The composition of the medusa fauna in offshore waters off Plymouth is reviewed. A table is given showing the months in which the different species have been recorded and those months in which they are most abundant. A preliminary list has been drawn up attempting to discriminate between those species which are regularly indigenous off Plymouth and those which are brought from elsewhere by water movements. The results of weekly hauls with the 2 m. stramin ringtrawl during the years 1930-7 inclusive are given. The years 1935, 1936 and 1937 have been noteworthy for the paucity of all species of medusae. Observations on the structure and biology of certain species of medusae are recorded.

In September 1937 Muggiaea atlantica Cunningham was very abundant off Plymouth. The presence of large numbers of eudoxids in the plankton afforded an opportunity to rear the early stages of this siphonophore. The development of M. kochi (Will) was worked out by Chun (1882, 1913), and he also described the eudoxid of that species. The eudoxid of M. atlantica has, however, never been described for certain. The allocation of eudoxids collected in the plankton to their respective species is necessarily fraught with uncertainty when more than one species is present. Throughout the whole of September 1937 M. atlantica was the only species of siphonophore to be found off Plymouth, and there can be no reasonable doubt that the eudoxids described below belonged to that species and that the development of the early stages from their eggs is that of M. atlantica.

For the terminology used in the description of the eudoxid I have followed Totton (1932), to whom I am grateful for helpful advice.

The Eudoxid

The bract (Fig. 2 a) is cone-shaped with a broad flat suture running from its apex to the base. The edges of the suture are raised into a slight flange. The right edge is continued downwards into a sweeping curve while the left edge cuts away horizontally at its lower end; the bract is thus asymmetrical. There is a slight cavity on the lower surface in which the somatocyst is centrally placed. The general form of the bract can also be seen in the different views of the whole eudoxid given in Fig. 1.

Kellia suborbicularis (Montagu) has long been known to be viviparous. Lovén (1848) mentions this fact and Jeffreys (1863, p. 227) quotes Mr Clark who “found in the ovary of one individual ova in an advanced state with fully formed testaceous fry.” It is probable, however, that these shelled larvae were in reality contained in the gill pouch where later workers have found them. Pelseneer (1935, p. 517) says that the eggs are incubated in the internal gill lamella, and this was also observed by myself.

It is striking that although the shelled larva is extremely small when liberated (shell 0.064–0.08 mm. in length), the late planktonic larva is very large (shell 0.30–0.37 mm. in length) and one of the most conspicuous bivalve veligers in the plankton. This indicates a prolonged free-swimming life quite unlike that of the oyster, Ostraea edulis, whose shelled larvae are liberated only to lose the velum in a few days. These late larvae of Kellia have been kept in bowls and plunger jars in the Plymouth Laboratory until they metamorphosed and for months afterwards, growing into beautifully clear and transparent individuals which undoubtedly belonged to this species.

Kellia suborbicularis is placed by Winckworth (1932) in the Erycinidae, but this family is closely related to the Leptonidae, Montacutidae and Galeommatidae, most or all of whose members are viviparous. Lovén (1848) described the young shelled stage of Montacuta ferruginosa and M. bidentata, the latter species now being referred by Winckworth toMysella. Both keep their young in the gill pouch until the shelled stage and Lovén's beautiful figures show that the early shelled stage in each of them is very like that of Kellia.

1. Ciliary currents concerned with rejection of sediment from the mantle cavity of pectinibranch Prosobranchia consist of A, currents carrying heavier particles to the inhalent opening; B, currents carrying medium particles across the floor of the mantle cavity; C, currents carrying fine particles over and between the gill filaments for later consolidation by the mucus from the hypobranchial gland. Material in currents B and C is rejected from the exhalent opening.

2. The feeding currents in ciliary feeding Prosobranchia represent modifications of these rejection currents.

3. In Vermetus novae-hollandiae currents B and C only are modified, material being passed to the mouth region, where it is mixed with mucus from the large pedal gland, by way of a food groove.

4. In Crepidula fornicata and other Calyptraeidae all currents are modified for feeding and there is an endostyle for mucus secretion.

This is the first part of a study of the metamorphosis of the larva of the European flat oyster (Ostrea edulis) and deals with the fate of the larval velum, foot, adductor muscles and eyespots.

The apical area of the larval velum forms the basis of the upper labial palps of the spat.

The bulk of the velar tissue is broken up and cast off or eaten; the total dissolution of the velum being complete within 48 hr. of attachment.

The larval foot after effecting attachment rapidly shrinks in size and disappears within 72 hr. of attachment. Partial loss of its tissues probably occurs.

