The evolution of the pulmonate genital system is traced from a prototype exemplified by the tectibranch Acteon. Many features remain almost unchanged in the genera studied, but there is a progressive degree of separation and specialization of the male and female ducts. Some genera of the Ellobiidae, such as Pythia, are undoubtedly primitive. However, it is suggested, following an investigation of the reproductive system of Chilina robustior, that the Chilinidae may stand closest to the ancestors of the Stylommatophora and the Hygrophila.
The sexual cycle in Physa is compared with the protandric sequence found in the primitive Ellobiidae. Similarly, the course of the vas deferens through the mantle wall and the development of the penis from a separate anlagen are also thought to be relies of the ancestral condition.
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