Pulmonate land snails are hermaphrodites, which means that each individual makes both sperm cells and eggs and normally has both a vagina and a penis. The organ that produces sperm cells and eggs is appropriately called the ovotestis (there's another good word).
Evolution, however, doesn't care much for rules; if a species will be better off by breaking a rule, the rule will be broken. Thus, there are many species of pulmonate snails that have ovotestes, but lack penises. Sometimes, some individuals of a species may have fully developed penises, while others may entirely lack penises. In such cases, a snail with a normally developed penis is said to be euphallic and an individual that lacks a penis is aphallic.
Finally, a snail may have an underdeveloped or a degenerate penis. Such a snail is hemiphallic1.
The genitalia of euphallic (left) and aphallic (right) specimens of the European snail Aegopinella nitens. In the aphallic specimen the vas deferens is a blind tube. Drawings of genitalia from Riedel2.
The reason why I brought up these terms is that some authors have associated aphally with miniaturization of land snails. In future posts I will return to this subject in greater detail.
1. The use of the terms hemiphallic and euphallic was first suggested by Hugh Watson in 1934 (Genital dimorphism in Zonitoides, Journal of Conchology, 20:33-42).
2. Riedel, A. 1953. Male copulatory organs deficiency in the Stylommatophora with a special reference to Retinella nitens. Annales Musei Zoologici Polonici, 15:83-98.