There are many reasons why sea slugs, especially those without shells, are difficult to preserve in a way that makes them useful for future research. In most mollusc groups, the shape and colour of the shell is a very useful, and often all that is needed to identify a species. In slugs however, it is necessary to know the shape and colour of the living animal, two characters often lost on preservation.
Slugs are without an internal or external skeleton.
When dissected, the internal organs of such animals are usually so misplaced and jammed together, that an understanding of their anatomy is very difficult. Preserving and fixing specimens . The first step is to narcotise the animals so they don't contract into an unrecognisable ball when placed in preservative. Two commonly used techniques are menthol crystals and magnesium salts. Menthol crystals Put slugs in a shallow dish and float crystals on the surface of the sea water.