Although "phylum" is often used as if it were a clearly defined term, no satisfactory definition of it exists. In fact, "phylum" may be a misnomer indicative of ignorance. Consequently the number of phyla varies from one author to the next.
The relationships among phyla are becoming increasingly well known, and larger clades can be found to contain many of the phyla.
Informally, phyla can be thought of as grouping organisms based on general body plan, as well as 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, along porifera cnidaria platyhelminthes nematoda és annelida all other vertebrate species, as well as some invertebrates such as the lamprey.