After the summer rain, the children's favorite thing to do is to go to the flower bed to find the snails that crawl out. The snail is one of the oldest animals in the world, carrying a heavy shell for hundreds of millions of years.
An amber snail fossil on display at the Colmar Museum in France.
Ancient animals evolved from the ocean
Snails are one of the oldest animals in the world. The earliest fossilized snails have been found in Canada and the northern United States, about 350 million years ago, indicating that they were already living on Earth at that time.
In October 2018, paleontologists from China, the United Kingdom, Australia, Canada and other countries announced in Beijing that they had discovered the world's first Cretaceous snail amber that preserved soft tissues such as tentacles. This is a small fluffy snail that lives in ancient amber forests in the Hukang River Valley in Kachin State in northern Myanmar, dating back to the middle Cretaceous period, about 100 million years ago.
Snails belong to the mollusk phylum Gastropods, and gastropods are the largest category of molluscs, which is often called shellfish, with more than 100,000 species in the world, distributed in the ocean, freshwater and land, with the most marine life, and snails are shellfish that have been carried out on land throughout their entire life history.
There are more than 25,000 kinds of snails in the world, distributed all over the world, different species of snails of different sizes, the largest up to more than 20 centimeters, generally only a few centimeters. The body of the snail is soft, except for a spiral shell, the body is divided into head and feet; The head has 2 pairs of antennae, the posterior pair is longer, the posterior antennae have 1 pair of eyes at the apex, and the mouth has a jaw and a file-like toothed tongue for chewing and chopping food. Snail breathing takes place in lung-like tissue, and air enters through the suction holes. It is named snail because it has two antennae like bull's horns on its head when crawling.
In terms of scientific classification, the species of snails are roughly divided into anterior gills and lung, the latter accounting for the majority. There are fewer species of snails in the pregill class, so the classification is relatively simple, while there are many species with lungs, and the classification is more complicated. In the past, snails with lungs were divided into three orders: primitive lung, basal and stalked, but the latest taxonomy reassigned them to the order Shrunken , Basal Eye and True Lung , and some nude suborder snails were added.
The biggest difference between the body structure of the anterior gill snail and the lungs is the gender and respiratory system. The anterior gill snail is dioecious, breathing with the gills, which is also one of the markers of the evolution of snails from the ocean; Lung snails, on the other hand, are hermaphroditic and breathe with their lungs. Most known snails fall into the latter category.
The meanings in Eastern and Western cultures are mixed
Ancient Chinese texts record the history of people's cognition of snails. In the pre-Qin and Western Han dynasties, the words snail, snail, snail, and slugs could refer to all spiral shells, or mollusks with mucus. "Erya Shi Worm" said, "A slug, a slug, is one and the same as a snail." However, with the gradual deepening of people's understanding of mollusks such as snails, from the Eastern Han Dynasty, "cockroaches" were gradually abandoned, and snails and slugs began to be referred to differently. "Shuowen" said that "those with shells are called snails, and those without shells are slugs", and "snails" specifically refer to a type of creature with spiral shells.
Due to its heavy shell and slow movement, snails represent different meanings in Eastern and Western cultures.
In ancient times, people often interpreted it as a negative image of slowness, conservatism, and backwardness. The ancients observed that snails crawled leaving traces of slime on the ground or walls, resembling seal scripts, so they called them "snail seals", and snails were therefore named "seal sorrowful kings". The Song Dynasty poet Su Shi's "Snail": Fishy saliva is not satisfied with the shell, and the chat is enough to self-indulgence. I don't know how to rise, but it is a sticky wall. This allegorical poem depicts us a poor insect with fishy saliva that can only wet its own body - a snail. It did not measure itself, only climbed up, and eventually salivated and died on the wall, satirizing those who had no talent and no virtue but blindly chased high-ranking officials and houlu, and finally died in Vanity Fair.
The snail was also a symbol of poverty. During the Three Kingdoms period, Jiao Xian and Yang Pei built round houses, shaped like snail shells, called "snail houses". Later, the literati used to call their residence "Worm House", and later also called "Worm House" and "Worm House", which referred to simple and narrow houses. Lu You once sighed, "The four walls of the snail are empty, and it has been a hundred years." ”
However, the image of the snail in modern culture is very different, symbolizing the "snail spirit" of active efforts, perseverance, perseverance and down-to-earth. For example, the song "Snail" composed by Jay Chou at the low point of his career is to encourage himself at the same time, but also to encourage young people to move forward step by step for their goals like snails, not to be crushed by reality, even like snails, they should strive to move up and have their own piece of the sky. Later, this positive song was selected as a patriotic song for Shanghai middle school students in 2005.
Symbolizing slowness, patience and tenacity in Western culture, the snail is often seen as a quality that signifies the perseverance and patience required in the pursuit of success, making it popular abroad. Snails in the United States mean happiness, while Scots believe that if the snail's tentacles stretch out very long, it means that there is good weather tomorrow.
