Abstract. Ontogenetic development of the pore-systems of two species of Loxoconcha, phytal L. japonica and bottom-dwelling L. uranouchiensis, were examined. Adult pore-systems differ between the two species in number and distribution of “smooth”-type bristles, the basal structure of “twisted”-type bristles and the existence of microhairs. These specific features start to differentiate after the A-2 or A-3 juvenile stage, a fairly late moulting stage, and continue to differentiate until the adult stage. These characters are identified as lower phylogenetic ones based on the ontogeny-phylogeny relationship. The result is consistent with the estimate that the characters were regarded as adaptive and specialized through the consideration of their function to the respective habitats. The mode of increase of the two types of pore-systems was revealed. The “twisted”-type bristle, whose number is common between the two species at all stages, reaches the final number, except for one, at the A-3 juvenile stage. In contrast, the “smooth”-type bristle continues to increase in number until adult stage, and, as mentioned above, the specific differences become more obvious as the moulting stages advance. This type of pore-system is valuable for specific differentiation. Two types of pore-system that differ from each other in the pattern of increase were recognized in other ostracod taxa. The two types of pore-systems have different significance in ostracod taxonomy and the recognition of the type is indispensable for future taxonomy.