Description of Animalia
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 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.