**amphibians**: A group of animals that includes frogs, salamanders and caecilians. Amphibians have backbones and can breathe through their skin. Unlike reptiles, birds and mammals, unborn or unhatched amphibians do not develop in a special protective sac called an amniotic sac.
**aquatic**: An adjective that refers to water.
**caecilian**: A type of amphibian that has no legs. Caecilians have ring-shaped folds of skin called annuli, small eyes covered by skin and sometimes bone, and a pair of tentacles. Most of them live underground in the soil, but some spend their entire lives in water.
**carnivore**: An animal that either exclusively or primarily eats other animals.
**chemical**: A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.
**diffuse**: adj.) To be spread out thinly over a great area; not concise or concentrated. (v) To spread light or to broadly release some substance through a liquid (such as water or air) or through some surface (such as a membrane).
**eel**: A fish with a snake-like body and no scales. Many migrate from freshwater to salt water when it’s time to spawn. Note: Electric eels are not true eels but a type of knifefish.
**egg**: A reproductive cell that contains half of the genetic information necessary to form a complete organism. In humans and in many other animals, ovaries produce eggs. When an egg fuses with a sperm, they combine to produce a new cell, called a zygote. This is the first step in the development of a new organism."
**fat**: A natural oily or greasy substance occurring in plants and in animal bodies, especially when deposited as a layer under the skin or around certain organs. Fat’s primary role is as an energy reserve. Fat also is a vital nutrient, though it can be harmful if consumed in excessive amounts.
**insect**: A type of arthropod that as an adult will have six segmented legs and three body parts: a head, thorax and abdomen. There are hundreds of thousands of insects, which include bees, beetles, flies and moths.
**limb**: (in physiology) An arm or leg.
**liquid**: A material that flows freely but keeps a constant volume, like water or oil.
**mammal**: An animal distinguished by possessing hair or fur, the secretion of milk by females for the feeding of their young, and (typically) the bearing of live young. They also are warm-blooded (or endothermic).
**oxygen**: A gas that makes up about 21 percent of Earth's atmosphere. All animals and many microorganisms need oxygen to fuel their growth (and metabolism).
**prey**: (n.) Animal species eaten by others. (v.) To attack and eat another species.
**reptile**: Vertebrate animals belonging to a group known as Reptilia. Most reptiles have skin that is covered with scales or horny plates. Snakes, turtles, lizards and alligators are all reptiles. Once considered their own separate group, birds now belong to this order. With the exception of modern birds, most reptiles are cold-blooded (or ectothermic).
**species**: A group of similar organisms capable of producing offspring that can survive and reproduce.
**tissue**: Made of cells, it is any of the distinct types of materials that make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues.