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The phylum contains about 7,600 living species, making it the second-largest group of deuterostomes after the chordates, as well as the largest marine-only phylum. The first definitive echinoderms appeared near the start of the Cambrian. Echinoderms are important both ecologically and geologically.
The class Asteroidea belongs to the phylum Echinodermata. As well as the starfish, the echinoderms include sea urchins, sand dollars, brittle and basket stars, sea cucumbers and crinoids. The larvae of echinoderms have bilateral symmetry, but during metamorphosis this is replaced with radial symmetry, typically pentameric. [12]
[21] [22] That implies that the protostome and deuterostome lineages split long before Kimberella appeared, and hence well before the start of the Cambrian 20] i.e. during the earlier part of the Ediacaran Period (circa 635-539 Mya, around the end of global Marinoan glaciation in the late Neoproterozoic). It has been proposed that the ancestral ...
The water vascular system develops from their middle coelom or hydrocoel. Echinoderms use this system for many things including movement by pushing water in and out of their podia or "tube feet". Echinoderms tube feet (including sea cucumbers) can be seen aligned along the side of their axes. While echinoderms are invertebrates, meaning they do ...
Sea urchins or urchins (/ ˈ ɜːr tʃ ɪ n z /) are typically spiny, globular animals, echinoderms in the class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal to 5,000 metres (16,000 ft; 2,700 fathoms). [1]
They graze near the substrate, and their diet includes algae, periphyton, and seagrass. [5] Most collector urchins feed on seagrass fronds; this has an ecological impact varying with the season and abundance of the urchins.
The tubercles are imperforate and do not have crenulate edges. There are few tubercles on the interambulacral plates. The buccal notches are reduced in size and, their most significant distinguishing feature, the pedicellariae are globiferous and have one or two pairs of lateral teeth on the narrow tubular blades.
Brittle stars are not used as food, [1] though they are not toxic, because of their strong skeleton. Even if some species have blunt spines, no brittlestar is known to be dangerous, nor venomous. There is no harm evidence towards humans, and even with their predators, brittlestars' only means of defense is escaping or discarding an arm.