Croatia hosts a snail race.
The animal with the most teeth in the world
In the long process of evolution, snails did not produce legs, let alone wings, and could only rely on rubbing or climbing, coupled with the threaded shell on the back, slow action is inevitable.
The snail's thick shell characterizes its body composition, but it also reveals its evolutionary time and course. Snail shells are formed by depositing calcium carbonate, suggesting that the ancestors of snails lived in the sea and evolved hard shells to protect themselves. After landing, the snail still maintains this trait, and the shell has evolved into a protective barrier for its weak body and internal organs, like the scales of fish, the thorns of porcupines, and the shell of turtles, once it encounters natural enemies and other damage, it can immediately shrink into the shell for refuge.
Don't underestimate the snail shell, which is also an important organ to ensure its survival. Snails can live without water, but they need to be damp, otherwise mucus is secreted that is used to keep the body moist. When the weather is dry, it seals the shell mouth with mucus and goes into hibernation.
Another magical feature of the snail's evolution is its feet. Humans have two legs, animals generally have four legs, insects have many legs, and snails only have a muscle in the abdomen that prompts it to move - the abdominal foot. Only a snail in the world is a foot, which can be called a veritable "one-legged fairy", so the speed of the snail crawling at full speed is only about 8.5 meters per hour, which is equivalent to 3 minutes for a person to take a bite of food from a plate and send it to his mouth.
Observing the crawling snail, you can see that the contraction of the abdominal foot muscles allows the snail to move forward like a wave, because the fluctuations generated by muscle contraction are only in a single direction, so they cannot retreat, and must move forward. If it encounters an obstacle to change direction, the snail can only modify the forward angle, and this movement that can only advance and not retreat also constitutes the behavior characteristics of the snail. In addition, the mucus secreted by the abdominal feet when crawling is not only to reduce friction, but also to protect the film, even if it passes on the blade, it will not be scratched on the body.
Don't look at the snail born harmless to humans and animals, but it is actually one of the top ten pests of agriculture, which can harm a variety of crops. This may puzzle many people: what does a soft snail do to harm crops? In fact, snails are the animals with the most teeth in the world. There is a small hole in the middle of the snail's tentacles, which is its mouth, although it is about the same size as the tip of the needle, and it cannot be distinguished by the naked eye without a magnifying glass, but there is actually a jagged tongue inside the small hole, which scientists call "tooth tongue". The tongue is surprisingly full of teeth, there are more than 26,000, each thinner than a human hair.
When endangering crops, snails scratch plant leaves or fruit surfaces through their toothed tongues, scratch the plant epidermis with serrated teeth, send leaves or pulp into the mouth, and then use their toothed tongues to transfer food to the digestive tract. If in a very quiet environment, you can even hear the sound of snails eating "crunching". The leaves of crops that have been harmed by snails will appear porous or net-like, and the peel will also have very obvious scars, which affect the normal photosynthesis of plants and bring trouble to agricultural production. What's even more amazing is that the snail's teeth can continue to circulate, replacing the old ones with new ones, usually growing new rows on the tooth tongue at the back of the mouth, and then gradually moving forward to replace the worn teeth in front.
Snails' eyes are similar to humans, and are also composed of cornea, lens, retina and nerves, but the structure is very simple, so they can only feel changes in light and shadow, and cannot really see the appearance of objects. The antennae are the most important sensory organs of snails, which can sense changes in the environment and air humidity and detect routes for their own movement. Antennae shrink when danger is perceived, when it touches something, or when the air humidity changes, and only stretches out when danger is not perceived. Moreover, the process of extending the antennae is much slower than contraction.
There is a familiar nursery rhyme in old Beijing: "Buffalo buffalo, first out of the horn and then out of the head..." "Buffalo" is a common name for a small snail in Beijing, when it is caught by a child, the body is immediately frightened and retracts into the shell. Children sing nursery rhymes while waiting for it to adapt before slowly sticking its head out and resuming its crawl.
French baked snail.
Further reading
"Invading Monsters"
Snails have been eaten in ancient times, and there are many opinions about whether they are delicious or not, but they are really good in terms of nutrition - high protein, low fat, low cholesterol, and a variety of nutrients that the human body needs. In recent years, snails and their nutritional value have been recognized by people all over the world, so they have also become a delicacy in restaurants.
"French snail" is an appetizer of French cuisine and one of the world's famous dishes. In France, snails are an indispensable delicacy on the plate of major festivals, consuming more than 300 million snails a year. In addition to France, the rest of the world can buy canned snails and process them. In supermarkets, a preliminarily processed snail meat is also sold, which is placed in the shell of the snail, sealed with butter, and after the customer buys it, as long as the snail is heated in the oven, the snail meat will be fried in melted butter, and the snail meat is picked out of the shell and eaten.
