1. In the Lamellibranchs, as typified by Pecten maximus, Mya arenaria and Ensis siliqua, the entire, gut has an acid reaction, the stomach being the most acid region and the pH rising along the mid-gut and rectum.

2. The origin of the acidity of the gut lies in the style. This has a low pH (5·4 in Pecten and Mytilus, 4·6 in Ensis and 4·45 in Mya), and, after it has been artificially extracted from Mya or induced to disappear, by keeping the animals under abnormal conditions, in Mytilus, Tapes and Pecten, the pH of the stomach invariably rises (by as much as 0·825 in Mya and 0·72 in Tapes), although the pH in the mantle cavity has fallen.

3. The style, which dissolves rapidly in alkaline or weakly acid media, is not dissolved in fluids below a certain pH—4·4 for Ensis, 4·2 for Mya, 3·6 for Pecten and Mytilus.

4. The style is never absent, even though animals are starved, so long as they are kept under otherwise healthy conditions. The disappearance of the style under abnormal conditions is probably due to a lowering of the vital activities, which include the secretion of the style substance, and the consequent dissolution of the style by the less acid contents of the stomach.

5. The style is only maintained as a result of a balance between the rate of its secretion and the rate of its dissolution.

6. There is a well-marked correlation between the tolerance of the presence of hydrogen ions possessed by the cilia from the various regions of the gut and the degree of acidity of the fluid with which they are normally surrounded.

7. The pH of the gut in five Gastropods has been investigated. The fore-gut and stomach have invariably the lowest pH.

8. This acidity may be caused by the salivary glands (Patella and Buccinum), the digestive gland (Doris and Aplysia), or the style (Crepidula).

9. The mid-gut and rectum have a high pH, except in Doris, where there is little secretion of mucus, the gut being free and muscular.

10. The style of Orepidula has similar properties to those of the Lamellibranchs. It has a pH of 5·8, and is not dissolved in fluid of pH 3·6 or lower.

11. The cilia from the gut of Buccinum and Doris can function in a pH of 5·0, but there is little difference in the toleration of the various cilia to the presence of hydrogen ions.

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.

It was observed during the summer of 1920 that specimens of the Long legged Spider-crab, Stenorhynchus phalangium Penn., which happened to be in the crowded tanks of the Roscoff Laboratory, were always to be found in the neighbourhood of specimens of Anemonia sulcata Penn. (Anthea cereus Ellis). That this was not merely accidental was shown when crabs, removed to a distance of over three feet, returned time after time to an Anemonia, passing on the way anemones of various species, as well as all manner of objects which might conceivably afford shelter. Of fifteen healthy crabs of this species observed during the past three summers, every one showed this tendency in greater or less degree. At least two of these crabs were found clinging to anemones of this species on the shore, between tide marks; but the subsequent behaviour of crabs dredged from deeper water was in no way different.

The material upon which the following account of the development of P.canaliculata is based was mainly obtained in 1902 when working in the Laboratory at Plymouth. The larvæ were first noticed, and their identity suspected, in April of that year, and identification was made certain by keeping the larvæ in the “mysis” stage until they moulted into the post-larval form, some individuals being kept also through several subsequent moults. The intermediate stages were all taken from the plankton, since I was unable to hatch the larvæ from the egg. The only egg-bearing female seen at that time was taken on the Eddystone Ground on April 29, and her eggs were so far advanced that, although she was unfortunately killed, a few larvæ escaped from the eggs and made it possible to obtain some idea of the structure of the first zoæa.

During a short stay in Plymouth during April, 1922,1 was able not only to pick out a few more specimens from the plankton then brought in, but also, by working through a large number of preserved plankton samples, to obtain a considerable amount of additional material, mainly of later stages. A few specimens have also been found in samples of plankton from the North Sea preserved at the Fisheries Laboratory at Lowestoft, for the loan of which I am indebted to Mr. A. C. Hardy. But the larvæ of this species are much rarer in the North Sea than in the neighbourhood of Plymouth, where they are quite common.

