It may be useful here to give the chief differences between the genera Arion, Amalia, Limax, and Agriolimax. The shield in the first two is granulated, in the other concentrically striated. The breathing orifice in Arion is in front of the centre of the mantle margin; in the others behind. The shell is distinctly formed in all but Arion, in which it is absent or represented by a few granules. Arion has no dorsal keel. Amalia has one all down the back. In Limax and Agriolimax it is confined to the caudal part. Other differences are only discovered by dissection.
One may also here note that to preserve slugs is difficult, and the best plan is to have a coloured drawing made of them when extended. Otherwise they may be drowned in cold water, cleaned of slime with a soft brush, and then preserved in glass tubes with diluted formalin or alcohol. Or, after drowning, they may be skinned and the skins dried on a card and varnished. Note also that most slugs have many variations in colour and markings.
Testacella haliotidea.—This genus of slugs forms a link between the naked slugs with rudimentary shells within, and the snails which live within their shells. The name Testacella, or little shell, was given by Cuvier in 1800, because this slug has a small shell at the end of the tail. Haliotidea means having a shell in the form of the marine shell Haliotis, the meaning of which again is the ear-shaped seashell, often called “Venus’ Ear.” It is subterranean in habit, and lives on worms. It should be looked for on the surface on damp nights, or is found when digging. Its length is 3 inches at most. Pale yellow in colour. See the illustration on page 35.
Testacella scutulum (a little shield).—Not so common as the former species, and differing chiefly in anatomy.
Testacella Maugei.—First found at Tenerife by M. Mauge. Reaches 4 inches in length. Deep brown in colour. Shell larger. Rarer and more western in habitat than the other species.
Testacella haliotidea.
Vitrina pellucida.—The Vitrinas in several ways afford a connecting link between the slugs and snails, having the same tooth-formation and mantle as the former, while the shell cannot contain the whole body. As the name indicates, the shell is like a bubble of clear greenish glass and very delicate. It is small, and found in damp places, coming out mostly at night. Omnivorous, it is often found feeding on dead worms, and, unlike nearly all our earth molluscs, can be found abroad in winter.