The distinction between retractor and protractor muscles of the freshwater snail's male organ has no physiological basis

Many animals are equipped with organs that can be everted, a notable example being male copulatory organs. The ability to protrude or evert an organ generally requires protractor and retractor muscles. Male copulatory behaviour of the pond snail Lymnaea stagnalis (L.) involves eversion (protraction) and retraction of the relatively large penis-carrying organ. For this preputium, protractor and retractor muscle bands have been defined, which implies eversion and retraction through the activity of these muscle bands. However, no physiological data are available that confirm that the terms protractor and retractor are appropriate. To test whether eversion and retraction are possible without protractor and/or retractor muscle bands, lesion experiments were performed. The results show that with either one or several muscle bands lesioned, snails were still capable of everting their preputium and using it for copulation. However, the majority of animals that had six or more muscle bands lesioned were unable to retract its preputium. Hence, retractor muscle bands serve their designated function whereas protractor muscle bands do not. We therefore suggest that a different terminology is used in which all muscle bands are retractors and, based on their location, are either called distal or proximal retractors. The findings furthermore indicate that the preputium muscle bands are normally contracted, possibly in a catch state, retaining the organ inside without high-energy expenditure.