Differentiate among the classes of echinoderms

The phylum echinoderms is divided into 5 extant classes: Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea (sea urchins and also sand dollars), Crinoidea (sea lilies or feather stars), and also Holothuroidea (sea cucumbers).

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The most famous echinoderms room members of course Asteroidea, or sea stars. They come in a big variety of shapes, colors, and sizes, with more than 1,800 types known therefore far. The key characteristic that sea stars that distinguishes them from various other echinoderm classes consists of thick arms (ambulacra; singular: ambulacrum) that prolong from a main disk whereby organs penetrate into the arms. Sea stars use their pipe feet not just for gripping surfaces, but likewise for grasping prey. Sea stars have actually two stomachs, one of which can protrude with their mouths and also secrete cradle juices right into or ~ above prey, even before ingestion. This process can basically liquefy the prey, making digestion easier.

Brittle stars belong come the class Ophiuroidea. Unequal sea stars, which have actually plump arms, brittle stars have actually long, slim arms that are sharply demarcated indigenous the central disk. Brittle stars move by lashing out their eight or pack them about objects and pulling themselves forward. Of all echinoderms, the Ophiuroidea may have the strongest propensity toward 5-segment radial (pentaradial) symmetry. Ophiuroids are usually scavengers or detritivores. Little organic particles space moved right into the mouth by the tube feet. Ophiuroids may likewise prey on tiny crustaceans or worms. Some brittle stars, such as the six-armed members the the family members Ophiactidae, are fissiparous (divide though fission), with the disk splitting in half. Regrowth that both the lost part of the disk and also the eight occur, yielding an pet with three huge arms and three tiny arms throughout the duration of growth.

Sea urchins and also sand dollars are instances of Echinoidea. This echinoderms do not have actually arms, however are hemispherical or planarization with 5 rows of pipe feet that aid them in sluggish movement; pipe feet room extruded with pores that a consistent internal shell referred to as a test. Like other echinoderms, sea urchins space bilaterans. Their early on larvae have actually bilateral symmetry, however they construct fivefold symmetry as they mature. This is most evident in the “regular” sea urchins, which have roughly spherical bodies, with five equally-sized components radiating the end from their main axes. Several sea urchins, however, including the sand dollars, space oval in shape, with unique front and also rear ends, giving them a degree of bilateral symmetry. In this urchins, the upper surface of the human body is slightly domed, however the underside is flat, when the sides space devoid of pipe feet. This “irregular” body kind has advanced to permit the animals to burrow v sand or various other soft materials.

Figure (PageIndex1): Sea urchins: Sea urchins perform not have arms, but have rows of pipe feet that have the right to be prolonged out the pores of the inner shell.

Sea lilies and feather stars are instances of Crinoidea. Both that these species are suspension feeders. They live both in shallow water and also in depths as good as 6,000 meters. Sea lilies refer to the crinoids which, in your adult form, room attached come the sea bottom through a stalk. Feather stars or comatulids describe the unstalked forms. Crinoids are identified by a mouth on the peak surface the is surrounded by feeding arms. They have a U-shaped gut; their anus is located next to the mouth. Although the straightforward echinoderm sample of fivefold symmetry deserve to be recognized, most crinoids have many much more than 5 arms. Crinoids usually have a stem offered to affix themselves to a substrate, however many live attached just as juveniles and become free-swimming together adults.

Figure (PageIndex1): Sea lilies: Sea lilies, favor feather stars, have a mouth on your upper surface ar that is surrounding by arms supplied for feeding.

Sea cucumbers of course Holothuroidea are prolonged in the oral-aboral axis and have 5 rows of tube feet. These are the just echinoderms that demonstrate “functional” bilateral symmetry together adults due to the fact that the uniquely-extended oral-aboral axis compels the pet to lied horizontally quite than was standing vertically. Prefer all echinoderms, sea cucumbers have actually an endoskeleton just listed below the skin: calcified frameworks that space usually lessened to isolated microscopic ossicles joined by connective tissue. In some types these have the right to sometimes it is in enlarged to planarization plates, creating armor. In pelagic species, such as Pelagothuria natatrix, the skeleton and also a calcareous ring space absent.

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Figure (PageIndex1): Sea cucumbers: Sea cucumbers room the just echinoderms that demonstrate “functional” bilateral symmetry together adults, as they lied horizontally as opposed come the vertical axis of various other echinoderms.