Animal

The Animalia are the animals. The word metazoa is also used for this group. Animals include sponges, cnidaria and all animals with epithelia (sheets of cells covering the outside of the organism, the gut system, and from which other organisms are derived). Animals are distinguished as organisms which may be multicellular, use extracellular collagen as a skeletal material, have a sexual developmental cycle that involves motile sperm, relatively immotile eggs, and development that involves the formation of a blastula (or are derived from organisms with these features). With our current understanding, this life form has diversified much more than any other group. Animals were often divided into the vertebrates (including fish, amphibia, reptiles and birds, and mammals), and the invertebrates. Most invertebrates and all vertebrates are organisms that are bilaterally symmetrical - with many organs such as appendages motion, sensory organs, nerves and muscle - similar on both sides of the body. Most animals have a head - a region with a concentration of sensory organisms and nervous system (brain). The animals evolved from a group of unicellular organisms - the choanoflagellates or collar flagellates. The first multicellular organisms were the sponges. Later organisms like jellyfish appeared, and these are represented in the fossil record. While sponges are filter feeders, the cnidaria (includes jellyfish) eat larger morsels of food. This style of feeding, coupled with the ability to actively move, set off the explosion of animal life. Worm-like organisms with appendages, heads, centralized nervous systems followed next and much of the animal diversity was established in the Cambrian geological period. Animals are the most successful (in terms of number of species) of evolutionary lineages that moved from unicellularity to multicelluarity - current estimates being that there are about 1,500,000 species, but this excludes fossil species and the myriads of so far undescribed animals.

The animalia are the animals. The word metazoa is also used for this group. Animals include sponges, cnidaria and all animals with epithelia (sheets of cells covering the outside of the organism, the gut system, and from which other organisms are derived). Animals are distinguished as organisms which may be multicellular, use extracellular collagen as a skeletal material, have a sexual developmental cycle that involves motile sperm, relatively immotile eggs, and development that involves the formation of a blastula (or are derived from organisms with these features). With our current understanding, this life form has diversified much more than any other group. Animals are often divided into the vertebrates (including fish, amphibia, reptiles and birds, and mammals), and the invertebrates. Most invertebrates and all vertebrates are organisms that are bilaterally symmetrical - with many organs such as appendages for senses, motion, nerves and muscle - similar on both sides of the body. Most animals have a head - a region with a concentration of sensory organisms and nervous system (brain). The animals evolved from a group of unicellular organisms - the choanoflagellates or collar flagellates. the first multicellular organisms were the sponges. Later organisms like jellyfish appeared, and these are represented in the fossil record. While sponges are filter feeders, the cnidaria (includes jellyfish) eat larger morsels of food. This ability coupled with the ability to actively move, set off the explosion of animal life. Worm-like organisms with appendages, heads, centralized nervous systems followed and much of the animal diversity was established in the Cambrian geological period. Animals are the most successful (in terms of number of species) of evolutionary lineages that moved from unicellularity to multicelluarity - current estimates being that there are about 1,500,000 species - but this excludes fossil species and the myriads of so far undescribed animals.

The Kingdom Animalia (=Metazoa) is one of a handful of lineages rooted far back in the branching "tree" that represents the history of life on Earth. This lineage that is composed of those organisms we know as "animals" represents one of the three major origins of multicellularity (the other two large and diverse groups of multicellular organisms are the fungi and the green plants).

It is difficult to list characteristics that apply to all animals, since various branches of the animal tree have undergone a range of significant modifications. However, most animals obtain energy from other organisms. They generally feed on them as predators (killing and eating a prey item); parasites, including herbivores feeding on plants (feeding on their "prey" without killing it, at least not immediately); or detritivores (ingesting tiny bits of decomposing organic material such as fallen leaves). In contrast to animals, most plants make their own food, through the extraordinary process of photosynthesis, using energy captured from the sun; most fungi break down decaying organic material (without ingesting it) into its chemical constituents and absorb released nutrients. Animal cells lack a rigid cell wall (some form of which is typical of plants and most fungi) and their cell biology and physiology differ in a variety of ways from other organisms.

The diversity of animals is impressive. Zhang (2011; 2013) recently coordinated an effort to outline a classification scheme for all known animals and to estimate species richness (i.e., number of species) in different parts of the animal tree. Results from this publication are enlightening. More than 1.5 million animal species have been described (and many more continue to be discovered and formally described each year). The phylum Arthropoda (insects, spiders, crustaceans, etc.) accounts for around 80% of this total; around 2/3 of the total is accounted for by the insects alone. Well over a third of all known insects (and around a quarter of all known animal species!) are beetles: nearly 400,000 different species of beetles have already been described. Among the known species of insects are also nearly 120,000 Hymenoptera (ants, bees, and wasps) and nearly 160,000 Lepidoptera (moths and butterflies). More than 40,000 spider species and over 50,000 species of Acari (mites and ticks) have been described. Nearly 70,000 species of Crustacea (crabs, shrimps, barnacles, pillbugs, and many groups completely unfamiliar to those who don't study them!) are known. The Myriapoda (millipedes, centipedes, and relatives) includes around 12,000 described species. The Mollusca (clams, snails, octopuses, and relatives) is among the largest of the animal phyla, with nearly 120,000 known species. There are over 17,000 known species of Annelida (segmented worms, including earthworms, "polychaete" worms, leeches, and their relatives), Even some groups most people have never even heard of are quite diverse. For example, there are over 1000 described Acanthocephala, over 3000 Pseudoscorpiones, and more than 1500 Rotifera species (and rotifer specialists believe this last number may represent just a tenth or less of the true global rotifer species diversity). By comparison with these invertebrate clades, the generally more familiar vertebrate groups are less diverse, but many people may still be surprised to learn, for example, that there are around 32,000 species of described "fishes" and nearly 6,000 described mammal species. The numbers presented here are merely an appetizer. Anyone seriously interested in biodiversity will thoroughly enjoy studying the original volume by Zhang and colleagues which is freely available online.

The Shape of Life. A revolutionary eight-part television series that reveals the dramatic rise of the animal kingdom through the breakthroughs of scientific discovery. Public Broadcasting Service (PBS). The History of Animal Evolution. The University of Waikato. Dawn of Animal Life. Miller Museum of Geology, Queens University, Kingston, Ontario. The Radiation of the First Animals. Jere Lipps, Access Excellence.

Cool Invertebrates. Enjoy the wonderful world of invertebrates, which comprise 95 percent of all living animal species. Rick Brusca.

Explore the diversity of with One Species at a Time, EOL's podcast series.

Each short audio story focuses on species and the scientists who study them, include multimedia extras and relevant educational resources.

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One Species at a Time Podcast Series

Students around the United States and other countries will collect samples from local ponds to answer the question: Are the organisms found in pond water the same all over the world?

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