From the snailagraph to SnailNet
Atlas Obscura ran an article on an 1850s invention to transmit messages instantly via snail. Innovator Jacques Toussaint Benoît believed that two snails which had come in contact would always be connected by an invisible "escargotic fluid" which which "can be uncoiled and prolonged almost indefinitely in space without its breaking". As a result, when one snail of the pair was stimulated, a galvanic current would travel along the fluid to the other snail. (In Jake's words, it's "quantum entanglement, but with snails".)
That's the theory. The practical application was la boussole pasilalinique sympathique, the "pasilianic-sympathetic compass", which transmitted long-distance messages by assigning a pair of snails to every letter of the alphabet.
(Atlas Obscura's primary source is Sabine Baring-Gould's 1890 Historic Oddities and Strange Events, which you can read for free on Google Books. Jules Allix's 1850 report on the device) is also available online in the original French.)
Atlas Obscura doesn't mention it, but research in snail-based communication continued into the 21st century. In 2005, researchers at Tel Aviv University announced SNAP, or SNAil-based data transfer Protocol, that outperformed ADSL. The researchers did warn of possible denial-of-service attacks in regions such as France. Unfortunately the SNAP paper doesn't cite Benoît's pioneering research — surely his Parisian labs found a solution to the problem.
(My ideas for nudibranch-based communication remain, at present, unfunded.)