Animal Groups

Updated February 21, 2017 | Factmonster Staff

Vertebrates and Invertebrates

Warm-blooded animals regulate their own body temperatures; their bodies use energy to maintain a constant temperature. Cold-blooded animals depend on their surroundings to establish their body temperatures.

Almost all animals fall into one of two groups. Adult vertebrates have a spinal column, or backbone, running the length of the body; invertebrates do not. Vertebrates are often larger and have more complex bodies than invertebrates. However, there are many more invertebrates than vertebrates.


  • Fish breathe through gills, and live in water; most are cold-blooded and lay eggs (although sharks give birth to live young).
  • Amphibians are cold-blooded and live both on land (breathing with lungs) and in water (breathing through gills) at different times. Three types of amphibians are frogs and toads, salamanders, and caecilians. Caecilians are primitive amphibians that resemble earthworms. They are found in the tropics.
  • Reptiles are cold-blooded and breathe with lungs. They have scales, and most lay eggs. Reptiles include snakes, turtles and tortoises, crocodiles and alligators, and lizards. Dinosaurs were reptiles, although some scientists believe that some were warm blooded.
  • Birds are warm-blooded animals with feathers and wings. They lay eggs, and most can fly (although many, including penguins and ostriches, cannot).
  • Mammals are warm-blooded, and are nourished by their mothers' milk; most are born live (however, the platypus lays eggs). Most mammals also have body hair.


  • Sponges are the most primitive of animal groups. They live in water (usually saltwater), are sessile (do not move from place to place), and filter tiny organisms out of the water for food.
  • Coelenterates are also very primitive. Their mouths, which take in food and get rid of waste, are surrounded by stinging tentacles. Some coelenterates are jellyfish, corals, and sea anemones.
  • Echinoderms include starfish, sea urchins, and sea cucumbers. They live in seawater and have external skeletons.
  • Worms come in many varieties and live in all sorts of habitats — from the bottom of the ocean to the inside of other animals. They include flatworms (flukes), roundworms (hookworms), segmented worms (earthworms), and rotifers (philodina).
  • Mollusks are soft-bodied animals, which often live in hard shells. They include snails, slugs, octopus, squid, mussels, oysters, clams, scallops, chitons, and cuttlefish. Mollusks are the second-largest group of invertebrates, with 50,000 living species.
  • Arthropods are the largest and most diverse of all animal groups. They have segmented bodies supported by a hard external skeleton (or exoskeleton). Arthropods include insects, arachnids (spiders and their relatives), centipedes, millipedes, and crustaceans like crabs, lobsters, and shrimp.
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