Download Phylum Echinodermata Reading And Questions

Phylum Echinodermata Reading and Questions
The phylum Echinodermata (echino- means spiny, and dermis means skin) is composed of the many familiar
marine animals such as sea stars (starfish), brittle stars, sea cucumbers, sear urchins and sand dollars, and sea
lilies and feather stars. In addition to having spiny skin, echinoderms are characterized by pentanumerous (fivepart) radial symmetry; an internal skeleton, a water vascular system and suction-cuplike structures called tube
feet. The internal skeleton or endoskeleton made up of hardened plates of calcium carbonate which are often
bumpy or spiny. The water vascular system, which is unique to this phylum, consists of an internal network of
fluid-filled canals connected to external appendages called tube feet. The water vascular system is involved
with many essential life functions including feeding, respiration, internal transport, elimination of waste
products and movement.
Although echinoderms possess certain characteristics found in no other animals, living or extinct, they share
several important features with members of our own phylum (Chordata). For example, certain stages in the
development of echinoderm larvae are remarkably similar to stages seen in some members of our own phylum.
In all other invertebrates, the first opening of the digestive system formed in the embryo is the mouth
(protostome). The opening for the anus later develops opposite the mouth. In the echinoderms, however, the
pattern of development is just the opposite. During embryological development, the first opening that forms is
the anus in both echinoderms and chordates, and the mouth forms at a later stage opposite the anus
(deuterostome). In addition, echinoderms have an internal skeleton (as do vertebrates) rather than an external
skeleton (as do other invertebrates). For these reasons, biologists believe that among invertebrates,
echinoderms are most closely related to humans.
Form and Function in Echinoderms
External Features
Adult echinoderms have a body plan with five parts organized symmetrically around a center. As a result of
this body plan, adult echinoderms have neither an anterior nor a posterior end and no brain. However, most
echinoderms are two-sided. The side where the mouth is located is the venter or ventral, and the anus is on the
dorsal surface.
Water-Vascular
All echinoderms have a unique system of internal tubes called a water vascular system. The water vascular
system opens to the outside through a sieve-like structure called the madreporite. In starfish, the madreporite
connects to a tube called the ring canal that forms a circle around the animal’s digestive system. From the ring
canal, five radial canals extend into each body segment. Attached to each radial canal are hundreds of movable
tube feet. The water vascular system operates like a series of living hydraulic pumps that can propel water in or
out of the tube feet. When water is pushed into a tube foot, the tube foot expands. When water is pulled out,
the cup on the end of the tube foot shrinks, creating a partial vacuum that holds onto whatever the foot is
touching. In this way, the tube feet act like living suction cups. A single tube foot alone cannot accomplish
much, but hundreds acting together create enormous force. All echinoderms move with their tube feet, and
some use their tube feet for feeding.
Digestive
Echinoderms have several methods of feeding. Carnivores, such as sea stars, use their tube feet to pry open the
shells of bivalves such as clams and scallops. Once the bivalve’s shell is opened, the sea star flips its stomach
out of its mouth, pours out digestive enzymes and digests its prey in the prey’s own shell. When the sea star has
finished eating, it moves its stomach back into its mouth, leaving behind an empty shell. Sea stars also eat
snails, corals, and even other echinoderms. Herbivores such as sea urchins, scrape algae from rocks by using
their five-part jaw. Filter feeders, such as sea lilies, basket stars and some brittle stars, use tube feet on flexible
arms to capture plankton that float by on ocean currents. Detritus feeders, such as sea cucumbers, move much
like a bulldozer across the ocean floor, taking in a mixture of sand and detritus. Then, like an earthworm, they
digest organic material and pass the sand grains out in their feces.
Solid wastes are released through the anus in the form of feces (except in brittle stars which have no anus).
Echinoderms, like most other marine invertebrates, excrete nitrogen-containing cellular wastes primarily in the
form of ammonia. Wastes seem to be excreted in many of the same places around the body in which gas
exchange takes place—the tube feet and skin gills.
