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An echinoderm (/ ɪˈkaɪnəˌdɜːrm, ˈɛkə -/) [3] is any deuterostomal animal of the phylum Echinodermata (/ ɪˌkaɪnoʊˈdɜːrmətə /), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". [4]
The Eocrinoidea are an extinct class of echinoderms that lived between the Early Cambrian and Late Silurian periods. They are the earliest known group of stalked, arm-bearing echinoderms, and were the most common echinoderms during the Cambrian. Eocrinoids were a paraphyletic group that may have been ancestral to six other classes: Rhombifera ...
Hemichordata (/ ˌhɛmɪkɔːrˈdeɪtə / HEM-ih-kor-DAY-tə) is a phylum which consists of triploblastic, eucoelomate, and bilaterally symmetrical marine deuterostome animals, generally considered the sister group of the echinoderms. They appear in the Lower or Middle Cambrian and include two main classes: Enteropneusta (acorn worms), and ...
Ctenocystoidea is an extinct clade of echinoderms, which lived during the Cambrian and Ordovician periods. Unlike other echinoderms, ctenocystoids had bilateral symmetry, or were only very slightly asymmetrical. They are believed to be one of the earliest-diverging branches of echinoderms, with their bilateral symmetry a trait shared with other ...
Another feature present in both the Hemichordata and Chordata is pharyngotremy — the presence of spiracles or gill slits into the pharynx, which is also found in some primitive fossil echinoderms . [16] [17] A hollow nerve cord is found in all chordates, including tunicates (in the larval stage). Some hemichordates also have a tubular nerve cord.
The annelids (/ ˈænəlɪdz /), also known as the segmented worms, comprise a large phylum called Annelida (/ əˈnɛlɪdə /; from Latin anellus 'little ring'). [3][a] It contains over 22,000 extant species, including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecologies – some in marine environments ...
Water vascular system. The water vascular system is a hydraulic system used by echinoderms, such as sea stars and sea urchins, for locomotion, food and waste transportation, and respiration. [1] The system is composed of canals connecting numerous tube feet. Echinoderms move by alternately contracting muscles that force water into the tube feet ...
Spider book lungs (cross section) Internal anatomy of a female spider, book lungs shown in pink. A book lung is a type of respiration organ used for atmospheric gas exchange that is present in many arachnids, such as scorpions and spiders. Each of these organs is located inside an open ventral abdominal, air-filled cavity (atrium) and connects ...