Ontogenesis of the outer demibranch of ctenidium was observed in the different groups of freshwater bivalvos: najades (Unionidae), fingernail and pill clams (Sphaeriidae), and Dreissena, with some data on Corbicula (Corbiculidae) and Hypanis (Cardiidae) also presented. Established are the sequence of stages as well as growth patterns. Three different strategies of the outer demibranch growth and development specific for unionids, sphaeriids, and Dreissena, are described. Probable phylogenetic transition between these strategies is discussed. Some trends of the evolution of gill ontogenesis within the family Sphaeriidae are traced up. It was shown that the stage at which the outer demibranch appears is an important morphogenetic factor determining size and position of the organ in fingernail and pill clams, and, presumably, in bivalve molluscs in general.
Variation of some structural and developmental characteristics of ctenidia was studied in bivalves representing the families Mytilidae, Unionidae, Dreissenidae, Sphaeriidae and Cardiidae. Fingernail and pill clams were investigated most thouroughly. Significant differences in the outer demibranch position were found in Sphaeriidae (in the genus Pisidium) and Cardiidae. In the first family, correlation between the outer demibranch position and the time of its ontogenetic origin is observed. Some taxonomic interpretations of the described differences are discussed. For Unio and Anodonta, the differences in the outer demibranch growth rate are demonstrated.