Round worms and flat worms biology lesson for kids: 1st, 2nd, 3rd, 4th, 5th, 6th and 7th grades biology lesson.
There are many kinds of animals that we call “worms”. They are invertebrates, meaning they don’t have backbones or even spinal cords. Two of these, roundworms and flatworms, have some things in common, but they are also very different. Both kinds of worms include parasites that live inside the bodies of other animals, and free-living species. Depending on the species, they can be so small that you can’t see them with the naked eye, or grow to be feet long.
Flatworms are flat because they don’t have any fluid-filled cavities at all. What they do have is a single opening that they use to eat and then get rid of the stuff they eat after they digest it. They don’t have lungs or blood either. All the nutrients and gases they need to survive move around their body by diffusion. Flatworms have a group of nerves in their heads, kind of like a brain, but not as developed as vertebrates’. They have muscles, too, and this allows them to move over solid surfaces.
Roundworms are round because they do have fluid-filled cavities. Like flatworms, though, roundworms don’t have blood or lungs, but they do have a straight digestive system with their mouths separate from their anuses. This kind of digestive system is the type that most complex animals on Earth have. Unlike flatworms, roundworms have tough outer coating on their skin to protect their bodies and give their muscles something to hold on to so they can move around, even over surfaces that aren’t very solid.
Both flatworms and roundworms produce sexually, but they do it in different ways. Most flatworms make are hermaphrodites, which means they have both male and female body parts. That means they can fertilize each other’s eggs, but can even fertilize their own eggs! Roundworms aren’t hermaphrodites, though; a single roundworm is either male or female. The male fertilizes the eggs inside the female. Both flatworms and roundworms lay eggs that hatch into larvae (singular: larva), that grow up over time into adults.
Some flatworms eat very small invertebrates and the dead bodies of big animals. Some roundworms eat bacteria and other very, very small organisms and rotting things. These flatworms and roundworms tend to live in water or wet soil. Other flatworms and roundworms live inside the bodies of bigger animals; they might live in the digestive system, on the liver, or other places, and get their food from the bodies of the animals they live in.
Though flatworms and roundworms are related, roundworms are more complex than flatworms. Still, they have a lot of similarities, and can be dangerous parasites to big animals, even though they’re so small.