There is a passage that says that a bored earthworm wants to play football, so he cut himself into 22 paragraphs and ended up being admitted to the hospital.
We know that even earthworms, which have a strong ability to regenerate, can't stop cutting themselves into "meat fillings". And there are many restrictions on the regeneration of the cut sections of earthworms.
For example, although it is possible to "split" Eisenia Fetida into two pieces, if one of them is particularly short, the short one is likely not to survive.
Most earthworms can't afford to cut their heads and tails, and all earthworms can only cut horizontally, and if they are cut longitudinally, they will not survive.
However, there is such a class of animals in nature that can really "divide into many", cut themselves out of two "football teams", the key is that every paragraph can survive!
Who is it? Next, let's get to know today's protagonist - the flat animal.
Bedford flatworm, image source: Wikipedia
<h1 class="pgc-h-arrow-right" >what is a flat animal? </h1>
Flat-shaped animals, as the name suggests, are flat-shaped animals, and are a general term for about 15,000 species of animals under the Platyhelminthes.
Flat animals are a relatively simple type of animal that has no spine, no circulatory system, and no respiratory system.
Because of this, such animals must maintain a relatively flat body shape in order to absorb oxygen and nutrients through osmosis through the body, and the name "flat animal" is derived from this.
If we directly say "flat animals", the friends may still be more confused, but when it comes to planarians and schistosomiasis, are you a little impressed?
Planarians, two eye spots can be seen on the head
The eye spots of flat animals are photoreceptor structures composed of pigment cells and photoreceptor cells, which can only distinguish the intensity of light and have no imaging function.
Flat animals can take advantage of the fine bristle movements on their bodies, which trigger their avoidant behavior when exposed to strong light, while they will move forward normally when there is no light.
<h1 class="pgc-h-arrow-right" > the "nasty ghost" in the aquarium and the master of regeneration</h1>
Friends who have raised ornamental shrimp may be deeply disgusted with planarians, and these flat bugs are often introduced into the aquarium with aquatic weeds.
Not only will they grab shrimp feed, but they will also attack shrimp fry and freshly molted shrimp, and the reproduction rate is particularly fast! Although they usually pose no threat to shrimp, they are still very annoying.
The slender light-colored animal in the picture is a planarian worm in the aquarium of ornamental shrimp aquarium, image source: flying
Although planarians are unpopular "nasty ghosts" in aquariums, they are "regulars" in biological laboratories.
Because of their strong regeneration ability, biologists often use planarians to do various experiments and cut planarians in different ways. It was found that basically every piece cut could slowly develop into a complete individual.
Planarians that are cut off
Geneticist Morgan (the one who keeps fruit flies) once "broke the corpse" of planarians in experiments.
He found that when a planarian was cut to only 1/279 of its original size, the few remaining planarian tissues could regenerate a complete planarian.
In other words, in theory, planarians can theoretically turn into 279 new planarians after being cut into 279 portions!
Planarians: The football team is nothing at all, give me some time, I can even get the audience together
In addition to this "normal" method of segmentation, biologists have tried some strange methods of segmentation.
For example, if the head is cut in half, but the body is not completely cut off, eventually the planarian will grow two heads on the same body. Similarly, there are experiments with "multiple bodies in one head".
Other scientists cut their bodies apart, but not completely, and eventually the long flatworms became starfish-like, full of bulges.
And if you cut the head every time, it will even grow into a star-shaped flatworm with multiple heads.
Changes in planarians and their nerves that become multi-headed after being cut
<h1 class= "pgc-h-arrow-right" > "witty" planarians are not witty or even brainless</h1>
There is a very magical-looking flat-shaped animal with a head that resembles a hammerhead shark [click to review the hammerhead shark], and it has a brightly colored pattern.
Its common name is also very magical, called "Heavenly Snake", also known as "Earth Worm". This weird guy is a leech (pronounced "witty") planarian.
Is the head of the leech planarian, is its head a bit like a hammerhead shark?
Although it has the name of "wit", but like other flat animals, the pentarian planarian has a very simple neural structure, is not witty, and does not even have a "brain".
On both sides of the flat animal's body there are two abdominal nerve cords, which are the main channels for them to transmit nerve signals, and there are transverse neural connections between the two abdominal nerve cords, which look like ladders, so their nervous system is also called the trapezoidal nervous system.
They have no brains on their heads, but have denser neural tissue than the rest of the body.
The nervous system of the planarian, well, rounds it up to have a "brain" too
Among the flat animals, the planarian leeches are also a fierce player. They wrap their bodies around prey such as earthworms and secrete digestive juices from the opening in the middle of their bodies to digest them and suck them up, and the hunting scene of this small animal looks quite thrilling.
Snail-eating planarian worms
Flat animals have no circulatory system, and nutrients in food can only be absorbed by body tissues near the intestine. As a result, they evolved a branched intestine that spread throughout the body in order to allow the whole body to access the nutrients in food.
Incidentally, they don't have an anus yet, and food is "eaten" from the opening in the middle of their body, and after the body has absorbed nutrients, food residues will be excreted from this opening (friendly reminder: see here, don't continue to imagine).
<h1 class= "pgc-h-arrow-right" > not just a weak critter, but a dangerous parasite</h1>
The trematodes, represented by schistosomiasis, are a very well-known parasite. Schistosomiasis has been widely spread in 12 provinces in the south of China, causing a large number of residents to fall ill and causing very serious loss of life and property.
Chairman Mao Zedong once used the verse "A thousand villages of Xueli people are left behind, and ten thousand households are singing" to describe the tragic situation of large-scale infection of schistosomiasis before liberation.
Eggs of Schistosomiasis japanensis
After the eggs of schistosomiasis are excreted from the host, if they enter the water, the hatched caterpillars will look for nail snails in the water and parasitize on the snails.
Hairy cymbals first develop into female cinders on the snails, and the female cilis reproduce asexually to produce sub-capsiculates, and then reproduce again to produce a large number of tail cymbals.
Tail clyalis can leave the snail, swim freely in the water, when it comes into contact with the human body, it will drill into the human body through the skin, and after a certain period of development, it will become an adult worm, settling in the blood vessels of the liver and intestines.
History of Japanese bloodsuckers
Both China and Japan have adopted the method of eliminating the intermediate host snail of schistosomiasis to control schistosomiasis infection.
At present, schistosomiasis in China and Japan and other places has been effectively controlled, but schistosomiasis will still cause serious harm in many developing countries and underdeveloped areas.
In addition to schistosomiasis, The Caryodidium warfarinus (Liver Flukes) with freshwater fish as the intermediate host and Weschens (Lung Flukes) with freshwater crustaceans as the intermediate host are also potential health killers.
Therefore, when we eat freshwater fish, shrimp, crabs and other aquatic products, we must cook them thoroughly and avoid eating raw freshwater aquatic products. Flat animals are not very difficult to see around us, and they are found in many ponds and streams.
If you are interested, you can collect the planarians you find, raise them in a small sink, and do a small experiment of "dividing into two" by yourself!