The ophiuroid, Ophiocomina nigra, is noted for its variability of colour, a phenomenon most pronounced where the species also has its greatest distribution in depth. Observations have been made on two geographically distinct populations, from the vicinities of Millport and Plymouth. The body colour of the observed animals ranges in a continuously graded series from black to orange-yellow. While both populations exhibit analogous variations, there is a distinct difference in the range and tone of coloration which each displays. It is suggested that this geographical difference is correlated in some degree with differences in colour tone of the substratum. It was observed that the incidence of light-coloured phases in each population increased with increasing depths, and some statistical evidence supports this conclusion. Thus, at least two factors, depth and substratum, may affect the distribution of colour variations in O. nigra

In addition to the variations in general body colour, there are two distinct colour patterns found on the disk. These are probably truly polymorphic and may be of cryptic value.

A scheme of colour classification is described which facilitates observations in the field and in the laboratory.

The ophiuroid, Ophiocomina nigra, displays a colour variation ranging from black to yellow-orange. The pigment responsible for the dark colours has been studied histochemically and by means of chemical techniques applied to extracts. The solubility, oxidation-reduction, fluorescence and other chemical properties of this pigment indicate that it is a true melanin. The visible colour of die melanin depends upon its oxidation state in the integument. Fully oxidized pigment is dark brown; partially reduced pigment is fawn coloured.

The red to orange-yellow colours of the ophiuroid, Ophiocomina nigra, are due to carotenoid pigments contained in specialized lipocytes. These cells are dendritic, with their cell bodies located in the dermis below a melanocyte layer and with dendrites approaching, but not penetrating, the epidermis. The lipocytes are either yellow or orange-red.

Five principal pigments were detected in addition to two minor constituents, at least one of which may be an artifact. These major pigments are: β-carotene, an acid carotenoid similar to mytiloxanthin, and four xanthophylls of uncertain identity.

The perivisceral and ambulacral fluids of Asterias rubens have been shown to be not only isosmotic but isoionic with sea water, even if that medium is diluted by nearly half. However, there is a slight accumulation and regulation of calcium in the perivisceral fluid and to a much more marked extent, potassium in the water vascular system. The rapidity with which the potassium diffuses away when the ambulacral fluid is dialysed against sea water suggests that its presence in the water vascular system is due to an active accumulatory mechanism. This mechanism is capable of functioning when the animal encounters sea water of significantly reduced salinity, over the temperature range 0—20° C and extends throughout the season apparently uninfluenced by the breeding cycle or sex of the animal. Smaller animals tend to have a higher concentration of potassium in the water vascular system than do larger ones. Direct and indirect measurements of salt loss under various conditions, together with observations on the rate of lithium transfer, suggests that the integument is very permeable to many ions as well as to water. This supports the hypothesis that the ionic regulation observed results from an active process and not merely from an impermeability of the integument, although measurements with potassium itself have not yet been made.

Sporelings of the marine red alga Plumaria elegans, grown under continuous illumination from a fluorescent tube, show a maximum rate of cell production at a light energy flux of 10 ergs/sec/mm2 in the range 380—720 mμ. This amount of light energy is low, but compares closely with that illuminating Plumaria in its natural habitat at Wembury during the period of the year when the plant is producing spores.

An undulating movement of the body was observed in Entobdella soleae, a monogenean found on the blind surface of a mud-dwelling flat-fish, Solea solea, at Plymouth. The movement is described and shown to have a breathing function, the rate of undulation increasing with decreasing oxygen content of the ambient sea water and vice versa.

The relationship between the movement and micro-habitat is discussed and the phenomenon is compared with breathing movements in other muddwelling animals.

A similar movement was noted in three other skin-parasitic monogeneans: Acanthocotyle sp. from Raia clavata, Pseudocotyle squatinae from Squatina squatina and Leptocotyle minor from Scyliorhinus canicula.

An analysis has been made of plaice fecundity at four localities in Norway. The fish tended to be larger than those used in other fecundity investigations, and the comparison between localities has been made by considering the average egg number expected in a 50 cm fish. In general the plaice off Norway had more eggs than in the North Sea or round the British Isles. At Arendal in the Skagerrak the fecundity was 355 thousand for 50 cm, which is lower than has been found in the Kattegat, but a little higher than I found at Bergen (317 thousand eggs). At Arendal and Bergen the fecundity was higher than has been found in the North Sea but lower than in localities farther north in Norway. In Trondheim Fjord the plaice fecundity was very high indeed. Among the eleven fish examined it varied from 673 thousand to nearly ½ million eggs, and for an average 50 cm plaice would be 819 thousand. The female fish on the coast outside the fjord were quite different and contained about half the number of eggs (407 thousand for 50 cm). These coast plaice were very similar to those caught in the vicinity of Tromso where a 50 cm fish would have about 400 thousand eggs.

The byssal apparatus appears during post-larval life when its secretion permits brief attachment during metamorphosis when the animal assumes the form which fits it for life in the adult habitat.

The byssus persists for continued temporary attachment in animals which may lose it in adult life, e.g. Mya arenaria, or in which it becomes an organ of permanent fixation, e.g. Mytilus edulis, Pododesmus macroschisma and Tridacna crocea