Download Echinoderms: Features

Echinoderms: Features
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Invertebrates with a deuterostome pattern of
development and five-part radial
symmetry.
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All the 7,000 or so living species inhabit
the sea or intertidal zone, and they include
starfish, sea urchins, sea cucumbers, sand
dollars, brittle stars, sea lilies, and feather
stars.
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Internal transport of nutrients, wastes,
and other metabolites is accomplished by
circulation of coelomic fluid within a system
of channels called the hemal system, which
runs alongside the water vascular system.
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Respiratory gases and wastes diffuse across
the body surface, and there are no excretory
organs.
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The body shape is highly variable; for
example, starfish (sea stars) are star-shaped,
with five distinct arms, whereas sea
cucumbers are wormlike.
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The nervous system consists essentially of a
nerve ring beneath the epidermis around the
mouth, from which arise radial nerves to
supply each of the five body regions.
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There is no distinct head, and the mouth lies
at the center of the animal – generally on the
lower surface in motile forms and on the
upper surface in attached forms.
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Sense organs can include eyespots,
statocysts (organs of balance), and chemical
and touch receptors.
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The anus is usually on the surface opposite to
the mouth.
The sexes are usually separate, with eggs
and sperm shed into the seawater, where
fertilization take place.
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Many live by collecting organic detritus from
sediment or seawater, whereas others are
scavengers or predators, or graze algae.
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The body wall contains an endoskeleton of
calcium carbonate plates (ossicles) that often
bear spines or other surface protuberances.
The egg hatches into a bilaterally
symmetrical, ciliated, planktonic larva,
which usually undergoes several stages of
development before metamorphosis into
the adult.
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Some brood their young; for example, sea
cucumbers retain the larvae among their
tentacles or inside their body.
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Adults can often regenerate lost arms or other
body parts, and some species divide into two
or more parts to produce new individuals.
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Part of the coelom forms a water vascular
system, consisting of a network of seawaterfilled canals, connected to the exterior via a
stone canal and button-like madreporite. This
serves as a hydraulic system for extending
numerous tube feet (podia), used in
locomotion, feeding, and gaseous exchange.
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Echinoderms can also move their arms or
spines, for crawling, swimming, burrowing,
or feeding.
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The gut varies in form. In sea stars it
comprises a large cardiac stomach and
smaller pyloric stomach; paired digestive
glands in each arm discharge enzymes into
the pyloric stomach, from where a short
intestine leads to the anus.
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