| MadSci Network: General Biology |
Whilst this seems like an easy question, the correct answer would be "we don't really know". The eight largest phyla in terms of the numbers of described species may be radically different to the actual largest phyla, because many of the phyla, including Nematoda, Kinorhyncha and Rotifera, consist mostly of tiny animals, and have been subject to little research or exploration. For example, the comb jellies (phylum Ctenophora) and arrow worms (Chaetognatha) inhabit the little explored open ocean, often in deep water. Many species of all the phyla probably remaim undiscovered.
The arthropods (spiders and their relatives, insects and their relatives and crustaceans) are by far the most diverse group of living things, and a large proportion of all described species (including plants and bacteria) are insects, which account for approx. 1,000,000 of the arthropod total. Within the insects, thelargest group are the beetles (Coleoptera) with approx. 300,000 species, almost a third of all known animals!
Unsurprisingly, the top 8 are well known and familiar groups, but a number of less well known types of animal are suprisingly diverse, including the Entoprocta, commonly called bryozoans (see http://www.civgeo.rmit.edu.au/ bryozoa/index.html), the rotifers, a group of tiny animals that are most common in freshwater, but can also be found in damp soil or moss, and in marine haitats - see this movie, a beautifully illustrated site, and the Urochordata or sea-squirts (see pictures). Online information about most of the invertebrate phyla is available at the University of California Museum of Paleontology (http://www.ucmp.berkeley.edu/ help/taxaform.html)
REFERENCES
Brusca, R. C. and G. J. Brusca (1990) Invertebrates. Sinauer Associates, Sunderland, Massachusetts.
Nielsen, C. (1994) Animal Evolution. Oxford University Press.
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