Given that new species are continually being discovered and existing ones are being reclassified, it is challenging to pinpoint the precise number of animal groupings that exist on Earth. However, there are typically thought to be 35–40 major animal phyla, or groups, in existence. This includes the mollusks (snails, clams, squid), arthropods (insects, spiders, crustaceans), and chordates (vertebrates and certain invertebrates) (earthworms, leeches). There are several species and subgroups within each phylum.