With over 2 million species, Kingdom Animalia is the largest of the kingdoms in terms of its species diversity. But when you think of an "animal", what image comes to mind? While mammals, birds, reptiles, amphibians, and fish are the most familiar to us , over half of all the animal species belong to a group of animals known as arthropods. Arthropods include animals such as centipedes, crabs, insects, and spiders. And with over a million species of arthropods, this means that the majority of animals species come from a group of critters that give most folks the creeps!
So, what exactly is an "animal"? With so much diversity among different animal species, it's difficult to imagine what they all might have in common. First, animals are "multicellular" (composed of many cells). In most animals, these cells are organized into tissues that make up different organs and organ systems. Second, all animals are heterotrophs (= "other feeder"), meaning that they must get their food by eating other organisms, such as plants, fungi, and other animals. (Plants are referred to as autotrophs or "self feeders") because they produce their own food by the process of photosynthesis.) In addition, all animals require oxygen for their metabolism , can sense and respond to their environment, and have the capacity to reproduce sexually (though many reproduce asexually as well). During their development from a fertilized egg to adult, all animals pass through a series of embryonic stages as part of their normal life cycle. Though frogs and humans don't look very much alike, we share many features in common during our embryonic phase.