Under the most frequently used classification scheme, there are 38 animal phyla. Certain systematicists claim there are a different number of phyla, although always between 35 and 40. Only three phyla have been discovered in the last century, the most recent in 1993. No known phyla have ever gone completely extinct: at least some of their representatives always survive major extinctions.
Most modern animal phyla show up in the fossil record during or shortly after the Cambrian explosion, an episode of adaptive radiation 630-620 million years ago. There remains disagreement as to how many of these phyla existed prior to the Cambrian explosion, during the Ediacaran period, though most scientists agree that at least eight were already established.
The vast majority of animals belong to just nine phyla: Mollusca, Porifera, Cnidaria, Platyhelminthes, Nematoda, Annelida, Arthropoda, Echinodermata, and Chordata. In informal terms, these are the molluscs (112,000 species), sponges (5,000 species), cnidarians (jellyfish and friends: 11,000), flat worms (25,000), nematodes (80,000 - 1 million), annelids (segmented worms: 15,300), arthropods (1,134,000+, may be as many as 8 million), echinoderms (starfish and friends: 7,000), and chordates (vertebrates and lancelets: 100,000+).
Every phylum is monophyletic, meaning it includes all the descendants of a single species, and no species that are not descendants of that so-called stem taxa. This can be confirmed through genetic analysis. Among the phyla, the most diverse groups are the insects (over a million species, probably much more), and the mites (an arachnid with 45,000 described species, but may be as many as 1 million), both arthropods.
All the remaining phyla have fewer than about 2,000 members, the rarest phyla with just three (Cycliophora: odd sacs represented by Symbion pandora), two (Xenoturbellida: strange flatworm) or one species (Micrognathozoa: tiny jawed animal, and Placozoa, an animal that resembles a multicellular amoeba). Most are simple marine organisms, often referred to as worms or nanoplankton. Among the most interesting are the rotifers (Rotifera, small ciliated organisms), tardigrades (Tardigrada, water bears, the hardiest of all animals), dicyemids (Rhombozoa, lozenge-shaped squid parasites), gastrotrichs (Gastrotricha, tiny tube-like animals), acoels (Acoelomorpha, very small worms lacking a gut, basal to all bilaterians), and moss animals (Bryozoans, superficially similar to coral).
