"Terrestrial pulmonate snails have got a hermaphroditic gonad producing egg cells as well as sperm cells. " The word "got" is not necessary in this sentence.
"While most snails have a definite sex, or in case of the terrestrial pulmonates, generally are hermaphrodites, it may also happen in some species, that the sex changes during the lifetime. " The final comma is not needed in this sentence (the one between species and that).
"The sperm cells the snail has received during several copulations, are collected in a sperm pouch" The comma is not necessary in this phrase.
"Only later the juvenile snail managed to harden its shell by accumulating lime, to achieve a shell so hard, that the hardness of helmet shells (Cassinae, Tonnidae) or conches (Strombidae) only can be imitated with great effort by modern material research." Another unneeded comma: the second one (between hard and that). Also, I think you meant to say manages rather than managed (it's a common typo).
Other than those minor issues, I found the web page to be just fine. ;D