"Ravenworks69" at the Halloween Forum gives us a tutorial on creating quick and easy lab specimens using toy animals. For such a simple technique the results are surprisingly good and ideal for a quick and dirty bottled specimen. For the price of a cup of coffee and a few minutes worth of effort you can't beat it.
If you're willing to expend a little more effort these could be downright amazing. The first thing I thought of was my go-to material for wet specimen props: liquid latex carpet adhesive. Just sponging on a layer and rubbing it once it dries would produce some interesting webbing and texture effects.