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There are 38 animal phyla, with most appearing in the fossil record during or after the Cambrian explosion. The most diverse groups are insects and mites. The majority of animals belong to nine phyla, with the rarest having only one or two species. No known phyla have gone extinct completely.
According to the most widely used classification scheme, there are 38 animal phyla. Some systematics argue that there are a different number of phyla, although always between 35 and 40. Only three phyla have been discovered in the last century, the last one in 1993. No known phyla has ever gone completely extinct: at least some of their representatives always survive the great extinctions.
Most modern animal phyla appear in the fossil record during or shortly after the Cambrian explosion, an episode of adaptive radiation 630-620 million years ago. There remains disagreement about how many of these phyla existed before the Cambrian explosion, during the Ediacaran period, although most scientists agree that at least eight have already been 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 molluscs (112,000 species), sponges (5,000 species), cnidarians (jellyfish and companions: 11,000), flatworms (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 lancets: 100,000+).
Each phylum is monophyletic, meaning that it includes all descendants of a single species and no species that is not descended from that so-called stem taxa. This can be confirmed through genetic analysis. Among the phyla, the most diverse groups are insects (over a million species, probably much more) and mites (an arachnid with 45,000 described species, but can go up to 1 million), both arthropods.
All remaining phyla have fewer than about 2,000 members, the rarest phyla having only three (Cycliophora: odd sacs represented by Symbion pandora), two (Xenoturbellida: strange flatworm), or one species (Micrognathozoa: tiny jawed animal and Placozoa, an animal resembling 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 most resistant of all animals), diciemids (Rhombozoa, lozenge-shaped squid parasites), gastrotrichis ( Gastrotricha, tiny tube-shaped animals), acoels (Acoelomorpha, very small worms without guts, basal to all bilateria), and mossy animals (Bryozoans, superficially similar to coral).
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