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The main conclusions drawn in this thesis are summarised below: (1) Haemolysins. Haemolytic activity for a variety of erythrocyte types is found in M. edulis and X. edulis. These activities were heat labile, being inactivated by heating at 40°C for 30 minutes. Gel-filtration suggests that there may be more than one lytic agent in M. edulis haemolymph. This is supported by kinetic evidence and by the response to temperature of lytic activity to different erythrocyte types. This activity does not appear to be related to vertebrate complement with which it is compared. Evidence drawn from kinetic studies indicates that haemolytic activity may be due to an enzyme, possibly a phospholipase. Lysozyme-like activity and lipase activity are found in the haemolymph of M. edulis. These are not related to the haemolytic activity. (2) Haemagglutinins. Erythrocyte agglutination was found in all the bivalve species studied. The haemagglutinins of G. gigas were particularly investigated as this species lacks haemolysins. Agglutinating activity was also found for a range of particles including micro-organisms. The haemagglutinins are proteins, they are inactivated by heating at 60°C for 30 minutes, they require Ca+l"ions for activity and have a pH optimum for activity at pH7. C. gigas and M. edulis haemolymph contain separable agglutinating activities for horse and human erythrocytes. The carbohydrate binding specificity of the human agglutinin of 0. gigas is for sialic acids. This agglutinin shows high affinity for substances containing many, United, sialic acid residues. This high affinity is considered to be a result of multi-point binding. The agglutinin also agglutinates bacteria lacking sialic acids, it is suggested that the agglutinin binds to residues sterically related to sialic acids on micro-organisms. Purified human agglutinin material from C. gigas could be demonstrated to consist of homogenous subunits with a subunit molecular weight of 15,000. The total MW of the intact molecule appears to lie 5 6 between 3 x 10 and 1 x 10 from gel filtration evidence. (3) Phagocytosis. A small proportion of the haemocyte population of C. gigas will phagocytose fluorescently labelled bacteria. The numbers of bacteria phagocytosed could be enhanced by pretreating bacteria with oyster haemolymph or purified oyster agglutinin. This was a species-specific reaction as a non-oyster agglutinin of related specifity failed to enhance uptake. Agglutinin was not necessary for phagocytosis to proceed as there was considerable uptake of bacteria in the absence of opsonisation. The possible evolutionary significance of this observation is discussed.

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The pattern of life is a discontinuous one: both fossils and modem taxa show a clustering of forms separated by gaps. Evolutionary theory explains these gaps by relatively rapid transitions happening long ago, leaving few identifiable intermediates. One mechanism for this 'punctuated equilibrium' process suggests that these dramatic changes take place in small isolated populations at the edges of a species' range. This notion was tested in the land snail Cepaea nemoralis in the French and Spanish Pyrenees. Geographical patterns of genetic differentiation in enzyme polymorphisms ('molecular area effects') suggest that the population was once fractured into smaller units which, after a process of genetic disruption, recolonised their original range. The interface between two of these areas was mapped by an analysis of enzyme polymorphisms. Pure populations and their hybrids were then compared for adaptational differences (shell and body colour, thermal behaviour and mortality rates), and for reproductive isolation (as calculated by estimates of selection against hybrids from the enzyme dine width using field determinations of dispersal rate). No significant adaptive differentiation or reproductive barrier could be established between the two molecular areas. Instead, the same processes of adaptation seem to have taken place within the areas, in response to altitude and insolation. Furthermore, I investigated the possibility of a hybrid advantage effect taking place within the zone of hybridisation, where mean shell size is larger, resulting in more eggs being laid. Hybrid genotypes are thus likely to disperse faster than those of the pure types, dissipating the genetic differentiation between area effects more quickly than by random mixing. Present day selection is unlikely to determine the pattem of molecular area effects, as they do not coincide with geological, climatic, and vegetational discontinuities. Rather, they are relics of history, arising from the expansion of glacial refuges. The dynamics of this process were investigated by tracking vegetation patterns from ancient pollen cores, and by an analysis of mtDNA and enzyme variant distribution. The results of this study counter the expectations of the proposed mechanism of punctuated equilibrium. They are a reminder that, for most of the time, living organisms are robust to dramatic evolutionary change.

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