![]() In addition to having pentaradial symmetry, adult echinoderms have two unique structural features. Nearly all of the 7,500 extant species of echinoderms live only in marine environments. Many clades of echinoderms became extinct during the periodic mass extinctions that have occurred throughout Earth’s history (see Table 18.1). Only 6 of the 23 major groups known from fossils are represented by species that survive today. They are probably only a small fraction of those that actually lived. Rather than having an anterior–posterior (head–tail) and dorsal–ventral (back–belly) body organization, echinoderms have an oral side (containing the mouth) and an opposite aboral side (containing the anus).Įchinoderms have unique structural featuresĪbout 13,000 species of echinoderms in 23 major groups have been described from fossil remains. ![]() As is typical of animals with radial symmetry, echinoderms have no head, and they move equally well (but usually slowly) in many directions. ![]() Echinoderms, however, undergo a radical change in form as they develop into adults ( FIGURE 23.32B), changing from a bilaterally symmetrical larva to an adult with pentaradial symmetry (symmetry in five or multiples of five). Adult hemichordates also are bilaterally symmetrical. The echinoderms and hemichordates (together known as ambulacrarians) have a bilaterally symmetrical, ciliated larva ( FIGURE 23.32A). Echinoderms evolved their adult forms with unique symmetry (in which the body parts are arranged along five radial axes) much later, whereas other deuterostomes retained the ancestral bilateral symmetry. The earliest deuterostomes were bilaterally symmetrical, segmented animals with a pharynx that had slits through which water flowed. Go to ANIMATED TUTORIAL 23.3 An Overview of the Deuterostomes Note that neither of the above factors is apparent in the morphology of the adult animals. ![]()
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