Schmalhausen Institute of Zoology 252601 Kiev, B. Khmalnitasky str. 15, Ukraine

Abstract

Spatial and temporal ontogenetic relationships between inner and outerdemibranchs of the gill (ctenidium) were studied in some species of the familiesMytilidae, Cardiidae, Myidae, Scrobiculariidae and Corbiculidae. They werecompared to the published data on Unionidae, Dreissenidae, Sphaeriidae, andPisidiidae in order to check some hypotheses concerning evolution of the bivalvectenidium. In all species studied, the outer demibranch was formed at the posteriorend of the gill with ctenidium axis lying free in the mantle cavity, whereas theanterior portion of the axis fused with the mantle cavity wall. This fusion probablytook place in the ancestral bivalve group causing a delay of outer demibranchdevelopment. Different modes of outer demibranch lamellae differentiation werealso described. Ontogenetic precedence of the ascending lamella over the descendingone in the outer demibranch was shown for several Eulamellibranch groups. Inparticular, homology of the only outer demibranch lamella in Pisidium to theascending lamella of the other bivalve groups is supported.