Animals and Their Phylogeny
All of us have animals in our lives that we love and take pleasure in. We observe, love, care for and eat their products. We share our homes with pets and companion animals. We enjoy their companionship, friendship, cuddling and company. In essence, all of us become animal lovers when we have friends, family members, or pet animals in our lives.
Multicellular organisms are defined as single-celled organisms in the Kingdom Animalia. With few exceptions, all animals breath oxygen, produce protein by means of photosynthesis, eat organic matter, can multiply, move, can reproduce, and live in an environment that is composed of a living host (an animal) with which they interact. The cells in animals are made up of complex molecules which contain amino acids, fatty acids, sugars and other building blocks of life. The cells in animals are enclosed by extracellular matrix which is a network of fibers woven into a sheath. The cells need an outer covering to protect them from external injury and infections.
A variety of multicellular organisms make up the animal kingdom. They include reptiles, amphibians, birds, mammals, fish, insects, and even crustaceans. Each animal has a set of physiologic, structural and behavioral characteristics that distinguish it from all other animals. A well-developed immune system produces cells that help in the development of the various organs and systems of the body while a well-developed nervous system enables animals to communicate with each other and with members of the animal kingdom.
One major class of animals is the Cenozoic (co-existing) taxa which includes the dinosaurs, their descendants (theropods), and numerous present day birds and mammals. The dinosaurs are classified as Class Reptile, meaning they originated in the Age of the dinosaurs. The most common dinosaurs are Ornithischia (or Amphibolopoda) and the Sauropodids. The most recent speculations on the ancestry of modern birds are currently based on comparative studies of the DNA of modern birds and the fossils of Archaeopteryx.
The largest animal is the earth, and even in this earth there are a wide variety of organisms. In fact, it appears that there is a great variety of animal species out there in space. If we were to list all the existing organisms out there, it would be huge. But there are many interesting exceptions to the rule, including some very large unicellular organisms such as algae and plankton. Earthworms are the only known unicellular organisms with a diameter of more than one micrometer.
The diversity of life on earth is astonishingly diverse, and the animals kingdom contains many diverse kinds of unicellular organisms and more than ten thousand different types of organisms which are both photosynthetic and aerobic. Most living things are either photosynthetic or aerobic, but some invertebrates are considered to be strictly biocompatible, meaning that they can live both as photosynthesizers and as respiration-based organisms. Some of the animals we consider to be both photosynthesizers and respiration-based include certain sharks, cephalopods, eels, sponges, snails, mantis shrimp and some coral reefs.