Because French cuisine is made from snails, a large, meaty Burgundy snail has been bred locally, each only four or five centimeters long and weighing about 40 grams, and this edible snail has spread around the world. However, snails mainly live in soil and rotting leaves, and are likely to be contaminated with bacteria or other harmful substances to humans, so edible snails must go through a two-week detoxification period before being sold in order to excrete bacteria and toxic species from their intestines.
Not all large snails are edible, such as the African giant snail native to East Africa, which is one of the world's top 100 invasive alien species, which is disliked by most countries and was included in the list of the first invasive alien species on the mainland as early as 2003.
The African giant snail is the largest of the living land snails, with a lifespan of up to 10 years, and some larger individuals can have shells longer than 20 cm. As early as the 70s of the last century, Guinness World Records certified a huge African snail with a shell length of 27.3 cm, a length of 39.3 cm from head to tail, and a weight of 1.8 pounds.
Some people may ask, even if it is large, what is so scary about snails that move slowly and have no offensive power?
It turns out that this large snail eats an astonishing amount and can eat more than 500 important crops, including cassava, cocoa, papaya, peanuts, rubber, most beans and melons, as well as ornamental plants and herbs, and even some inedible substances can become its plate, such as cardboard, sand, small stones, plaster or wall skin, which seriously threatens various buildings.
In addition, its reproductive capacity is very strong, it can lay one or two thousand eggs a year, and the survival rate is very high. Moreover, compared to ordinary snails, they can survive in many extreme environments. Even in extremely arid places, African giant snails can remain buried in the soil for about three years. As an invasive species, the African giant snail is rapidly expanding in the wild, squeezing the living space of native gastropods and seriously threatening native biodiversity.
Since the '60s, Floridians have spent decades eradicating giant snails, once for 7 years and once for 10 years, at a cost of tens of millions of dollars. But who would have thought that after more than half a year, the third large-scale eradication operation was launched again last year, and it is no wonder that the docile snail is regarded as an "invasion monster" in Florida.
The African giant snail first appeared in Florida in 1966. At that time, a young boy in Miami went on a trip vacation, and when he came back, he put three large snails in his pocket as pets, and the family put the snails in the garden, but they did not expect to breed tens of thousands of them a few years later, and they were eating everywhere, and everywhere they went.
Don't think that the eradication operation is a fuss, the illegal invasion of the African giant snail will not only bring disaster to the local booming agriculture, but more frighteningly, the African giant snail carries a large number of parasites, among which rat pneumocystis can cause human meningitis. Therefore, the United States uses the highest quarantine standards to prevent and control this highly combative invasive species, and the agricultural department even warns residents not to move snails, soil, plants or yard garbage at will, and all vehicles and machinery must be inspected when they go out to avoid snails sticking to them and spreading to other places with vehicles.
The discovery of large African snails in Florida, USA, has caused headaches for local residents.
Biological mysteries
Magical slime
The mucus marks left by snails crawling make some people feel uncomfortable, thinking that they are dirty and smelly. But scientists have found that secreting mucus when crawling is not only the way snails act, but also suggests that snails' mucus may contain many mysteries, and snails' mucus is priceless.
Now, whether in Europe or Southeast Asia, it has become fashionable to use snail mucus extract for beauty, which has both historical roots and the results of modern biomedical research.
Hippocrates, the father of medicine in ancient Greece, has understood the medical role of snail mucus, and believes that snail mucus has moisturizing, swelling, anti-inflammatory and analgesic effects, and continuous use can make the skin surface smooth and translucent. He mixes milk with crushed snails to treat skin scars.
Later, snail skin care products began to appear in South America, because local farmers raised snails to export to France, they found that after contact with snails, the skin on the hands became smoother and softer, so the beauty effect of snail mucus slowly spread.
As early as 1738, the French Pharmacopoeia published recorded the use of snails to treat wounds and skin diseases. The more scientific basis for the proposal is Spanish radiation oncologist Raphael. In the 60s of the 20th century, while doing experiments, he accidentally discovered a Chilean snail that has the ability to repair its own body damage. When rays irradiate a snail, the snail twists its body, continuously secretes mucus, and covers and attaches to the surface of the body. After a while, the ray-irradiated wounds on the snail's body healed. Raphael then collected some mucus from the snail and tested it, and found that the mucus did have the effect of accelerating wound healing and repairing the skin. Later, Raphael developed a snail mucus extract product for beauty.
A large number of modern studies have found that snail mucus extract contains a variety of substances, such as collagen, elastin, allantoin, multivitamins, calcium, glycolic acid, etc. In addition to this, the researchers also found that snail mucus contains antimicrobial peptides, and although different snails contain different components of antimicrobial peptides, they all have antibacterial effects. It can be seen that the snail mucus that does not look amazing is extracted and used in the human body, which not only helps to protect the skin, but also effectively prevents other diseases.
(Original title: Snails shellfish living on land)
Source: Beijing Daily
Author: Zhang Tiankan
Photo courtesy of this edition: Visual China
Process editor: u008