| self-fertilization in the freshwater snails helisoma duryi and helisoma trivolvis. | fifty specimens of five strains (10 per strain) of helisoma duryi from lima (peru), st. croix (virgin islands), formosa (brazil), cartago (costa rica) and st. vincent (lesser antilles), reared in isolation for about 150 days, laid 103 eggs. the numbers of eggs laid by the 10 specimens of each strain were respectively (viable eggs in parenthesis): 44 (26), 1 (1), 5 (0), 15 (7) and 38 (0). egg production widely varied between the individuals of each strain, there being in all strains, except st. v ... | 1988 | 3271940 |