During the month of September, 1888, we availed ourselves of the facilities afforded by the Laboratory for the purpose of continuing the investigations began by us the year before, of the function of the electrical organ of the skate. In the record of the work done by us in 1887 at St. Andrews, published in the Journal of Physiology, vol. ix, p. 137, we indicated several new lines of investigation which we hoped to pursue if the opportunity offered. Two of these indications we have now been able to fulfil satisfactorily, namely, those relating to the electromotive force of the shock, and to the way in which the function of the electric organ is controlled and influenced by the central nervous system. In the first of these inquiries, we used apparatus which was brought from the Oxford Physiological Laboratory, and temporarily fitted up in the room at Plymouth, which is set apart for physiological researches, and which we found well adapted for this purpose. For the second, a large number of experiments and consequently a considerable number of fish were requisite. Forty skates of various species (Raia Batis, R. clavata, R. microcellata, and R. maculata) were supplied to us and used in our researches, of which the result will shortly be ready for publication.

We desire to express in the strongest terms our appreciation of the advantages afforded by the Laboratory for physiological researches. We would also record our personal obligation to the Director for his uniform courtesy and untiring zeal in obtaining for us, in spite of considerable difficulties, the material required for our work.

The crustacean moult cycle is a convenient model system in which to study calcium (Ca) homeostasis as vectorial movement across Ca transporting epithelia (gills, gastric epithelium, cuticular hypodermis, antennal gland) which occurs in either direction at different stages of the moulting cycle. Intermoult crustaceans are in relative Ca balance. During premoult, at the same time as the cuticle decalcifies, epithelia involved in Ca storage (e.g. gastric) calcify and/or increase their intracellular Ca stores. Premoult Ca balance is typically negative as Ca is excreted. During postmoult the soft new cuticle is remineralized largely with external Ca taken up across the gills and gastric epithelium (positive Ca balance); conversely during this time internally stored Ca is remobilized. This review (1) compares the relative roles of Ca transporting epithelia in Ca balance for crustaceans from different habitats; (2) proposes up-to-date cellular models for both apical to basolateral and basolateral to apical Ca transport in both noncalcifying and calcifying epithelia; (3) compares kinetics of the Ca pump and exchanger during intermoult; (4) presents new data on specific activity of calcium adenosinetriphosphatase (Ca2+ATPase) during the moult cycle of crayfish and (5) characterizes a partial cDNA sequence for the crayfish sarcoplasmic reticular Ca2+ATPase and documents its expression in gill, kidney and muscle of intermoult crayfish. The physiological and molecular characterization of Ca transporters in crustaceans will provide insight into the function, regulation and molecular evolution of mechanisms common to all eukaryotic cells.

Squids (teuthoids) fall into two distinct groups according to their density in sea water. Squids of one group are considerably denser than sea water and must swim to stop sinking; squids in the other group are nearly neutrally buoyant. Analyses show that in almost all the neutrally buoyant squids large amounts of ammonium are present. This ammonium is not uniformly distributed throughout the body but is mostly confined to special tissues where its concentration can approach half molar. The locations of such tissues differ according to the species and developmental stage of the squid. It is clear that the ammonium-rich solution are almost isosmotic with sea water but of lower density and they are present in sufficient volume to provide the main buoyancy mechanism of these squids. A variety of evidence is given which suggests that squids in no less than 12 of the 26 families achieve near-neutral buoyancy in this way and that 14 families contain squids appreciably denser than sea water [at least one family contains both types of squid]. Some of the ammonium-rich squids are extremely abundant in the oceans.

Generation of ion electrochemical potential differences by primary active transport can involve energy inputs from light, from exergonic redox reactions and from exergonic ATP hydrolysis. These electrochemical potential differences are important for homoeostasis, for signalling, and for energizing nutrient influx. The three main ions involved are H+, Na+ (efflux) and Cl− (influx). In prokaryotes, fluxes of all three of these ions are energized by ion-pumping rhodopsins, with one archaeal rhodopsin pumping H+into the cells; among eukaryotes there is also an H+ influx rhodopsin in Acetabularia and (probably) H+ efflux in diatoms. Bacteriochlorophyll-based photoreactions export H+ from the cytosol in some anoxygenic photosynthetic bacteria, but chlorophyll-based photoreactions in marine cyanobacteria do not lead to export of H+. Exergonic redox reactions export H+ and Na+ in photosynthetic bacteria, and possibly H+ in eukaryotic algae. P-type H+- and/or Na+-ATPases occur in almost all of the photosynthetic marine organisms examined. P-type H+-efflux ATPases occur in charophycean marine algae and flowering plants whereas P-type Na+-ATPases predominate in other marine green algae and non-green algae, possibly with H+-ATPases in some cases. An F-type Cl−-ATPase is known to occur in Acetabularia. Some assignments, on the basis of genomic evidence, of P-type ATPases to H+ or Na+ as the pumped ion are inconclusive.

When electro-osmosis is observed through a substance (or tissue) certain inferences can be made about the fine structure of the substance. These include the presence of a zeta potential whose sign is the same as that of the electrode towards which water moves, and the presence of interstices or channels large enough to allow ions to move through them and to sweep along more water molecules than those carried as hydration shells. Channels too large may permit both positive and negative ions to move or at least allow a counter flow of water to reduce the net (observable) movement of water molecules.

The measurements recorded for marine algæ of various groups show that the reaction of the sap is in most cases almost neutral, and in no case is the sap of the pronounced acid character met with in many land plants. This being so it follows that the enzymes concerned in the metabolism of these algæ must be quite different from those which effect corresponding changes in land plants, as may be seen on referring to the optimum pH values for various enzymes quoted in the writer's previous paper on the reaction of plant cells (1922).