THE GENITAL SYSTEMS OF THE FRESHWATER BASOMMATOPHORA

Abstract

Following a comparative histological investigation of the genital systems of representatives of the four families of the Hygrophila, a standardized nomenclature for the different parts of the female duct is proposed and homologies are suggested.The reproductive systems of Physa fontinalis, Lymnaea peregra, Planorbarius corneus and Ancylus fluviatilis are described and all four species differ in the relationship of the fertilization pockets to the union of hermaphrodite, male and female ducts. In the Physidae alone, the ova pass into the albumen gland and there is no albumen gland duct, a condition which is similar to that of Succinea.The Chilinidae probably stand close to the ancestral condition of the Hygrophila and the prostatic histology of Chilina resembles that of the Physidae, Ancylidae and Planorbidae, whilst the cells of the prostate of the Lymnaeidae are fundamentally different.The degree of development of the different parts of the oviduct in the four families is illustrated. Oviduct I is a short conduit and oviduct II, which is extensively developed in Physa and Lymnaea, is comparable to the posterior portion of the female tract of Chilina robustior. In oviduct III the fertilised ova are embedded in viscous fluid and enclosed in the capsular membrane and this region of the female duct becomes greatly folded in the Lymnaeidae and Planorbidae to form the oothecal gland. Oviduct III is absent in Ancylus where oviduct I opens directly into oviduct IV, a region which has a specialized histology. Oviduct IV is also found in Planorbarius, following oviduct III, and is therefore a common feature of these two families.It is concluded that the genital system of the Physidae is nearest to the ancestral condition, from which the Planorbidae and Ancylidae are later offshoots. The reproductive system of the Lymnaeidae, while retaining several features in common with that of the Physidae, has probably followed a separate evolutionary path.