Anesthetizing the Slug
So Aplysia are to a first approximation bags of seawater. This is important to understand in terms of anesthetizing them. First, yes, we're sticking several inches of needles into them, but they don't have nerves in all the portions that are basically empty spaces full of seawater. Second, the anesthetic goes everywhere pretty quickly. Third, yes, if you stick holes in them in the wrong places, they will leak. This usually doesn't kill them, but it can mess up your experiment pretty darn well.
Despite the anasthetic goes everywhere bit, I mostly put the MgCl into their heads. My surgeries are pretty fast, but it's really important that the muscles be well and truly relaxed. This injection point creates an interesting effect - when you stick the needle in, they (slowly) recoil from the site. Except the anesthetic is spreading quickly, so it's like you have a spreading sphere of recoil that is slowly overtaken by a spreading sphere of numb. If you start in the middle of the animal, this effect isn't so obvious, but when you start at one end, well, it can be pronounced.