Planarians are about 2 to 3 cm long and are flattened phylums that live in water. There is a head and a body, yes, it has a brain, but it is more primitive, it is U-shaped, and it has a complete digestive system, but no anus. There are a pair of eyes, but basically invisible, just to feel the light. On each side is a well-developed auricular process, which is its sensory organ, with many sensory cells processing taste and smell. There are also many haptic cells distributed on the surface of the body.
Planarians have a special function, that is, super regenerative ability. Its body is symmetrical on both sides, and its body shape is divided into front and back, left and right, and dorsal abdomen. Amazingly, the cut planarian tissue blocks are still able to re-form this body type after losing these features. For example, after the two ends of a planarian are cut off, the anterior part of the trunk still forms the head and the posterior part forms the tail; If cut from the central axis, the two parts re-establish the central axis, each regenerating a small planarian. Numerous studies have shown that whether it is longitudinal, transverse, or mitered, cut planarian fragments can always readjust their body shape and regenerate the lost part.
The regenerative ability of planarians is amazing, even as small as 1/279 of the worm's body, the planarian tissue can still regenerate a complete individual. How did that number come about? American geneticist Thomas cut the planarians into 279 parts, each less than 1 millimeter. As a result, most of them grew into new ones. This is the highest recorded planarian cut. This super regenerative power is related to the fact that neoblasts are the only somatic cells in adult planarians with the ability to proliferate and self-renew, and the only cells that can be found in the body that can differentiate into nearly 40 types of cells including nerve cells and germline cells. Neoblasts of planarians are mainly distributed within the parenchymal tissues on both sides of their subcutantal bodies, with less distribution near the midline, and no distribution at the front of the eye point and outside the pharynx.
After the planarians are stimulated by cutting, the rods within the planarian epidermis (a specialized cell in the epidermis) immediately release the inclusions, forming a protective mucosa on the trauma surface At the same time, the muscles on the traumatic surface contract violently, so that the wound is narrowed, and after a few hours, the proliferating active cells at the wound can be observed under the wound, neoblasts begin to gather, and continue to divide and migrate, forming a white regenerative tissue, called "blastema", the embryo base regenerates the lost part of the planarian. The formation of regenerative embryos is the result of a combination of cell division and neoblasts cell migration. The regeneration capacity and regeneration rate of different parts of the body are different, from front to back, and the regeneration rate decreases.
Planarians with such a powerful regenerative ability are also afraid of X-rays, and it was found that a certain dose of X-rays can be used to irradiate the worm body to kill all the neoblasts in the planarians. The irradiation dose of 15 Gy is generally considered to be the minimum lethal dose. (At this intensity, people will die quickly.) But in fact, it is easier to kill it, just slap it flat. And if the surrounding environment is not good, it will die. If it is cut open while it is eating, the digestive juices secreted in its body will corrode itself and burn itself to death. Therefore, when doing the cutting experiment on it, it should be starved for a week to ensure that there is no food in the body and then cut it off.
When it comes to coping with hunger, planarians can be said to be very good at it. When food is abundant, planarians grow to their maximum body length, but when hungry for a long time, an adult planarian with a length of 2 cm is obscene to the size of the hatching time (about 1 mm), and once the food is obtained, it resumes growth. What changes in the process is not the size of its cells but the number of cells. It's also a way of regeneration.
So are you curious about how long it lives? It is a bit more powerful than the Tardigrade, somewhat similar to the Lighthouse Jellyfish, and its lifespan can be infinite. Whenever it was about to die, it cut itself into two pieces and then into two small ones. The two small ones grow again, and when they grow up, they cut themselves off, and the cycle repeats. So you have no idea how long it lives. This is actually one of its breeding methods, called fracture reproduction. By breaking yourself, turn 1 into 2 and 2 into 4. In addition to fracture reproduction, it can also reproduce sexually. It is hermaphrodite and can reproduce sexually and lay eggs on its own. Then the eggs can also grow on their own. Generally speaking, as long as there is sufficient food, planarians will develop sexual organs for sexual reproduction, the environment is indeed very harsh, and it is the first choice for hungry and asexual reproduction.
One of the most powerful things about it is that it is cut into many pieces, and then it grows new and then has the memory of the original body. Not only is the body exactly the same, but the memory is also exactly the same. Researchers at Tufts University in Boston did an experiment on the memory of planarians. They trained planarians to ignore the light in the lab, allowing them to find food under concentrated attention. The scientists found that even when planarians were decapitated, they remembered the training. This means that these worms must be trained to overcome the fear of light to find food. Once they learn this skill, they are beheaded. Two weeks later, when most planarians were back up, the team put the worms back in the Petri dish. Using video tracking techniques, the scientists found that trained true planarians found food that was illuminated by bright light faster than ordinary true planarians. Although they failed to find food on their first attempt, they immediately regained their previously learned skills after one training session, ignoring the light. They move much faster than decapitated true planarians that were not trained before losing their heads.
There are really many treasures on its body, and people yearn to live like it, even if it is divided into two, it can maintain the same memory.