Snail facts for kids
Quick facts for kidsSnails
|A grapewine snail (Helix pomatia)|
Snails and slugs which breath with a kind of lung are members of a group, the Pulmonata, which was a well-known order in traditional taxonomy. Their fossil records extends back into the Carboniferous period.
The term "snail" is also sometimes used for aquatic snail-like gastropods, which usually have gills. Actually, most snail species are marine snails: they have much greater diversity and a greater biomass. Numerous kinds of snail can also be found in fresh water.
Close relatives of the snails are the slugs, which are basically snails without shells. Both slugs and snails are numerous and successful on land. Most land snails and slugs are herbivorous. Aquatic snails and slugs are usually omnivores or predatory carnivores.
The biggest snail is the giant African snail. Their foot is up to 35 cm long. The fastest snail is the Helix aspersa. It can reach speeds up to 0.03 mph. There are known more than 43000 species of snails all over the world.
Land snails are nocturnal animals and move to food and partners in the late evening or at night. They prefer a damp, but not a wet environment and bury themself up to two weeks in hot and dry phases. Their enemies are birds and mammals like mice and similar predators normally.
There are also many water snails. Some of them live in rivers or freshwater, but most are resident on oceans ground. Ocean snails are colorful, so they can be pink, blue, red, grey, yellow and also with many other colors. The colors protect them, because this colours scare potential enemys away. Water snails are not as active as land snails in general, but they mostly do not distinguish different daytimes. Water snails' most abundant enemies are fishes of prey.
Snails are distinguished by an anatomical process known as torsion, where the visceral mass of the animal rotates 180° to one side during development, such that the anus is situated more or less above the head. This process is unrelated to the coiling of the shell, which is a separate phenomenon. Torsion is present in all gastropods, but the opisthobranch gastropods are secondarily de-torted to various degrees.
Torsion occurs in two stages. The first, mechanistic stage, is muscular, and the second is mutagenetic. The effects of torsion are primarily physiological - the organism develops an asymmetrical growth, with the majority occurring on the left side. This leads to the loss of right-paired appendages (e.g., ctenidia (comb-like respiratory apparatus), gonads, nephridia, etc.). Furthermore, the anus becomes redirected to the same space as the head. This is speculated to have some evolutionary function, as prior to torsion, when retracting into the shell, first the posterior end would get pulled in, and then the anterior. Now, the front can be retracted more easily, perhaps suggesting a defensive purpose.
However, this "rotation hypothesis" is being challenged by the "asymmetry hypothesis" in which the gastropod mantle cavity originated from one side only of a bilateral set of mantle cavities.
Gastropods typically have a well-defined head with two or four sensory tentacles with eyes, and a ventral foot, which gives them their name (Greek gaster, stomach, and pous, foot). The foremost division of the foot is called the propodium. Its function is to push away sediment as the snail crawls. The larval shell of a gastropod is called a protoconch.
The principal characteristic of the Gastropoda is the asymmetry of their principal organs. The essential feature of this asymmetry is that the anus generally lies to one side of the median plane.; The ctenidium (gill-combs), the osphradium (olfactory organs), the hypobranchial gland (or pallial mucous gland), and the auricle of the heart are single or at least are more developed on one side of the body than the other ; Furthermore, there is only one genital orifice, which lies on the same side of the body as the anus.
Most shelled gastropods have a one piece shell, typically coiled or spiraled, at least in the larval stage. This coiled shell usually opens on the right-hand side (as viewed with the shell apex pointing upward). Numerous species have an operculum, which in many species acts as a trapdoor to close the shell. This is usually made of a horn-like material, but in some molluscs it is calcareous. In the land slugs, the shell is reduced or absent, and the body is streamlined.
Some sea slugs are very brightly colored. This serves either as a warning, when they are poisonous or contain stinging cells, or to camouflage them on the brightly colored hydroids, sponges and seaweeds on which many of the species are found.
Lateral outgrowths on the body of nudibranchs are called cerata. These contain an outpocketing of digestive gland called the diverticula.
Sensory organs and nervous system
Gastropods have no hearing.
The majority of gastropods have simple visual organs, eye spots either at the tip or base of the tentacles. However, "eyes" in gastropods range from simple ocelli that only distinguish light and dark, to more complex pit eyes, and even to lens eyes. In land snails and slugs, vision is not the most important sense, because they are mainly nocturnal animals.
Many marine gastropods are burrowers, and have a siphon that extends out from the mantle edge. Sometimes the shell has a siphonal canal to accommodate this structure. A siphon enables the animal to draw water into their mantle cavity and over the gill. They use the siphon primarily to "taste" the water to detect prey from a distance. Gastropods with siphons tend to be either predators or scavengers.
Almost all marine gastropods breathe with a gill, but many freshwater species, and the majority of terrestrial species, have a pallial lung. The respiratory protein in almost all gastropods is hemocyanin, but one freshwater pulmonate family, the Planorbidae, have hemoglobin as the respiratory protein.
In one large group of sea slugs, the gills are arranged as a rosette of feathery plumes on their backs, which gives rise to their other name, nudibranchs. Some nudibranchs have smooth or warty backs with no visible gill mechanism, such that respiration may likely take place directly through the skin.
The primary organs of excretion in gastropods are nephridia, which produce either ammonia or uric acid as a waste product. The nephridium also plays an important role in maintaining water balance in freshwater and terrestrial species. Additional organs of excretion, at least in some species, include pericardial glands in the body cavity, and digestive glands opening into the stomach.
The main aspects of the life cycle of gastropods include:
- Egg laying and the eggs of gastropods
- The Embryonic development of gastropods
- The larvae or larval stadium: some gastropods may be trochophore and/or veliger
- Estivation and hibernation (each of these are present in some gastropods only)
- The growth of gastropods
- Courtship and mating in gastropods: fertilization is internal or external according to the species. External fertilization is common in marine gastropods.
Snails are found all over the world. Generally speaking, land snails live on damp habitats. They live in caves and dark places. Snails can be found in dark places such as in a garden under plant's foliage leaves. Some species live in cold places like the Arctic and a few are found in warm places like beaches and deserts.
Some snails are aquatic and live in water. They live in the sea, fish tanks, rivers and oceans. They do not swim in the water because most of the time they stick to rocks.
Land snails eat vegetables and fruits, such as lettuce, carrots, cucumber and apples. Aquatic snails are often carnivorous. Snails use their radula to cut food. The radula is a hard, rough plate in the mouth. Radula teeth are like little pieces of sandpaper. They are good for cutting up plants and if the snail eats meat they are good for tearing the meat apart. Radula teeth look like little fangs.
Many animals eat snails. Fireflies, snakes, beetles, fish, insects, turtles and people eat snails too. To defend themselves, snails pull back into their shells.
Slugs evolved from snails which reduced, and finally lost, their shells. They live in similar habitats.
Images for kids
The use of love darts by the land snail Monachoides vicinus is a form of sexual selection
Snail Facts for Kids. Kiddle Encyclopedia.