Section Objectives: Compare similarities and differences among the classes of echinoderms. Interpret the evidence biologists have for determining that echinoderms are close relatives of chordates. What is an echinoderm? Echinoderms move by means of hundreds of hydraulic, suction-cup-tipped appendages and have skin covered with tiny, jawlike
pincers. Echinoderms are found in all the oceans of the world. Echinoderms have endoskeletons If you were to examine the skin of several different echinoderms, you would find that they all have a hard, spiny,or bumpy endoskeleton covered by a thin epidermis.
Echinoderms have endoskeletons Sea stars, sometimes called starfishes, may not appear spiny at first glance, but a close look reveals that their long, tapering arms, called rays, are covered with short, rounded spines. The endoskeleton of all echinoderms is made primarily of calcium carbonate, the compound that makes up limestone.
Echinoderms have endoskeletons Some of the spines found on sea stars and sea urchins have become modified into pincerlike appendages called pedicellariae (PEH dih sih LAHR ee ay). Echinoderms have endoskeletons An echinoderm uses its jawlike
pedicellariae for protection and for cleaning the surface of its body. Pedicellariae Echinoderms have radial symmetry
You may remember that radial symmetry is an advantage to animals that are stationary or move slowly. Radial symmetry enables these animals to sense potential food, predators, and other aspects of their environment from all directions.
The water vascular system The water vascular system is a hydraulic system that operates under water pressure. Water enters and leaves the water vascular system of a sea star through the madreporite (mah druh POHR ite), a sievelike, diskshaped opening on the upper surface of the echinoderms body. The water vascular system
The underside of a sea star has tube feet that run along a groove on the underside of each ray. The water vascular system Tube feet are hollow, thin-walled tubes that end in a suction cup. Tube feet look somewhat like miniature droppers. The round, muscular structure called the
ampulla (AM pew lah) works something like the bulb of a dropper. The water vascular system Each tube foot works independently of the others, and the animal moves along slowly by
alternately pushing out and pulling in its tube feet. Ampullae The water vascular system Tube feet also function in gas exchange and excretion. Gases are exchanged and wastes are eliminated by diffusion through the thin
walls of the tube feet. Echinoderms have a simple nervous system Echinoderms have no head or brain, but they do have a central nerve ring that surrounds the
mouth. Ring canal Echinoderms have a simple nervous system Nerves extend from the nerve ring down each ray. Each radial nerve then branches into a nerve
net that provides sensory information to the animal. Echinoderms have cells that detect light and touch, but most do not have sensory organs. Echinoderms have a simple nervous system Sea stars are an exception. A sea stars body consists of long, tapering rays that extend from the animals central disk. A sensory organ known as an eyespot and
consisting of a cluster of light-detecting cells is located at the tip of each arm, on the underside. Echinoderms have bilaterally symmetrical larvae If you examine the larval stages of echinoderms, you will find that they have bilateral symmetry. Through metamorphosis, the free-swimming
larvae make dramatic changes in both body parts and in symmetry. Echinoderms are deuterostomes Echinoderms are deuterostomes. This pattern of development indicates a close relationship to chordates, which are also deuterostomes. Diversity of Echinoderms
Approximately 6000 species of echinoderms exist today. About one-fourth of these species are in the class Asteroidea (AS tuh ROY dee uh), to which the sea stars belong.
