnation with lowered pH. In the quarry pond of Borough Farm, Antony, this condition persisted till the autumn; in Staddon reservoir it was succeeded by a period of high alkalinity, lasting till October. These changes are associated with the spring increase in plankton, and the development of masses of floating algæ in Staddon reservoir; they may be traced in Figs. 1 and 3.