Respiration
Echinoderms, like other animals, need to exchange carbon dioxide for oxygen. In most species, the thin-walled
tissue of the tube feet forms the main respiratory surface. In some species, like sea stars, small outgrowths
called skin gills also function in gas exchange. The functions of transporting oxygen, food and wastes—which
are performed by the circulatory system in most animals- are shared by different systems in echinoderms.
Because respiration (gas exchange) and the removal of metabolic wastes occur through the skin gills and tube
feet located all over the body, a system to deliver oxygen and carry away carbon dioxide and nitrogenous waste
is not essential. The distribution of nutrients is performed primarily by the digestive glands and the fluid within
the body cavity.
Nervous
As you might expect in animals that have no head, echinoderms have primitive nervous systems. Most
echinoderms have a nerve ring that surrounds the mouth and radial nerves that connect the ring with the body
sections. Scattered sensory cells that are sensitive to chemicals released by potential food are also characteristic
of most species. Sea stars additionally have up to 200 light sensitive cells clustered in eyespots at the tip of
each arm. These eyespots can sense light or dark, but do not allow for visual images. Some echinoderms
possess statocysts (simple organs for balance that tell an organism whether it is right side up). The spines of
most species provide little protection from predators, and many species are nocturnal.
Skeletal
Echinoderms use tube feet and thin layers of muscle fibers attached to the plates of the endoskeleton to move.
An echinoderm’s mobility is determined in part by the structure of its endoskeleton. In sand dollars and sea
urchins, the plates are fused together to form a rigid box that encloses the animal’s internal organs. These
animals have movable spines attached to their endoskeleton, which they use along with their tube feet to creep
from one place to another or to burrow in the sand. In sea stars, brittle stars and feather stars, the skeletal plates
move around a series of flexible joints, enabling these echinoderms to use their arms for locomotion. Feather
stars can swim for short distances by flapping their arms, but sea stars and brittle stars are only able to crawl. In
sea cucumbers, the plates are reduced to tiny vestiges inside a soft, muscular body wall. The loss of the plates
makes the body of sea cucumbers very flexible. Some species are able to crawl along the ocean floor like large,
fat worms by contracting the muscles of the body wall.
Reproductive
Most echinoderms are either male or female, although some are hermaphrodites. In sea stars, the sperm are
produced in testes and the eggs are produced in ovaries, which fill the arms during the reproductive season.
Males and females shed their gametes into the water, and fertilization occurs externally. The larvae swim
around for some time as members of the huge community of plankton that swarm in the ocean. Eventually the
larvae, which have bilateral symmetry, swim to the ocean bottom, where they mature and metamorphose into
adults that have radial symmetry.
Many sea stars have incredible abilities to repair themselves when damaged or regenerate individuals from
fragments. In fact, if a sea star is pulled into pieces, each piece can grow into a new animal as long as it
contains a portion of the ring canal (located in the central part of the body). This ability of a sea star to
regenerate (a form of asexual reproduction) itself has caused a great deal of trouble to people who earn their
living fishing for bivalves. In the past, angry fishermen who were aware that starfish ate bivalves would tear
the animals into two or three pieces and toss them overboard. Obviously, this only exacerbated their problem.
Today fishermen know that they best way to kill a starfish is to remove it from the water.
Echinoderms in the Biosphere
Sea stars are important carnivores that control the populations of other animals. Sea urchins control the
distribution of algae. Both organisms, however, can do extensive damage if their numbers become too high. In
various parts of the world, sea urchin eggs and sea cucumbers are considered delicacies by some people. Many
more echinoderms are useful as research subjects and as possible sources of medicine. Several chemicals
extracted from starfish and sea cucumbers are currently being studied as potential anti-cancer and anti-viral
drugs. Sea urchins have been the subject of pioneering studies in embryology. These animals are easy to study
because they produce large eggs that are fertilized externally and develop in plain seawater. Sea urchin
embryos also make excellent subjects for testing the effects of drugs on cell division and development.
Questions:
1. Based on the reading, sketch a diagram of a sea star labeling the five major characteristics that define
echinoderms.
2. Why do biologists think that echinoderms and chordates show an evolutionary relationship?
3. What is the purpose of the echinoderm’s water vascular system?
4. What organs are used for respiration and excretion in echinoderms?