|Most animals fall into one of two groups -- vertebrates and invertebrates.
Vertebrates have a spinal column, or backbone, running the length of the body. Invertebrates do not have a backbone. Vertebrates are often larger and have more complex bodies than invertebrates. However, there are many more invertebrates than vertebrates.
VertebratesVertebrates are grouped into cold-blooded and warm-blooded animals. 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.
Some cold-blooded animal groups are fish, amphibians, and reptiles.
Fish breathe through gills and live in water. Most fish lay eggs. Sharks, though are considered fish, give birth to live young.
Amphibians live both on land and in water. When they are on land, they breath with lungs and when they are in water, they breath through gills. 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 breathe with lungs. They have scales, and most lay eggs. Reptiles include snakes, turtles and tortoises, crocodiles and alligators, and lizards.
Birds and mammals are the only two warm-blooded animal groups.
Birds have feathers and wings. They lay eggs, and most can fly. There are some birds like penguins and ostriches that cannot fly.
Mammals are fed by their mothers' milk. Most mammals have body hair. Most mammals give birth to live young, except the platypus which lays eggs.
InvertebratesArthropods 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.
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.
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).
Echinoderms include starfish, sea urchins, and sea cucumbers. They live in seawater and have external skeletons.
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.
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.
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