In this article we are going to go from describing fish to sea worms. In this case, we will talk about the planarians. They are a group of flatworms (hence their name) whose class was previously the Turbellaria. Therefore, they are also called mobsters. About 4500 species are known of these worms, hence their importance. The vast majority of them are aquatic and live in benthic ecosystems. There are some other species adapted to humid environments.
If you want to know these sea flatworms in depth, in this post we will delve into their biology, classification and grapevine modeto. Would you like to meet them?
Classification
The Turbellaria class it was considered a group of flatworms that took in all those that are not strictly parasites. However, with the passage of time and the development of the taxonomy, this class has disappeared. Therefore, planarians are considered paraphyletic groups that include flatworms that are not parasites and the acellomorphs. These changes are due to a more in-depth study of the evolutionary line of these animals.
Key features
They are invertebrates of very small size and quite variable length. We can find specimens ranging from millimeters to 600mm in length. Larger planarians are shaped like a leaf or ribbon.
Most of these species are aquatic. Benthic species are those that inhabit the seabed. Therefore, these worms are considered benthic organisms. Regarding its morphology, we can find a large number of cilia throughout its body.
The cilia are used to create small movements that generate swirling microscopes around them.
Similar to flatworms
Planarians resemble flatworms morphologically by having bilateral symmetry. This means that they have a longitudinal axis that separates two symmetrical body parts. They are triblastic because they have a third layer of non-embryonic cells. The same goes for human beings, we are triblastics.
Unlike other organisms with bilateral symmetry, planarians and flatworms do not have any internal cavity. This is due to its flatness. They have no coelom, which is why they are classified as cellophane.
Another characteristic that differentiates flatworms from other species is the absence of respiratory and circulatory systems. By not having these devices, their ability to exchange oxygen and carbon dioxide with the environment is reduced and limited. It is the reason why its size is smaller. If it had a larger body, it would have increased gas exchange needs with the outside world and could not survive. The larger ones are flat so as not to alter this exchange for a larger surface area.
So if these gadgets, how do they exchange oxygen and CO2? They do it through their body surface. In addition, the digestive tract has ramifications so that nutrients are able to more easily reach all parts of the body. This gas exchange through the skin makes the planarians very susceptible to dehydration. For this reason they have to live in aquatic and humid environments where it is almost impossible to get dehydrated.
The nervous system is concentrated in the head where several nodes appear. From these ganglia branches of nerves leave that are dispersed throughout the body. In the case in which they are damaged, they can regenerate their body if they lose any part. They are even capable of regenerating the head.
Special characteristics of planarians
As you can see, these animals are really special and unique. Most of them have a free life, unlike the flatworms that are parasites. Living on the seabed they have to feed on other smaller animals or decomposing organic matter.
There are some planarians that frequent the coastlines and reach their greatest diversity on coral reefs. They can be seen forming some large communities. Other species have colonized freshwater habitats and some are adapted to humid environments on land. Those who venture to live on land choose dark and humid places. This can be covered with litter and almost always of nocturnal habits, where the humidity is better kept.
They have no cuticle and the surface of the body is a single layer of cells with cilia. In some species that are larger, they do not have cilia. Under the skin it has a very small layer of muscles and some glands that are connected to the surface through pores. They are continually secreting mucus and other substances that help them stay moist at all times.
To move they have several options. The smaller aquatic ones use the cilia on their skin to propel themselves out of the water and move around. On the other hand, the larger ones that do not have cilia need to perform muscular movements to crawl or move through the water. Those who live on earth are capable of throwing threads of snot to be able to climb high areas such as rocks and branches.
Some planarians have structures similar to the spicules of corals and sponges (calcareous or siliceous structures) and give the body of the planaria an annular appearance.
Reproduction
Planarians are capable of reproducing both sexually and asexually. Some of them reproduce by fragmenting their bodies. They are capable of cloning themselves by transverse division. They can also do it asexually by budding.
However, the best known and most common form of reproduction is sexual. To do this, all planarians must fertilize eggs inside by copulating with another individual. They are all hermaphrodites so they can self-fertilize.
Planarians that do not have a coelom do not have gonads. But the rest of them do have one or more pairs of ovaries and testicles. Seminiferous tubes start from the testicles that flow out into muscular penises.
In most species the eggs hatch to give specimens very similar to adults, but with a smaller size. In other species the eggs give larvae that mature in the aquatic environment.
As you can see, these animals are quite special and curious.