Diversity of Echinoderms The five other classes of living echinodems are Ophiuroidea (OH fee uh ROY dee uh), the brittle stars; Echinoidea (eh kihn OY dee uh), the sea urchins and sand dollars. Diversity of Echinoderms Holothuroidea (HOH loh thuh ROY dee uh), the sea cucumbers; Crinoidea (cry NOY dee uh), the sea lilies and feather stars; and
Concentricycloidea (kon sen tri sy CLOY dee uh), the sea daisies. Sea Cucumber Class Asteroidea Sea Stars. of starfish species are in this class (1500 species). Found along the shorelines aggregated on rocks. Can be of many colors including many bright colors. Examples include Asterias. Sea
Stars typically have 5 rays but can have more. Classic starfish Class Ophurioidea Brittle Stars. The largest of the Major groups of echinoderms with over 200 species. They are probably the most abundant also. They abound in all types of benthic marine habitats, even carpeting the abyssal sea bottom in many areas. Brittle stars typically have 5 arms but they are slender and sharply
set off from the central disc. They have no pedicellarae or papulae. Their tube feed are without suckers. They aid in feeding but are of limited use in locomotion. Brittle stars Brittle stars are extremely fragile. This adaptation helps the brittle star survive an attack by a predator. While the predator is busy with the broken
off ray, the brittle star can escape. A new ray will regenerate. Brittle stars Brittle stars propel themselves with the snake like, slithering motion of their flexible rays. They use their tube feet to pass particles of food
along the rays and into the mouth in the central disk. Class Echinoidea Sea Urchins and Sand Dollars. About 950 species which generally have a compact body enclosed in an endoskeletal shell. These lack arms or rays. A majority of species of sea urchins are regular with a hemispherical shape, radial symmetry and medium to long spines. Sand dollars and heart
urchins are irregular because they have become secondarily bilateral. Their spines are very short. Regular urchins move means of tube feet with some assistance from spines. Irregular urchins move chiefly by their spines. They can be quite colorful and some have painful toxins. They can be found in all seas from intertidal regions to deep seas. Examples include Dendraster. Sea urchins and sand dollars
Sea urchins and sand dollars are globe or disk-shaped animals covered with spines; they do not have rays. A living sand dollar is covered with minute, hair-like spines that are lost when the animal dies. A sand dollar has tube feet that protrude from the petal-like markings on its upper surface.
Sea urchins and sand dollars Sea urchins look like living pincushions, bristling with long, usually pointed spines. Sea urchins have long, slender tube feet that, along with the spines, aid the animal in locomotion. Sea urchins and sand dollars
These tube feet are modified into gills and are used for respiration. Tube feet on the animals bottom surface aid in bringing food particles to the mouth. Class Holothuroidea
Sea Cucumbers. This class contains members that both structurally and physiologically are the strangest. These animals resemble a vegetable which they are named for. They are elongated and soft bodied. Some species crawl on the sea floor, others are found beneath rocks, and some are burrowers. There are about 1150 species. Examples include Cucumaria.
Sea cucumbers When sea cucumbers are threatened, they may expel a tangled, sticky mass of tubes through the anus, or they may rupture, releasing some internal organs that are regenerated in a few weeks. Sea cucumbers reproduce by shedding eggs and sperm into the water, where fertilization occurs.
Sea cucumbers Sea cucumbers are so called because of their vegetable-like appearance. Their leathery covering allows them flexibility as they move along the ocean floor. Class Crinoidea Sea Lillies and Feather Stars. Consists of about 625 species. They differ from other echinoderms by being
attached during a substantial part of their lives. Se lilies have a flower-shaped body that is placed at the tip of an attached stalk. Feather stars have long, many branched arms and adults are free moving. They remain in the same spot for long periods. During metamorphosis feather stars become sessile and stalked, but after several months they detach and become free moving. Many are deep-water forms, but they can inhabit shallow waters as well. Examples include Comantheria.
Sea lilies and feather stars Sea lilies and feather stars resemble plants in some ways. Sea lilies are the only sessile echinoderms. Feather stars are sessile only in larval form. The adult feather star uses its feathery arms to swim from place to place. Class Concentricycloidea
Sea Daisies. Little disc shaped animals (less than 1 cm diameter). Only two species are known so far. There is a lot of disagreement as to their relationship to other echinoderm classes. Sea daises do not have arms. Their tube feet are located around the center disc. Once species does not have a digestive tract but has a membranous velum which absorbs nutrients. The other species has a shallow, saclike stomach but no intestine or anus. Examples include Xyloplax.
Sea stars Most species of sea stars have five rays, but some have more. Some species may have more than 40 rays. Sea stars Pedicellariae Endoskeleton
Ray Madreporite Radial canal Radial nerve Tube feet
Eyespots Anus Ring canal Ampullae Nerve ring
Stomach Mouth Reproductive organ Endoskeletal plates Digestive gland
Origins of Echinoderms The earliest echinoderms may have been bilaterally symmetrical as adults, and probably were attached to the ocean floor by stalks. Another view of the earliest echinoderms is that they were bilateral and free swimming. Origins of Echinoderms The echinoderms represent the only major
group of deuterostome invertebrates. This pattern of development is one piece of evidence biologists have for placing echinoderms as the closest invertebrate relatives of the chordates. Origins of Echinoderms Most echinoderms have been found as fossils from the early Paleozoic Era. Fossils of brittle
stars are found beginning at a later period. Not much is known about the origin of sea daisies.
