The morphology of a complete series of developmental stages of Lernaeocera branchialis is described. Seven stages are passed through before reaching the gadoid (final) host.

The structure of the mouth tube and appendages is discussed in detail and compared with those of related genera in which the resemblances are close.

The free single nauplius and copepodid stages are immediately followed by four chalimus stages on Pleuronectes flesus; these show some dedifferentiation though segmentation is not lost. An explanation is given for the peculiar suspensory mechanism of the chalimus.

A second dedifferentiation follows the second well-developed pelagic phase in which the adult form is attained and copulation takes place. Some irregularity is noted in the details of the retrogressive metamorphosis of the maturing female on the gadoid host.

The rate of increase in body length in Lernaeocera decreases regularly up to the assumption of the adult form (stage VII). In this respect it is similar to the free-living copepods Diaptomus, Eurytemora and Calanus, but is in strong contrast to Caligus centrodonti which has an increasing growth rate up to the fourth chalimus stage. In the latter at this stage there is a marked acceleration in the growth rate unaccompanied by any change in shape or any other discontinuity, whereas in Lernaeocera there are early discontinuities in development, yet the growth rate is continuous when expressed graphically: it is a straight line rather than a logarithmic curve as would be expected from our knowledge of the higher Crustacea.

The size of the female is greater than that of the male: that of the female overlapping the male of the succeeding stage. Variations in size of the larvae in a stage group are not seasonal, and an explanation is offered for giant larvae.

Similar lernaeocerid larvae and males to those of Lernaeocera branchialis on Pleuronectes flesus were found on Solea solea and Cyclopterus lumpus. No characters of generic or specific significance could be found which would serve o t separate them from Lernaeocera branchialis so that their identity is not determined.

1. The number of species found in this investigation is six, of which four are marine and two brackish water.

2. The mean value of the body weight and the fecundity varies with the species. The size of a species bears no relation whatever to the reproductive capacity.

3. The fecundity is subject to seasonal variation, being generally higher in the spring than in the winter months.

4. In general, a positive correlation exists between body weight and fecundity of the individuals of each species. This rule does not, however, apply to Gammarus duebeni, in which the fecundity appears to fall after reaching its maximum at the 19 mg. body-weight class.

The American whelk tingle, Urosalpinx cinerea (Say), a gastropod enemy of oysters, has been introduced to Britain with American oysters and is now established in the rivers Blackwater, Colne, Crouch and Roach in Essex and on the Kentish Flats off Whitstable. Evidence available concerning the migratory powers of Urosalpinx is conflicting, but it is probable that migration is of local significance only in the spread of this pest. To infest new areas tingles or their spawn must be transported by human or other agency, for Urosalpinx has no free-swimming larval stage, the eggs being deposited in capsules from which fully formed young tingles emerge.

Females of Urosalpinx grow more quickly than males and reach a larger size. In males the annual increase in shell height never exceeds 0·2 cm. after the first 3 years, and is generally rather less. In females an annual increment of 0·2 cm., or slightly more, is general up to the age of about 7 years, after which it gradually falls. Males and females of the same age differ in height by about 0·25 cm. Growth marks are frequently to be seen on the shell and are of value in assessing growth rates. In both sexes a maximum age of 13–14 years may be reached. Urosalpinx reaches a much greater average size in Britain than in its natural habitat on the Atlantic coast of the U.S.A.

Females may deposit some spawn during the first 2 years of life, but the amount is insignificant in relation to the production of older tingles. Spawning begins when the water temperature in its seasonal rise reaches 12° C. Adult females may deposit an average of 25 egg capsules at a single laying, but it is possible that further capsules are deposited later in the season. The bulk of the spawn is deposited in May and June on British beds, but a few freshly laid capsules may sometimes be found in August and September. The average period of incubation is about eight weeks. Young tingles usually begin to emerge early in July. The average number of young emerging from each capsule is 11–74. Urosalpinx appears to be more prolific on British oyster beds than in America.

The distribution of Arachnactis albida in the Celtic Sea in 1937–9 is described. The species appears, to have a definite centre of distribution in the neighbourhood of lat. 50° N., long. 10° W. The larvae have been taken as far south as lat. 48°, and eastwards up to the mouth of the English Channel, but they are absent from the tongue of low salinity water continuous with the St George's and Bristol Channels. Their distribution in the remaining area can be correlated with the cyclonic circulation.

The larvae obtained from the Celtic Sea are specifically identical with those found off the west coast of Ireland and with the typical A. albida of the Faroe-Hebrides region.

An attempt has been made to deal briefly but quantitatively with all size groups of the fauna and flora in a marine mud deposit.

It has been necessary to propose the following new terminology: the Macrobenthos, which is here equivalent to the macrofauna, the Meiobenthos, under which term are included copepods, nematodes, foraminifera, etc., and the Microbenthos, comprising the rest of the protozoa, bacteria, bottom diatoms and other algae; planktonic diatoms and coloured flagellates also occur on the bottom in the region investigated but are not regarded as true microbenthos.

