Invertebrates are animals that lack a vertebral column, or backbone. Many invertebrates have a fluid-filled skeleton, like the jelly fish or worm. Others have a hard outer shell, like insects and crustaceans. More than 90 percent of living animals are invertebrates. Worldwide in distribution, they range in size from minute protozoans to giant squids.

Sharks and their biological cousins, the rays, are among the highest-profile animals of the ocean, thanks to the popularity of books and films that have cast them in leading roles. Despite such worldwide media glare, sharks and rays are among the least-understood creatures that inhabit the world's oceans.

Fish are cold-blooded aquatic vertebrates that include the bony fishes and usually the cartilaginous and jawless fishes and that have typically an elongated somewhat spindle-shaped body terminating in a broad caudal fin, limbs in the form of fins when present at all, and a 2-chambered heart by which blood is sent through thoracic gills to be oxygenated.

Amphibians, which include salamanders, newts, toads and frogs, are vertebrate animals that spend at least part of their life cycle in water. They are cold-blooded, which means that they do not produce their own body heat like birds, people or other mammals. They remain the same temperature as their surroundings and seek out cooler or warmer spots to avoid temperatures too high or too low for their survival. Like amphibians, reptiles are vertebrates . Unlike amphibians, reptiles do not have to live part of their life in water.

Birds are two-legged and are warm-blooded. They are considered vertebrates and they lay eggs. Birds are characterized primarily by feathers and have forelimbs modified as wings. They also have hollow bones.

Mammals are warm-blooded vertebrates. Female mammals give birth to babies, not eggs. Humans, dogs, Hippopotamuses and dolphins are all mammals, but birds, fish and crocodiles are not.