THE GENITAL SYSTEM OF ACOCHLIDIUM FIJIENSE (OPISTHOBRANCHIA: ACOCHLIDIOIDEA) AND ITS INFERRED FUNCTION

摘要

The genital system of the adolescent-gonochoric freshwater opisthobranch Acochlidium fijiense is described from histological serial sections of five individuals and dissection of a sixth animal in full detail. The penis has a characteristic armature consisting of an ascending spiral of chitinous spines on the edge of the glans. The basal finger in association with the parapros-tate probably functions as stimulatory organ analogous to the gypsobelum of pulmonate gastropods. The presence of sperm in the haemocoel and the kidney of one specimen and the penial armature suggest that A. fijiense transfers sperm through hypodermic impregnation. The most peculiar feature is the connection of the genital system with the digestive system. A duct with unknown function connects the digestive gland with the distal gonoduct. In addition, in one individual the ampulla which stores autosperm had an opening into the digestive gland. This opening is interpreted as a temporary structure established only when required in order to digest excess autosperm, thus compensating the lack of a gametolytic gland. However, it cannot be ruled out that this connection seen in a single individual was an abnormality.