The statocysts persist in the spat.

Following fixation the anterior adductor muscle rapidly degenerates.

The so-called pigment spots of the larval oyster have the structure of eyes. During metamorphosis they degenerate rapidly in situ..

Taschenberg (1879) briefly described, under the name of Didymozoon scombri, a trematode which he recorded as occurring in pairs in cysts on the inside of the gill-covers of Scomber colias at Naples. Odhner (1907) found what he believed to be the same species not uncommonly in small specimens of S. scombrus off the west coast of Sweden, but was unable to find it in the larger fish taken in the early summer. He gives a minute description and excellent figures of the worm. Johnstone (1914) has also given a good, though diagrammatic, figure of its anatomy. Odhner describes the worm as occurring most commonly in paired cysts on the upper pharyngeal bones, but also in exceptional cases in cysts on the inside of the gill-covers and on the outside of the head, close behind the eye. About half the cysts examined by him contained two worms, the rest from three to seven. Johnstone describes the cysts as being most commonly situated ” “on the roof of the mouth beneath the pharyngo-branchials”, but also sometimes on the basi-branchials, on the external surfaces of other parts of the gill-bars, or on the internal surface of the operculum. One cyst contained as many as sixteen worms. According to Dollfus (1926), he and Monod found cysts containing one, two, three or more worms in the mucosa of the mouth, palate and gills of S. scombrus, and also single unencysted worms under the transparent skin behind the eye.

Thus at least two authors record the same worm as occuring both inside the mouth and on th e outside of the head, behind the eye.

The present report continues the observations on the seasonal abundance of the pelagic young of teleostean fish in the plankton off Plymouth(Russell, 1937) and gives the results for the year 1937. These results are published in the same form as in previous reports. The dates on which collections were made with the 2-metre stramin ringtrawl are given in Table I and the monthly average catches of the young fish per half-hour haul in Table II. In Fig. 1 is given the curve for the average catches for each fortnight of all young fish, excluding clupeids, and superimposed upon this the corresponding curve for the average catches over the period 1930–1934 inclusive.

During 1936 and 1937 extensive physical and faunistic studies of the Tamar Estuary were made by several workers at the Plymouth laboratory. These investigations, though conducted independently and with different immediate aims, proved to have considerable bearing on one another, and each worker benefited by occasions for co-operation. As the work proceeded it became clear that one of the most important aspects of each investigation was the bearing it had on the balance of life in the estuary as a whole. It was decided to publish the results of these studies in a series of papers in which each contributor should lay due emphasis on data having possible bearings on the problems studied by the others, so as to present as complete a picture as possible.

Studies have been made of salinity, temperature, penetration of daylight, macro- and micro-fauna of the intertidal zone, the sessile flora and fauna of the buoys, the fishes and their food, and the birds which frequent the flats and channels. Other studies are contemplated. In the work on light penetration and on quantitative estimates of the fauna of the mud-flats, no comparable data have hitherto been procured in this country.

The Tamar is the largest of the rivers which flow into the west end of Plymouth Sound. It is tidal for nearly 19 miles (30 km.) of its course. Rather over 3 miles (5.5 km.) from the sea it is joined by the River Lynher to form the Hamoaze, an estuary common to the two rivers.

The Zeiss Pulfrich photometer, with its eight spectral niters covering narrow spectral bands within the visible spectrum, has been used to investigate the penetration of light into an estuary under winter and summer conditions.

Throughout the visible spectrum, extinction coefficients in Hamoaze are always higher, in winter often very much higher, than in the English Channel. In marked contrast to the open sea, red light penetrates as well or better than green, and blue is cut down most rapidly

An attempt has been made to give the range and rate of fluctuation of salinity (and, to a lesser extent, of temperature) that organisms experience at various points on the bed of the lower Tamar Estuary and in the overlying water especially near the surface. Such work until now does not seem to have been attempted. The following general facts emerge: On proceeding from high-water mark to low-water mark there is an increase in the range, rate and duration of fluctuation in salinity. Also the upper limit of the salinity range rises very slightly while the lower limit falls by a large amount, i.e. there is a more or less uniform gradient from high-water mark to low-water mark. Near high-water mark the fluctuation is less and of shorter incidence and is distributed over a higher level of the total salinity range (for the intertidal area) than near low-water mark. Owing to stratification, the farther down the intertidal area the (slightly) longer is the duration of the high salinities, especially in winter. At mid-tide mark the fluctuation is very similar to that on the bottom in mid-river, but is of twice the rate.