A comparative study on the internal defence system of juvenile and adult Lymnaea stagnalis.

The immunological immaturity of juvenile specimens of some snail species, e.g. Lymnaea stagnalis, may contribute to their greater susceptibility to infection by schistosome parasites. In a comparison between juvenile and adult specimens of the pond snail L. stagnalis, we have shown that the blood cells (amoebocytes) of juvenile snails are less efficient at phagocytosing: fewer amoebocytes are competent and the average number of particles engulfed per cell is lower. This functional immaturity seems to correlate with morphological immaturity of the amoebocytes. Opsonic and haemagglutinating activities are low in juvenile snail plasma, but much higher in adult plasma. Finally, however, the initial rate at which injected bacteria are eliminated from the circulation seems only slightly slower in juvenile snails than it is in adults.