(warning not yet finished!!!)
There are many ways of fixing free-living flatworms, and what I give here are only suggestions. Generally, the quality of the fixation in these tiny animals is very important.
The procedures are a little different for marine, brackish and freshwater species. The higher osmolarity of sea water affects both the relaxation and fixation step.
Fixation:
Before fixation the worms should be relaxed, so that they do not contract and assume unnatural postures when placed into the fixative.
Marine (30-40‰): place the worm into a mixture of about 1/3 the water in which the worms were collected or extracted and 2/3 magnesium chloride solution (71.4 g/L of MgCl2 *6H2O in distilled water or tap water).
Brackish (5-30‰): place the worm into a mixture of about 1/3 the water in which the worms were collected or extracted and 2/3 magnesium chloride solution (71.4 g/L of MgCl2 *6H2O in distilled water or tap water).
We usually fix worms for 1h in 2.5% glutaraldehyde in 0.1M sodium cacodylate buffer (adjusted to pH 7.2) at 4°C (i.e. in the refrigerator).
Such worms could later be postfixed with Osmium Textroxide, if one want to investigate them with transmission electron microscopy.
fixed for 1 h in 2.5% glutaraldehyde
in 0.1 M cacodylate buffer (pH 7.2) containing 10% sucrose,
postfixed for 1 h with 1% osmium tetroxide in 0.1 M
cacodylate buffer,