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
Pacific spiny dogfish Pacific spiny dogfish The Pacific spiny dogfish ("Squalus suckleyi") is a common species of the Squalidae (dogfish) family of sharks and are among the most abundant species of sharks in the world. This species is closely related to "Squalus acanthias" and for many years they were treated as a single species. Recent research, using meristic, morphological and molecular data led to the resurrection of Pacific Spiny Dogfish as a separate species. The American Fisheries Society recommends the common name "Pacific Spiny Dogfish" for "Squalus suckleyi" over alternatives such as "Spotted Spiny Dogfish" and "North Pacific Spiny Dogfish" and "Spiny Dogfish"