Ask Mr. Smarty Plants

Slugs or snails are the likely culprits. Jack-in-the-pulpit (Arisaema triphyllum), contains chemical compounds which most herbivores - including deer and rabbits - find unpalatable. Of course, deer are famous for eating plants which common knowledge says they won't eat, but deer typically browse such plants only when other, preferred food sources are scarce and when the normally-distasteful plant is producing new, tender growth with low cellular concentrations of the offending chemical compound. Snails and slugs are not that picky and Jack-in-the-pulpit seems to be a preferred plant for them.

The best way to forestall another slug attack next year is to remove the things in your garden that attract them. Since you will want to keep your Jack-in-the-pulpit plants, you will want to take a different strategy. The most attractive garden elements for snails and slugs are good hiding places. These creatures spend most of their time lurking in moist sheltered spots and do their hunting and feeding during nighttime hours. If you remove or otherwise make unsuitable the places snails like to hide, they will simply go elsewhere.

There are several non-toxic (to humans and other vertebrates) control strategies that you might employ to kill or repel your garden snails. Here is a link to an excellent article from The University of California on snail and slug management. One of the copper-based strategies described in the article, along with removal of hiding places, will likely yield the best results for you.