A quantitative ecological study of the microbenthos has been started and the habitat is first described, stress being laid on points of importance to the microorganisms.

The census method for bacteria was an agar-plate method and that for the protozoa and diatoms was a dilution culture method modified from that used by soil microbiologists.

These methods give minimal values for the total population and may safely be used for comparative purposes. The results are still tentative.

Typical figures for the minimal numbers and volumes of living protoplasm per gram of dry mud in the top ½ cm. layer are given in Tables I-IV.

The surface layer is much richer in all types of organisms than is the mud deeper in the cores. There is considerable local horizontal variation in numbers of bacteria and protozoa, suggesting dense aggregations.

Diatoms contribute by far the greatest bulk to the total volume of living micro-organisms; planktonic diatoms, particularly during the summer, completely outweigh the true microbenthos.

The method by which the weight, volume, density and sinking factor of living aquatic organisms can be determined has been extended. By sinking factor is meant the ratio of the density of the organism to that of its environment at the time of the actual determination. The density of the environment is always raised to 1000, and the density of the organisms computed on this basis.

A quick and accurate method for determining the percentage of water in a living organism has been described and discussed.

On the assumption that a sponge consists of protoplasm and spicules only, the density of the protoplasm has been computed.

The sinking factor of several fish and of the embryos of some Crustacea at different stages has been determined. One effect of the swim-bladder in teleost fishes is to reduce the sinking factor and thus facilitate swimming.

The effect of the sinking factor on swimming in general and the effects of spinous outgrowths in pelagic Crustacea are discussed.

Gammarus zaddachi is perhaps the most prolific and widespread of all the estuarine amphipods known to occur in northern Europe, and inhabiting, as it does, the low-salinity estuarine zone and adjacent coasts, it has come to be recognized in recent ecological work as a ‘salinity indicator’.

Unfortunately, there has been constant confusion with the other common species of Gammarus, G. locusta, pulex, and duebeni, which has been greatly complicated by the difference in the appearance of zaddachi according as it lives in a freshwater or a saline habitat. It is shown that this difference is entirely due to the sensory equipment, the greater production of hairs in freshwater conditions, and that the structure of the two ‘forms’ is identical.

The history of the species has been carried back as far as I have been able to trace it (1836) with the actual specimens, described in the different papers, and the more important of these papers are discussed. It will be seen that the material examined was derived from every country of northern Europe; from Russia, the White Sea, Crimea, and the Baltic, the coasts of Scandinavia, Germany, including the Hamburg water-supply, Denmark, the Netherlands, Great Britain and Ireland, and France as far up the Loire as Nantes.

Detailed descriptions and figures of both forms of G. zaddachi are given; and finally, a comparison is made between the species most commonly confused with it, the Arctic species G. wilkitzkii being included because of a suggestion recently made that it might be, not a distinct species, but merely the Arctic form of zaddachi.

Part I. After a brief statement of the present unsatisfactory position regarding the life history of Polystomella crispa, an account is given of experiments with various fixatives on P. crispa, collected near Drake's Island, Plymouth. A method is described for the cultivation of Polystomella in diatom cultures, especially with Navicula mutica var., with which a flagellate (Bodo sp.) was constantly present. The identification of individual Foraminifera, and determination of growth, were often possible by some irregularity in the shell, or by noting the attachment of one or more stalked egg cocoons of an unknown turbellarian worm. The feeding of Polystomella on diatoms seized by the pseudopodia outside the shell is described; the structures previously taken for ingested algal cells inside the shell are shown to be foraminal plugs (‘bouchons’ of le Calvez) which are discarded from time to time and thrown out of the shell along with the excretory granules (xanthosomes), mostly via the canal system.

Part II. Notes are given on the microspheric form and its reproduction, through the spring and summer, by schizogony. The rearing of two broods in laboratory cultures is described, with an account of the formation of a new chamber to the shell. It is concluded that a complete life cycle consisting of one microspheric and one megalospheric phase occupies a period of about two years.

Part III. Sporulation of the megalospheric form is described as seen during life; and various factors are discussed which might affect the process. The relatively small size of the reproducing Polystomella with which Lister worked is ascribed to their having lived under less favourable conditions. From a study of stained preparations, certain details are added concerning the opening up of the shell, presumably to facilitate the eventual escape of the flagellate swarm spores; also concerning the accompanying cytoplasmic and nuclear changes. Some account is given of the form and structure of the swarm spores, both alive and in permanent preparations. Experiments are described which constitute an attempt to carry the life cycle beyond the stage of sporulation. They are so far unsuccessful, with one possible exception, under all the various experimental conditions which are briefly discussed in their turn.