Ramshorn is a type of apple snail.
Here's what Drs. Foster & Smith say about ramshorns (and other apple snails):
Members of the Ampullariidae family are hermaphroditic. However, these snails will develop a dominant sex. The sex of a snail can be determined if they are the same age, have been kept in the same environment, and are fed the same foods in which case females will be larger than males.
And here's what biologist Jurgen Ziesmann says about hermaphroditic self-fertilization:
Self-fertilization very rarely occurs in animals; the norm is that even hermaphrodite animals cross-fertilize each other, with the parents functioning as male and female simultaneously, or as one or the other sex at different stages in their development. In most cases the spermatozoa and ova mature at different times (successive hermaphroditism), or the male and female external organs are located so that self-fertilization is impossible. Among the invertebrates, sponges, coelenterates, some mollusks, and earthworms are regularly hermaphroditic.
*shrug* I have a ramshorn that I put by itself in a tank, and it laid some eggs early on (was probably already producing them before I moved it), but hasn't again since.