MammalsOtters, whales, seals and humans are examples of mammals.
Mammals have five defining characteristics.
... are warm-blooded – their body temperature remains constant.
... have hair or fur.
... give live birth.
... breathe air through lungs.
... nurse their young.
InvertebratesCrabs, snails, anemones, jellies and sea stars are just a few examples of invertebrates.
Invertebrates are animals that do not have a spine or vertebral column. They do not have skeletons or bone. Overall, they are incredibly diverse: approximately 97% of all animal species are invertebrates. For this reason, they must be classified into more than 30 phyla - from sponges to arthropods.
Cartilaginous FishSharks, rays, skates and chimaera are cartilaginous fish.
The skeletons of these animals (Class Chondrichthyes) are composed of cartilage instead of bone. Their bodies are covered with dermal denticles, providing a smooth appearance. Depending on species, they will have 5-7 gills for breathing. Many must continuously swim to breathe.
ReptilesTurtles, alligators and snakes are examples of reptiles.
Members of the class Reptilia are covered in either scales or hard scutes. They are cold-blooded and, thus, depend on their environment and own activity levels to regulate their body temperatures. This allows the animal to live off much less energy and food than a comparably-sized warm-blooded animal.
Bony FishGrouper, sea horses and eels are a few examples of the more than 30,000 species of bony fish.
The skeletons of these animals (Class Osteichthyes) are composed of bone. All bony fish have gills, which are essential for breathing in most species. They also possess a swim bladder, which helps manage neutral balance so the fish neither sinks nor floats. These fish need not continuously swim to breathe.