The nature of the bivalve filter-pump was further elucidated in the n~ussel Mytilus edulis by investigating effects of temperature on pump and system characteristics. Pumping rates increased with temperature, linearly correlated with the temperature-determined decrease in viscosity of the water. The variation in pumping rate with temperature thus corresponded to the varying viscous resistance to water flow in the canal system of the mussel-pump. The increase in beat frequency of the lateral cilia with temperature therefore had no clear effect on the pumping rate. Also, spawning stimulation of beat frequency was without effect on pumping rates. In the back pressure-flow characteristic of fully open, active mussels, pump pressure at zero flow (AH;) was independent of temperature. Predictions based on the pump modelled as a leaky, viscous, constant-force pump were in good agreement with measured and estimated pump parameters, further supporting the concept of suspension-feeding bivalves as filter-pumps that process the ambient water autonomously -that is, in the absence of physiological mechanisms for regulating water pumping.

In the 1790s, Robert Townson established the main features of the water economy of terrestrial amphibians: rapid evaporative water loss in dry surroundings, 'drinking' by absorption of water through the abdominal skin pressed against moist substrates, and use of the urinary bladder as a reservoir from which water is reabsorbed on land. This knowledge was of little interest to the establishment in the first half of the nineteenth century of experimental physiology as a basic medical discipline, when frogs became models in the elucidation of general physiological processes. Townson's pioneer contributions to amphibian physiology were forgotten for 200 years (Jørgensen 1994b). During (1901) and particularly Overton (1904) restored knowledge about amphibian water economy to the level reached by Townson, but the papers had little impact on the young science of animal physiology because they primarily aimed at elucidating the transport of fluids across membranes. Frog skin remained a model preparation in such studies throughout the century. With the establishment of terrestrial ecology early in the century, the relations of animals, including amphibians, to water became a central theme. Concurrently with comparative studies of amphibian water economy in an ecological setting, the subject proceeded as an aspect of animal osmoregulation. Adolph (1920-1930) and Rey (1937a) established the highly dynamic nature of water balance in amphibians in water and on land. Their observations indicated functional links between environment, skin and kidneys, the nature of which remained to be explored. Thorson & Svihla (1943) reopened the ecological approach in a comparative study of the relations between amphibian habitat and tolerance of dehydration. By mid-century, the central themes of amphibian adaptations to terrestrial modes of life were re-established, except for the function of the bladder as a water-depot. During the following decades, a rich literature appeared, particularly focusing on adaptations of amphibians to arid environments. Thus, in the 1970s, it was found that 'waterproofing' of the highly permeable skins by means of skin secretions had evolved independently in several families of tropical arboreal frogs, and that a number of amphibians that aestivate whilst burrowed in dry soil could reduce evaporation by forming cocoons from shed strata cornea. In 1950-1970 the role of bladder urine as a water depot in terrestrial amphibians was recognized: this did not change the established view of water balance in terrestrial amphibians as alternating between dehydration on land and rehydration in response to the deficit in body water. Amphibians may, however, maintain normal water balance whether the ambient medium is water or air by means of little understood integrated mechanisms in control of cutaneous drinking behaviour, water permeability of the skin and bladder wall, and urine production.

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SUMMARY A description is given of the structure and function of the feeding organs in various aquatic invertebrate filter feeders (suspension feeders), especially in such forms as have also been used in experiments on feeding rate, on efficiency of feeding organs in retaining particles of different sizes, etc. Sponges ingest indiscriminately particles with and without food value. Particles that are too big to enter through the pores of the surface may be phagocytized by the cells of the epithelium. In primitive sponges with large flagellated chambers intake and digestion of food particles is mainly performed by the choanocytes, whereas in highly developed sponges a large part of the particles are phagocytized by the walls of the incurrent canals before they reach the flagellated chambers. The tube‐living polychaete Serpulimorpha are suspension feeders. They feed by means of the ciliated branchial crown which surrounds the mouth. The burrowing, tube‐living Chaetopterus variopedatus feeds by filtering water through a mucus bag. A similar feeding method has also been described in Nereis diversicolor. Within the Echiuroidea Urechis caupo likewise feeds by means of a mucus net. The net is attached to the walls of the burrow in which the worm is living. In suspension‐feeding lamellibranchs the gills both propel and filter the water. Most investigators assume that the filtration is performed mainly by the laterofrontal cilia of the gill filaments, whereas MacGinitie states that during normal feeding, water is filtered through sheets of mucus which cover the surfaces of the gills. A variety of sorting devices, especially on the gills, are developed in different lamellibranchs. Sorting is performed according to size, shape and density of particles. Qualitative sorting has, however, also been demonstrated in the oyster. Filter‐feeding habits have been adopted independently by several gastropod families, both sessile and free‐living. As in the lamellibranchs, it is generally the cleansing mechanisms of the unmodified gill and of the mantle cavity that have been developed into food‐collecting mechanisms. In Crepidula and other highly specialized suspension‐feeding Prosobranchia the filtering of the water is performed by mucus sheets, which are continuously carried over the gill surface. In vermetids from still water the importance of the gill as a food‐collecting organ is reduced, but long mucus threads, produced by the pedal gland and floating freely in the water, are used to catch food particles which adhere to the mucus. In suspension‐feeding copepods a filter chamber is enclosed between the ventral body wall and the maxillae which project ventroanteriorly. The maxillae carry long plumose setae extending antero‐medially towards the mouth and forming the lateral walls of the filter chamber. The feeding currents are produced chiefly by rapid vibratory movements of the antennae. Most if not all of the filter‐feeding copepods can feed in other ways too, e.g. by scraping or by catching larger food. Tunicates possessing a branc...

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