| MadSci Network: Zoology |
The information you've referenced sounds like it's from this MadSci answer that I wrote a few years ago.
That specific bit of information that you cited refers only to some marine snails. These snails do indeed reproduce by throwing their eggs and sperm into the water. Fertilization and larval development take place in the water, and eventually the larva metamorphoses into a juvenile snail and begins life on the bottom.
Terrestrial snails are different. They live on land, so throwing one's gametes into the water isn't a possibility. Land snails are simultaneous hermaphrodites and exchange sperm when they copulate. Terrestrial snails brood their babies and give live birth to little snails, miniatures of their parents complete with shell and foot.
I'd be willing to bet that the snail you saw with babies was a terrestrial snail. I'm not aware of any large marine snails that have internal development and give birth to live babies. There are some large snails (whelks and conchs) that copulate and lay egg cases, but the typical marine pattern for larval development still applies.
Anyway, I hope this answers your question!
Allison J. Gong
Mad Scientist
Try the links in the MadSci Library for more information on Zoology.