(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.

1. Putrefaction in sea-water sets free compounds toxic to Calanus. A portion of these compounds withstands oxidation to non-toxic forms in air saturated water for considerable periods.

2. “Deep water” from the English Channel contains a catalyst, probably an iron compound, which increases the rate of oxidation of a number of easily oxidisable organic compounds. Its action is inhibited in surface and inshore waters by dissolved organic matter.

3. Hydrogen peroxide decomposes relatively quickly in “deep water” from the English Channel, at a velocity corresponding to that of a monomolecular reaction. A catalyst, probably an iron compound, active towards hydrogen peroxide, occurs in sea-water. The action of the catalyst is inhibited by dissolved organic substances in surface and inshore water, until such substances are oxidised by the hydrogen peroxide.

1. A simple colorimetric method for plotting the dissociation curve of haemocyanin is indicated. The limits of error are within 5 per cent. The simplicity of the method commends it for laboratory class work.

2. The effect of hydrogen ion concentration on the dissociation of the hsemocyanins of the crustacean Palinurus and the pulmonate Helix have been compared. In the snail change of hydrogen ion concentration over a wide range was not found to affect the dissociation of the hsemocyanin: in 'the crustacean there is a marked effect similar to that seen in the dissociation of hæmoglobin.

3. The similarity of crustacean hsemocyanin to haemoglobin is also seen in that increasing temperature depresses the dissociation curve. The effects of certain salts upon haemocyanin. have also been recorded.

The well-known Peridinium ovatum (Pouchet) is a common and widely distributed species. It was first described by Pouchet (1883) from the Mediterranean, and later more detailed figures were given by Schütt (1895), Fauré Fremiet (1908), and Broch (1909). None of these figures agree with the plate arrangement found in the specimens from Plymouth. Jörgensen (1913) places the species in his section Humilia in the group Metaperidinium, on account of the supposed arrangement of the dorsal epithecal plates.

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.

Onchidella celtica (Cuvier) is the only representative of its genus found on our shores. Included in the Sub-order Stylommatophora of the Pulmonates, it is believed to be a land mollusc that has reverted to a marine habitat. It is quite devoid of a shell and closely allied to the land slugs.

A small coccosphere occurs abundantly in centrifuged water samples from the Plymouth district, the western part of the Channel, and outwards towards the Irish Sea and the French coast. This agrees with the species described by Ostenfeld (1899) as Coccosphœra atlantica, from the North Atlantic, which Lohmann (1902) considers identical with Wallich's Coccosphœra pelagica (1877). Lohmann, therefore, unites the two species as Coccolithophora pelagica, and in this he is followed by Ostenfeld (1908), the generic name of Coccosphœra being preoccupied by Perty (1852) for a small green flagellate.

The chief difference between C. pelagica and C. atlantica is in the number of coccoliths (16 to 36 in the former, 10 to 17 in the latter). The overlapping of the margins of the Coccoliths in C. atlantica is another difference, but it appears likely from Wallich's drawings that he had only taken the inner margins into consideration, and, therefore, regarded the coccoliths of C. pelagica as not overlapping. His measurements, which cover an extensive range, embrace those of C. atlantica. It seems, therefore, that C. atlantica is to be regarded as the same species as C. pelagica, the coccoliths having a similar form, and that Lohmann and Ostenfeld are justified in bringing them together.

Coccolithophora pelagica thus includes Murray and Blackman's C. pelagica (1898), Huxley's “Coccospheres” (1868, Plate 4, Figs. 6, c, d, e, and 7, b and c), and Ostenfeld's C. atlantica (1899, 1900). The coccoliths described by Joly and Dixon (1897) and the coccospheres by Dixon (1900) from the Irish Sea also belong to this species.

Owing to the combined action of bacteria, sunlight, sea-water, and general wear and tear the costly silk tow-nets when in constant use are only found to last a few months, or with less use for over twelve months. The young fish trawl-nets, made of a hemp material known as “stramin,” if used several times a week are expected to last over three months.