The simplest of all of these animals are the sponges. There are different kinds of invertebrates, including sponges, cnidarians, mollusks, arthropods, insects, worms, and echinoderms. Invertebrates depend on other living things for food. Most invertebrates reproduce sexually, meaning male and female invertebrates produce sex cells. Bilateral symmetry, or two-sided symmetry, means that if you were to draw a line down the middle of the animal’s body, the two sides, or halves, would look identical. But some, such as clams and oysters, spend their adult lives anchored, or attached to something solid, like rocks or even the bodies of other animals. Most invertebrates are mobile, meaning they are able to move freely. Other than sponges, cells within invertebrates are organized into tissues, or cells that share a common function. And unlike plant cells, animal cells have no cell walls. Invertebrates share common traits, such as being multicellular, or made of more than one cell. Among animals are those with backbones, or vertebrates, and those without backbones, or invertebrates.
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