Echinoderms | Classes of Echinoderms and their Examples: Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea (sea urchins and sand dollars), Crinoidea (sea lilies or feather stars), and Holothuroidea (sea cucumbers) are the five existing classes of phylum echinoderms.
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Classes of Echinoderms
Below are classes of Echinoderms and their examples
Asteroidea (sea stars)
Members of the Asteroidea class, or sea stars, are the most prominent echinoderms. With more than 1,800 species recognized so far, they come in a wide range of forms, colors, and sizes. Thick arms (ambulacra, singular, ambulacrum) extending from a central disk where organs penetrate the components are the main attribute of sea stars that differentiate them from other echinoderm classes. Sea stars not only use their tube feet to grip surfaces but also to grasp prey. Sea stars have two stomachs, one of which, even before ingestion, will protrude from their mouths and secrete digestive juices into or onto prey. The prey can essentially be liquefied by this process, making digestion easier.
Ophiuroidea (brittle stars)
The stars of Brittle belong to the Ophiuroidea class. Brittle stars have long, thin arms that are sharply demarcated from the central disk, unlike sea stars, which have plump arms. Brittle stars shift and pull themselves forward by lashing out their arms or wrapping them around objects. The Ophiuroidea may have the most significant tendency of all echinoderms toward 5-segment radial (pentaradial) symmetry. Ophiuroids are scavengers or detritivores in general. The tube feet move tiny organic particles into the mouth. Little crustaceans or worms can also be preyed on by ophiuroids. Some brittle stars, such as the six-arm members of the Ophiactidae family, are fissiparous (divided by fission), dividing the disk in half. There is a re-development of both the missing portion of the disk and the arms, creating an animal with three big arms and three small arms throughout the growth phase.
Echinoidea (sea urchins and sand dollars)
Echinoidea examples include sea urchins and sand dollars. These echinoderms have no arms but are hemispherical or flattened with five rows of tube feet that assist them in slow motion; the tube’s feet are extruded by the pores of a continuous inner shell called an examination. Like other echinoderms, bilateral are sea urchins. Their early larvae have bilateral symmetry, but when they mature, they develop five-fold symmetry. In the “regular” sea urchins, which have approximately spherical bodies, this is most obvious, with five equal-sized sections radiating from their central axes. However, most sea urchins are oval, like the sand dollars, with different front and rear ends, giving them a bilateral symmetry degree. The body’s upper surface is somewhat domed in these urchins, but the underside is smooth, while the sides are devoid of tube feet. To allow the animals to burrow through sand or other soft materials, this “irregular” body shape has evolved.
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Crinoidea (sea lilies or feather stars)
Examples of the Crinoidea are sea lilies and feather stars. Both of these animals are feeders for suspension. They live in shallow water as well as in waters as high as 6,000 meters. Sea lilies refer to crinoids connected to the sea bottom by a stalk in their adult form. The unstalked types apply to feather stars or comatulids. Crinoids characterize a mouth on the top surface that is surrounded by feeding arms. They have a gut that is U-shaped; their anus is situated near the mouth. While it is possible to identify the basic echinoderm pattern of fivefold symmetry, most crinoids have a lot more than five arms. Typically, crinoids have a stem used to bind to a substrate, but many live only as juveniles and become free-swimming as adults.
Holothuroidea (sea cucumbers)
In the oral-aboral axis, sea cucumbers of the Holothuroidea class are extended and have five rows of tube legs. These are the only echinoderms that, as adults, exhibit “functional” bilateral symmetry since the unusually extended oral-aboral axis causes the animal not to stand vertically but to lie horizontally. Sea cucumbers have an endoskeleton just under the skin, like all echinoderms: calcified structures typically reduced to isolated microscopic ossicles joined by connective tissue. These may often be expanded into flattened plates in individual animals, forming armor. The skeleton and a calcareous ring are absent in aquatic animals, including the Pelagothuria matrix.
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