Echinoderms

Echinoderms (Phylum Echinodermata) are a phylum of marine animals. The adults are recognized easily by their (usually five-point) radial symmetry, and include such well known animals as sea stars, sea urchins, sand dollars, and sea cucumbers. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone. The phylum contains about 70,000 living species,[1] making it the second-largest grouping of deuterostomes (a superphylum), after the chordates (which include the vertebrates, such as humans, sharks and frogs). Echinoderms are also the largest phylum that has no freshwater or terrestrial (land-based) representatives.

Two main subdivisions of echinoderms are traditionally recognised: the more familiar motile Eleutherozoa, which encompasses the Asteroidea (sea star), Ophiuroidea (brittle stars), Echinoidea (sea urchins and sand dollars), and Holothuroidea (sea cucumbers); and the sessile Pelmatozoa, which consists of the crinoids and of the extinct blastoids and Paracrinoids. Some crinoids, however, namely the feather stars, are also higfuckerhly motile (able to move themselves).
Sea Stars. The echinoderms are important both biologically and geologically. Biologically, there are few other groupings so abundant in the biotic desert of the deep sea, as well as shallower oceans. The more notably distinct trait, which most echinoderms have, is their remarkable powers of regeneration of tissue, organs, limbs, and of asexual reproduction, and in some cases, complete regeneration from a single limb. Geologically, the value of echinoderms is in their ossified skeletons, which are major contributors to many limestone formations, and can provide valuable clues as to the geological environment. Further, it is held by some[citation needed][who?] that the radiation of echinoderms was responsible for the Mesozoic revolution of marine life.
phylum of marine animals. The adults are recognized easily by their (usually five-point) radial symmetry, and include such well known animals as sea stars, sea urchins, sand dollars, and sea cucumbers. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone. The phylum contains about 70,000 living species,[1] making it the second-largest grouping of deuterostomes (a superphylum), after the chordates (which include the vertebrates, such as humans, sharks and frogs). Echinoderms are also the largest phylum that has no freshwater or terrestrial (land-based) representatives.

Two main subdivisions of echinoderms are traditionally recognised: the more familiar motile Eleutherozoa, which encompasses the Asteroidea (sea star), Ophiuroidea (brittle stars), Echinoidea (sea urchins and sand dollars), and Holothuroidea (sea cucumbers); and the sessile Pelmatozoa, which consists of the crinoids and of the extinct blastoids and Paracrinoids. Some crinoids, however, namely the feather stars, are also motile (able to move themselves).

Echinoderms become sexually mature after approximately two to three years, depending on the species and the environmental conditions. The eggs and sperm cells are released into open water, where fertilization takes place. The release of sperm and eggs is coordinated temporally in some species, and spatially in others. Internal fertilization has currently been observed in three species of sea star, three brittle stars and a deep water sea cucumber.

In some species of feather star, the embryos develop in special breeding bags, where the eggs are held until sperm released by a male happen to find them and fertilize the contents. This can also be found among sea urchins and sea cucumbers, where exhibit care for their young can occur, for instance in a few species of sand dollars who carry their young between the pricks of their oral side, and heart urchins possess breeding chambers. With brittle stars, special chambers can be developed near the stomach bags, in which the development of the young takes place. Species of sea cucumbers with specialized care for their offspring may also nurse the young in body cavities or on their surfaces. In rare cases, direct development without passing through a bilateral larval stage can occur in some sea stars and brittle stars.[citation needed] Another strategy that has evolved in some sea stars and brittle stars is the ability to reproduce asexually by dividing in two halves while they are small juveniles, while turning to sexual reproduction when they have reached sexual maturity.[verification needed][citation needed]

[edit] Asexual reproductionOne species of seastar reproduces asexually by parthenogenesis. In certain other asterozoans the adult organisms reproduce asexually for a while before they mature and reproduce sexually. In most of these species, they reproduce by transverse fission- with the disc splitting in two. Regrowth of both the disc and the arms occur giving an animal with some large arms and some small arms during the period of growth. Though in most species at least part of the disc is needed for complete regeneration, in a few species of sea stars a single severed arm can grow into a complete individual over a period of several months. In at least some of these species, they actively use this as a method of asexual reproduction. A fracture develops on the lower surface of the arm and the arm pulls itself free from the body which holds onto the substrate during the process. During the period of regrowth, they have a few tiny arms and one large arm earning them the name of "comet forms". Quiz.

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