The echinoderms represent a biological puzzle
for zoologists. Biologists generally agree that
bilateral symmetry is adaptive for free-moving
animals and radial symmetry is adaptive for sedentary
animals, but the echinoderms are freemoving
yet exhibit radial symmetry. Since the
echinoderm larval stages are bilaterally symmetrical,
they obviously evolved frombilateral ancestry;
however, the larvae metamorphose into radially
symmetrical adults. The echinoderms live in
marine environments and include a varied group
that is considerably different from all other members
of the animal kingdom. Echinoderms include
the starfishes, sea urchins, brittle stars, sea cucumbers,
and sea lilies. A calcareous skeleton is present
in all members of the phylum, and their name
is derived from the presence of external spines or
protuberances.
General Characteristics of the Echinoderms
Because of the tremendous diversity in this group
of animals, a complete description of all echinoderm
characteristics is beyond the scope of this article.
Externally, the arms or rays are joined at the
center to form a disc that bears the madreporite.
Many echinoderms are protected from predators
by a spiny skeleton made up of calcareous plates
just below the epidermis. Many of the plates bear
tubercles (bumps) and spines. In addition, predators
are often discouraged by the presence of
pedicellariae (tiny pincers) scattered over the body.
When stimulated, these snap vigorously and remain
shut for several days on anything they catch.
The mouth is in the center of the ventral surface
and is surrounded by a membranous area, the
peristome. The mouth opens directly into a large
cardiac stomach, which in turn opens into a
smaller pyloric stomach.Asmall intestine extends
upward from the pyloric stomach and ends at an
anus in the middle of the upper surface of the disc.
Five gastric glands extend out of the rays and
open into the pyloric stomach. When feeding, the
cardiac stomach everts through the mouth and
spreads over the food. A fluid containing digestive
enzymes is secreted and rapidly breaks down
the food materials. The nervous system is composed
of a nerve ring around the mouth and five
radial nerves to the lower epidermis. Other nerve
fibers are located in the walls of the digestive tract
and inside the upper body wall. The nervous system
exhibits reciprocal inhibition. The nerve centers on one side of the body inhibit those on the
other side, permitting the animal to move in a coordinated
manner in one direction.
The circulatory system is composed of circular
and radial vessels filled with a fluid similar to that
of the body cavity and which is very different
from sea water. The vessels lie above the nervous
system and are enclosed in their own body cavity.
Some of these vessels have been observed to contract.
Apair of gonads is located in the base of each
ray, with one on each of the gastric
glands. These gonads hang
free in the body cavity except
where they are attached by a
short duct to a reproductive pore
opening externally between the
bases of adjacent rays.
Representative Echinoderms
Most starfish (class Asteroidea)
have five rays and a relatively
small disc, but in some species
the body is pentagonal rather
than star-shaped and the disc is
large relative to the rays. The animals
in one genus, Leptasterias,
have six rays, while the number
of rays in other starfishes may
be as high as twenty-five or fifty.
In general, if the number of rays
is greater than seven, the number
will be variable within a species.
When the number exceeds
five, the embryo develops five
rays first and the others later.
Most starfishes eat only small bivalves
and other organisms that
are swallowed whole. Many of
the larger species, ranging from
one to three feet in diameter, feed
primarily on other echinoderms.
Sea lilies (class Crinoidea) are
echinoderms that attach to the
ocean floor by a stalk. The mouth
is directed upward, and the anus
is located to one side. The rays
generally branch to form graceful patterns. Ciliated
grooves flanked on both sides by tube feet
without suckers extend out of the mouth along the
upper surfaces of all the rays and branches. Food
is trapped and swallowed when tiny organisms
and food particles are pushed against the ciliated
groove by the action of the tube feet. Movement in
sea lilies is restricted to spreading and folding together
of the branches and postural changes of the
body. In the feather stars, the larvae attach and growa short stalk like that of the sea lilies,
but later break loose. The general anatomy
and method of feeding is unchanged,
but the feather stars differ from
the sea lilies in their locomotion. Feather
stars can crawl through vegetation using
the rays as prehensile organs, and they
can swim by raising and lowering the ten
arms more than one hundred times per
minute.
