The worms will often still move a lot when it is in a squeeze preparation (see Observation ), and this can make it hard to make detailed observations. Worms can be anaesthetised with a solution of MgCl2 in distilled water (tap or mineral water will often also do). For marine samples I usually us a 7.14% solution (i.e. 7.14g per 100ml), and I place the worm into a 50:50 (or even 60:40) mixture of MgCl2-solution and sea water. In contrast, fresh-water species are much more sensitive to magnesium chloride, and there I will often use only a few percent of solution (too high concentration will stop the beating of the cilia, and make the epidermis crumply and less transparent, which is not nice for observation).
Once you have the worm in a nice squeeze preparation go through a series of magnification steps to document more and more detailed structures of the worm. I usually start taking pictures with the 4x objective, even if the worm is very small. This makes it easier later to reconstruct the magnification at which the different pictures were taken. I usually try to take pictures of the following structures:
habitus, eyes, brain, pharynx, pharynxglands, testis, sperm in testis, ovary, forming eggs, eggs and or sperm in femaleantrum, eggs, vaginal cilia, shell glands, false seminal vesicle, seminalvesicle, copulatory stylet, prostate glands, vesicula granulorum, male pore, male porecilia, sperm, epidermis, cilia, sensory cilia, rhabdites, rhamites, adhesiveglands, and food items.
For taxonomy the most important structure is of course the copulatory stylet, but also the structure of the female antrum and the sperm morphology is important.
With a bit of practise you will be able to recover worms from the squeeze preparations if you want to do some other things with it. To do this place a drop of the water the worm came from next to the cover slip. The capillary force will draw in the water and the strength squeezing of the squeezing will be relaxed. Generally the worm will start moving around again below the cover slip and you can then lift the cover slip with pair of fine watchmakers forceps.