Systematic of Nudibranchia
All these animals are nudibranch in a strict sense.
Nudibranchs are soft or firm bodied elongate slugs which, in their adult form they have no shells, althought calcareous spicules may be present in the skin.
These animals look like slugs and have an external bilateral symmetry.
The mantle cavity is absent and the respiratory organs may take different form in the different species. The majority have externally retractable or contractible rear positioned gills arranged around the anus on the dorsal surface of the mantle. Some have their gills along each side of the body beneath the mantle edge, others respire by way of modified body ornementation.