The fourth family, Limacidæ, containing Testacella and Limax, are terrestrial pulmonary molluscs, entirely naked, or having only a very small shell. The Limax varies very considerably in appearance, in consequence of its extreme contractibility. When seen creeping along on the surface of the soil, it has nearly the form of a very elongated ellipse, at one extremity of which is the head; the surface of the body in contact with the earth is flat, the other convex. Towards the anterior extremity, and upon the middle of the back, a portion of the skin projects as if it were detached from the body, and is ornamented with transverse stripes of various convolutions. This part is named the cuirass, or buckler, under which the animal can hide its head.
The mouth is a transverse opening in the front of the head; above are two pairs of tentacles, or horns, immensely retractile, cylindrical, and terminating in a small button; the lower tentacles are the shorter; the upper present at their summit a black point, as in Helix, which have sometimes been mistaken for the eyes.
Upon the right side of the cuirass, and hollowed in the thickness of its edge, which is large and contractile, whose function it is to give access to atmospheric air, it abuts on an internal cavity, also large, and is intended to promote respiration. The outer skin, or epidermis, is rayed in brownish furrows, its surface covered with a viscous glutinous substance, which permits of the animal creeping up the smoothest surfaces, locomotion being produced by the successive contraction and extension of the muscular fibres of the feet.
The internal organization of the Limax is analogous to that already described in the snails. The taste and smell in the Limaceans differ only very slightly from those organs in Helix. They are, like the snails, deaf, and nearly blind. They love humid places; they lodge themselves in the holes of old walls, under stones, or half-decomposed leaves, in the crevices of the bark of old trees, and even underground, coming forth only at night and in the morning; especially after soft showers in spring and summer. In the garden, after one of these soft showers, many of these little creatures are sure to be met with in the more shaded alleys.
The Limax is mostly herbivorous. They seek, above all, for young plants, fruits, mushrooms, and half-decayed vegetables. They are very voracious, and cause great ravages in gardens and young plantations, and many are the devices of the watchful gardener to destroy them. Lime and salt are their abomination; ashes and fine sand they avoid. They dislike the noonday sun, and the gardener knows it; he arranges little sheltering tiles, or planks of wood and stone, under which they retire, where they are surprised to their destruction.
Fig. 191. Limax rufus (Linnæus).
There are thirty known species of Limax. Some are remarkable for their very striking colours. Limax rufus (Fig. 191) is common in woods, and well known for its large size and its colour of rich yellowish red; it is known all over Europe, from Norway to Spain.
Among the Limaceans nearly destitute of shells we find Testacella haliotidea (Fig. 192), which is provided with a very small shell placed at its posterior extremity, just over the pulmonary cavity. This shell becomes more important in Vitrina, already spoken of as forming the point of transition between Limax and Helix. This passage from Limaceans entirely destitute of shells to those furnished with a very small shell, as in Testacella, is very exactly indicated by Nature. Limax rufus, spoken of above, presents, under the posterior part of the cuirass, calcareous, unequal, isolated granulations, which are, so to speak, the elements, as yet internal, of a shell which is on the point of being built. Other species in the same genus present under the cuirass a little rough, imperfect scale, which seems to be produced by a great number of these calcareous granulations, which show themselves in an isolated state in Limax rufus.
