Science · Life Science

Classifying Animals — Reading Comprehension Lesson

Reading comprehension passage and printable worksheet about how scientists classify animals into groups — vertebrates, invertebrates, and the major animal classes — for grades 3–6.

Reading Passage

Classifying Animals

With millions of different animal species on Earth, scientists need a system to organize them into groups. This system of organizing living things is called classification. By grouping animals with similar characteristics together, scientists can more easily study them, identify new species, and understand the relationships between different animals.

The largest division in animal classification separates animals into two groups: vertebrates and invertebrates. Vertebrates are animals with a backbone, or vertebral column. This bony structure protects the spinal cord and provides support for the body. Only about 5 percent of all animal species are vertebrates, but this group includes many familiar animals.

Vertebrates are further divided into five major classes. Fish are cold-blooded vertebrates that live in water and breathe through gills. Amphibians, such as frogs and salamanders, begin life in water and can live on land as adults. Reptiles, including snakes, lizards, and turtles, have dry scaly skin and breathe air. Birds are warm-blooded vertebrates covered in feathers that lay hard-shelled eggs. Mammals are warm-blooded and nurse their young with milk.

Invertebrates are animals without a backbone. This group includes far more species than vertebrates. Insects are the largest group of invertebrates — there are more species of insects than all other animals combined. Other invertebrates include arachnids (spiders and scorpions), crustaceans (crabs and lobsters), mollusks (snails and clams), worms, and echinoderms (sea stars and sea urchins).

Scientists classify animals based on shared characteristics such as body structure, how they reproduce, whether they are warm-blooded or cold-blooded, and how they breathe. As scientists discover new information, the classification system is updated. Modern classification also uses DNA analysis to better understand how closely related different animal groups are to one another.

Printable Worksheet

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