Simple Animals biology lesson for kids: 1st, 2nd, 3rd, 4th, 5th, 6th and 7th grades biology lesson.
There are many different types of animals in the world. The ones that we are most familiar with, including humans, are complex animals, but there are entire groups of animals with very simple body structures. Animals’ bodies are divided into tissues, groups of similar cells that perform the same function. The numbers of tissues that an animal has its origin while it is still an embryo, even before it is hatched or born. The animals with very little complexity in their body structures are called simple animals. Let’s go through some of them.
Sponges are so simple that for a long time scientists weren’t even sure whether to call them animals! Their larval (baby) forms are like hairy balls of cells. They use their tiny hairs to swim around until they find a place to settle and grow. Then, when they’re in their adult forms, most are stuck to the sea floor, and can’t move around, filtering tiny food (bacteria, for example) from the water around them. Some of them can move, though, but they move very slowly; inching around at 1 to 4 mm a day. They have a few different kinds of cells, but not many, and they aren’t joined into tissues like more complex animals.
Cnidarians are another kind of simple animal. They aren’t as simple as sponges, since they do have tissues, those tissues don’t come together to make true organs, which means they as complex as much of the animal kingdom. Also, the cnidarian digestive system is kind of like a sack, with a single opening that they use as a mouth and an anus. They also don’t have a central nervous system or brain, but rather a nerve net. Jellyfish and anemones are examples of cnidarians.
And then, there are the roundworms. These are a few steps up from cnidarians in terms of complexity. They each have a straight digestive systems with a separate mouth and anus, and have fluid-filled cavities that helped the ancestors of more complex animals to involve more developed internal organs. They also have more complex nervous systems than cnidarians: their brains boast simple brains.
Finally, there are the molluscs. These include, clams, snails, slugs, and even octopi. These are the most complex of the simple animals. The fluid-filled cavity that all roundworms have is more well developed in molluscs, and contains most of the mollusc’s internal organs. Yes, organs! Everything from the small oyster to the octopus and squid has pretty well developed organs, including hearts, gills, etc. Although oysters don’t have much by way of brains, they do have groups of nerves. On the other end of the spectrum is the octopus, which does have a central brain, but also has a brain in for every single one of its arms; a network of brains!
From sponges to molluscs, simple animals take many forms, and show us a taste of the incredible diversity in the animals that live on our planet.