Sea cucumbers (class Holothuroidea)
creep or burrow in themudor sand. Since
the calcareous plates are small, the body
is soft and flexible. The body is elongated
between the mouth and anus, but one side
usually becomes the permanent lower
side, causing the radial symmetry to be
imperfect. While five rows of tube feet extend
frommouth to anus, often only three
of the rows have suckers and are used for
locomotion.Acircle of branched tentacles
form from the tube feet near the mouth.
Sea cucumbers are notable for their ability
to throw away their viscera. When environmental
conditions are unfavorable,
the sea cucumber contracts violently and
ejects the entire digestive system, which
can later be regenerated.
The skeletons of the echinoids (class
Echinoidea) form rigid boxes. Five grooves
with tube feet radiate from the mouth up
around the sides and end near the anus.
The tube feet on the upper surface are often
long and filamentous and apparently
used for respiration, while the lower tube feet usually
have suckers and are utilized in locomotion.
Sea urchins possess numerous long spines, some
of which are used to aid the tube feet in walking.
The urchins move about slowly, using their five
sharp teeth to scrape and chew whatever they encounter.
The sand dollars are a group of muchflattened
echinioids. They usually creep about
slowly on their short lower spines under the surface
of the sand.
Brittle stars (class Ophiuroidea) possess slender
rays attached to a circular disc. Each ray is
composed of a row of large cylindrical skeletal
pieces joined together by short, powerful muscles.
Each ray is very supple, and the tube feet are
poorly developed. Brittle stars move by pulling or
pushing on surrounding objects, and thus slither
like a snake. Most brittle stars feed on debris and
mud, but some capture prey with their prehensile
rays and bring it to the mouth, which opens into a
simple saclike stomach. Undigested remains are
eliminated through the mouth because no other
digestive organs are present.
The group commonly called sea daisies (class Concentricycloidea) is a class of echinoderms discovered
in 1986 off the coast of New Zealand.
They have no arms, and the tube feet are located
around the periphery of the disc rather than in
grooved areas, as in other echinoderms. Only two
species have been identified. One has a small,
saclike stomach but no intestine or anus, and the
other lacks any digestive organs. This latter species
apparently absorbs nutrients from the environment.
Classification:
Kingdom: Animalia
Subkingdom: Bilateria
Phylum: Echinodermata
Subphylum: Pelmatozoa
Classes: Crinoidea (sea lilies and sea feathers), Heterostella (extinct),
Cystidea (extinct), Blastoidea (extinct), Edrioasteroidea (extinct)
Subphylum: Eleutherozoa (stemless unattached echinoderms)
Classes: Holothuroidea (sea cucumbers; armless, poorly developed
skeleton); Echinoidea (sea urchins and sand dollars; armless, welldeveloped
skeleton); Asteroidea (starfishes; arms, well-developed
skeleton, locomotion with tube feet); Ophiuroidea (brittle
stars; arms, flexible skeleton, locomotion with prehension);
Concentricycloidea (sea daisies; armless, disc-shaped with marginal
skeleton); Ophiocistioidea (extinct)
Orders: Holothuroidea (five living orders); Echinoidea (three living
orders, five extinct orders); Asteroidea (three living orders, five
extinct orders); Ophiuroidea (two living orders); Concentricycloidea
(one living order); Ophiocistioidea (one extinct order)
Geographical location: Found all over the world
Habitat: Marine, from the shoreline to the ocean depths
Gestational period: Varies among species, but most species lay eggs
within a few days after fertilization; eggs usually hatch within a
few days to a few weeks after being deposited
Life span: Varies among species; can be as short as a year and up to
several years
Special anatomy: Body unsegmented with radial, pentamerous (five
or more radiating areas) symmetry; no head or brain and few specialized
sensory organs; endoskeleton of calcareous ossicles with
spines; a unique water vascular system that extends fromthe body
surface as a series of tube feet; excretory system is absent; development
through free-swimming bilateral larval stages that metamorphose
into radial adult
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