Although a phylum is often spoken of as if it were a hard and fast entity, no satisfactory definition of a phylum exists. Consequently the number of phyla varies from author to author. The relationship of phyla is increasingly well known, and larger clades can be erected to contain many of the phyla.
Informally, phyla can be thought of as grouping animals based on general body plan, developmental or internal organizations. For example, though seemingly divergent, spiders and crabs both belong to Arthropoda, whereas earthworms and tapeworms, similar in shape, are from Annelida and Platyhelminthes, respectively. Although the International Code of Botanical Nomenclature allows the use of the term "phylum" in reference to plants, the term "Division" is almost always used by botanists.
The best known animal phyla are the Mollusca, Porifera, Cnidaria, Platyhelminthes, Nematoda, Annelida, Arthropoda, Echinodermata, and Chordata, the phylum to which humans belong. Although there are approximately 35 phyla, these nine include over 96% of animal species. Many phyla are exclusively marine, and only one phylum, the Onychophora (velvet worms) is entirely absent from the world's oceans–although ancestral oncyophorans were marine.