Slugs are lung-breathing gastropods. They live on land and are almost worldwide in distribution. They are symmetrical animals with a well-defined head and an elongated body narrowing behind into a pointed tail. They have no spiral shell like snails. The common genus Limax found in India has only a small internal calcareous representing the shell. One characteristic of a land slug is that it has a hole on the right side of its body: this is the breathing aperture. The forepart of the body containing the lung, heart and other internal organs is covered over by the relatively small mantle which is sometimes called the shield. The head carries two pairs of tentacles. At the tips of the hinder pair are situated the eyes and the organs of smell. Both pairs of tentacles can be completely withdrawn into a hollow in the head. The mouth is at the lower part of the ahead and is a simple aperture. Land slugs are partly vegetarian and partly flesh-eating. Consistent with this feeding habit, their radula contains both large and small teeth - the marginal teeth are blade-like while the others are squarish. The radula is long and in Limax maximus, as many as forty thousand teeth are counted on it.
Slugs are more active by night than by day, and they seek moisture. In India they are common in areas receiving regular annual rainfalls. During the dry season the slugs remain dormant under the earth, that is, they estivate. During this period of inactivity the slug contracts its body and covers it with a double coating of tenacious slime or mucus which the animal copiously secretes. Unlike snails, slugs estivate separately and not gregariously.
Like Nassids or Dog-whelks slugs are sensitive to smell. Any putrid vegetable or decaying animal at once attracts them. They feed on these and also on earthworms, caterpillars, centipedes, etc. Certain species eat their own kind. Slugs have many natural enemies including thrushes, toads, frogs, lizards and carnivorous beetles.
A land slug has both male and female organs capable of producing sperms and ova. They lay only a limited number of eggs. The black slug - Arion - lays four hundred and seventy seven eggs in forty eight days, i.e. at the rate of ten eggs per day.
Belonging to the same sub-order of land slugs, there is a curious slug - Onchidium - which lives by the sea coast near brackish marshes and mangrove swamps. Unlike land slugs it has no internal calcareous plate. Viewed from above it is oblong shaped and from the side it looks more or less convexly arched. The mantle is thick and leathery and covers the entire body. The back is covered by warty growths or tubercles of different sizes. In addition to the normal pair of eyes on the tentacles a number of other eyes are developed upon some of the tubercles on the bank. These eyes are in structure similar to the eyes of Scallops. The animal attains a size of 4.8 centimetres long and 2.9 centimetres broad and 2.5 centimetres high. It feeds on tiny seaweeds and is common on rocks especially near coral reefs. The animal is known to be subject to the attacks of certain predatory fish; but the mantle eyes enable the mollusc to become aware of the shadow of its approaching enemy and escape in time.
It has been noted in the case of one genus - Prophysaon - that when it is annoyed by being handled, an indented line appears at a point about two-thirds of the length from the head. This line deepens and eventually the tail is shaken completely off. If the animal is let loose before the actual dismemberment takes place, the deepening of the indented line stops forthwith and after some time completely closes up.
Sea slugs are perhaps the most curious and wonderful of all gastropods. They are all small shell-less shell-fish. They are widely distributed on the weed-covered rocks and reefs of all seashores. In cooler waters they are especially rich in number and variety. Sea slugs are not really slugs but naked gilled gastropods called Nudibranchiates. As an embryo the animal has a tiny coiled shell which is lost soon after the sea slug emerges. An adult sea slug is symmetrical, with a soft and elongate body like that of a true slug and the mouth and anus at opposite ends. It has no shell, no lung, or internal gills. It breathes through outgrowths on its back which are often elegant and brilliantly coloured in green, blue, crimson, red or yellow. There are fantastic forms in different genera, some are branching filaments, some club-shaped, some rosette-like and others mere folds.
Sea slugs feed on sea-anemones, corals, sponges and tiny molluscs. Many of them have no radula but grind what they eat by the horny teeth with which the inside of their stomach is lined. The liver in these animals is not a compact organ as in other molluscs but broken up into a number of glands. In the plumed and crested genera like Aeolis, the liver glands are contained in the plumes and crests.
Sea slugs are highly elastic. They can shrink to half a centimetre and expand up to eight centimetres. Most of them are far more richly coloured than any of the land slugs and some are of truly remarkable appearance. One and the same individual changes its colour scheme while expanding or contracting, while feeding or while gliding through different coloured seaweeds, sea-anemones and corals. Thus their ornamental and colourful appendages serve the purpose of protecting them from their enemies by harmonizing with their surroundings. The colours are not merely protective but have also a warning significance, because in some species the colours conceal the spicules on their skin or the stinging cells on their dorsal processes.
Along the Indian shores, especially in the waters of Pamban, Krusadi and Shingle Islands, several attractive and curious species of sea slugs are very common. They are found attached to the underside of stones and rocks in shallows abundant in seaweeds and corals. The more interesting among them are; the plumed types - Bornella and Eubronchus, the filamentose Hervia and the star-marked Dendradoris and Trippa.