Animal mesoderm forms by enterocoelic pouching, through invagination of the endoderm. The main deuterostome phyla are the Echinodermata and the Chordata. Echinoderms are exclusively marine and include starfish, sea urchins, and sea cucumbers. The chordates are dominated by the vertebrates (animals with backbones), which consist of fishes, amphibians, reptiles, birds, and mammals. The deuterostomes also include the Hemichordata (acorn worms). The Ecdysozoa are protostomes, named after their shared trait of ecdysis, growth by moulting. They include the largest animal phylum, the Arthropoda, which contains insects, spiders, crabs, and their kin. All of these have a body divided into repeating segments, typically
Venom Other insects, such as true bugs and many ants, also produce venom. At least one ant species ("Polyrhachis dives") has been shown to use venom topically for the sterilisation of pathogens. There are venomous invertebrates in several phyla, including jellyfish such as the dangerous box jellyfish and sea anemones among the Cnidaria, sea urchins among the Echinodermata, and cone snails and cephalopods including octopuses among the Molluscs. Venom is found in some 200 cartilaginous fishes, including stingrays, sharks, and chimaeras; the catfishes (about 1000 venomous species); and 11 clades of spiny-rayed fishes (Acanthomorpha), containing the scorpionfishes (over 300 species), stonefishes