Phylum (plural: phyla) is a term in biological taxonomy, the classification and grouping of species. A kingdom is made up of phyla, and a phylum in turn is made up of classes. The plants or animals within a phylum exhibit enormous diversity, yet have fundamental similarities in body plan and physiology.
For example, the phylum Chordata includes all of the vertebrates: mammals, birds, fish, frogs, and many others—most of what we commonly think of as animals. Despite the enormous variety, almost all chordates have backbones, a nervous system, a heart, and bilateral symmetry.
The phylum Arthropoda includes insects, beetles, centipedes, spiders, lobsters, and many others. They are fundamentally similar to each other and fundamentally different from chordates. For example, arthropods don't have bones inside them, but instead have a stiff shell-like exoskeleton on their outside.
As with all classification systems, the list of phyla has varied over time, partly as a result of new knowledge and partly as the result of changes in biologists' opinions. Zoologists recognize some thirty or so animal phyla.
Some other large and familiar phyla are:
- the Porifera: sponges
- the Cnidaria: jellyfish, hydroids, etc.;
- the Mollusca: snails, clams, octopus, squid, etc.;
- the Echinodermata: starfish, sea urchins, sea cucumbers, sand dollars, crinoids, etc.
- the Platyhelminthes: flatworms, tapeworms, etc.;
- the Annelida: earthworms and other segmented worms, leeches, etc.;
- the Nematoda: nematodes and other round worms, etc.;