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Phylum Echinodermata

The content and activities in this topic will work towards building an understanding of the phylum Echinodermata.

Introduction to Phylum Echinodermata

<p><strong>Fig. 3.83.</strong> (<strong>A</strong>) Sea urchin (class Echinoidea)</p><br />
<p><strong>Fig. 3.83.</strong>&nbsp;(<strong>B</strong>) Sea star (class Asteroidea)</p><br />
<p><strong>Fig. 3.83.</strong>&nbsp;(<strong>C</strong>) Brittle star (class Ophiuroidea)</p><br />

<p><strong>Fig. 3.83.</strong>&nbsp;(<strong>D</strong>) Sea cucumber (class Holothuroidea)</p> <p><strong>Fig. 3.83.</strong>&nbsp;(<strong>E</strong>) Feather star (class Crinoidea)</p>

Echinoderms are named for the spines or bumps covering the outer surface of the bodies of many of them (Greek root word echino- meaning spiny; Latin root word -derm meaning skin). Examples of echinoderms include sea stars, sea urchins, sea cucumbers, brittle stars, and feather stars (Fig. 3.83). Although they may appear very different, echinoderms all have two major defining characteristics that set them apart from all other animals: a water vascular system and five-sided radial symmetry.


<p><strong>Fig. 3.84.</strong> Water vascular system of sea urchin</p><br />

The water vascular system is a complex series of canals running through an echinoderm’s body (Fig. 3.84). It is a hydraulic pressure system that aids in movement. The canals are water-filled tubes that open to the outside through a skeletal plate called the madreporite (from Latin root words madre meaning mother and pori meaning small hole) lying on the surface near the anal opening. Water enters and leaves the tubes through this sieve-like plate. An echinoderm moves by using many tube feet. Tube feet are small, delicate projections attached along the side of a water-filled tube called a radial canal. Figure 3.85 shows some examples of echinoderm tube feet. Tube feet extend through the small holes in the skeleton to the outside. These feet are grouped in five regions. Most sea stars, sea urchins, and sea cucumbers have suction cups at the tips of their tube feet. In some sea stars and brittle stars the tube feet are shaped like little paddles. Water gets from the madreporite to the tube feet through the radial canal. Valves keep water from flowing back into the radial canal (Fig. 3.84). The ampullae of the tube feet act like the bulbs of eyedroppers. When a valve closes and the ampulla muscles contract, squeezing the ampulla, water shoots into the tube foot, extending it (Fig. 3.85). When the tube foot comes in contact with hard substrate, its center withdraws, forming a cup and producing a vacuum much as a rubber suction cup does. The tube foot clings to the substrate because the water pressure on its outside edge is greater than the pressure inside its suction cup. When the muscles of the ampulla relax, water moves back into the ampulla, flattening the cup and releasing the vacuum in Fig. 3.85.

<p><strong>Fig. 3.85.</strong> (<strong>A</strong>) Oral or bottom side of a sea star</p><br />
<p><strong>Fig. 3.85.</strong>&nbsp;(<strong>B</strong>) Purple sea urchin (<em>Strongylocentrotus purpuratus</em>)</p><br />

<p><strong>Fig. 3.85.</strong>&nbsp;(<strong>C</strong>) Tube feet extending from the oral side of a <em>ha‘uke‘uke kaupali</em> or shingle sea urchin (<em>Colobocentrotus atratus</em>)</p><br />
<p><strong>Fig. 3.85.</strong>&nbsp;(<strong>D</strong>) Most species of sea cucumber have five rows of tube feet running along the length of their cylindrical bodies. The sea cucumber <em>Pseudocolochirus violaceus</em> is better known by its common name, the sea apple.</p><br />



Echinoderms are radically symmetrical, and the body is usually divided into five parts or multiples of five. This five-sided radial structure of echinoderms makes the body strong (Fig. 3.86). A five-sided skeleton is stronger than a four- or six-sided one because the line of weakness cannot run directly across the body. Even a three-sided body plan is weaker than a five-sided one.

<p><strong>Fig. 3.86. </strong>(<strong>A</strong>) Sand dollar</p><br />
<p><strong>Fig. 3.86.</strong> (<strong>B</strong>) Sea star</p><br />



Most animals that move around have bilateral symmetry, as described in the previous sections on molluscs, worms, and arthropods. But echinoderms, although they also move, are radially symmetrical, so the terms anterior, posterior, dorsal, and ventral do not apply. In the echinoderms there are two surfaces. One is the oral surface, where the mouth is and the tube feet project. The tube feet on the oral surface are limited to distinct regions called the ambulacral regions. The other surface is the aboral, which typically contains the anal opening of the digestive system. All echinoderms are variations on this oral-aboral body plan (Fig. 3.83). Sea stars (class Asteroidea) and brittle stars (class Ophiuroidea) have flat bodies with a broad aboral surface facing up and an oral surface facing down. Both groups have arms projecting from a central body disc and the ambulacral regions with the projecting tube feet extending along each of the arms (Fig. 3.83 B and Fig. 3.83 C). Like sea stars, the feather stars and sea lilies (class Crinoidea) have arms, but the oral surface faces up, away from the bottom (Fig. 3.83 E). The tube feet extend upward from the oral surface to capture particles of food floating by. Feather stars grasp the substrate with a series of rootlike projections from the aboral surface and sea lilies have long, stalk-like projections from the aboral surface with which they permanently attach themselves to the bottom. Sea urchins (class Echinoidea) have no arms (Fig. 3.83 A). The radial body plan is spherical. The oral surface, with ambulacral regions and tube feet, covers most of the sphere. The aboral surface is only a small disc at the top. In most sea urchins, spines also extend from the oral surface, usually between the rows of tube feet. The radial body plan of sea cucumbers (class Holothuroidea) is tube-shaped, with the aboral surface just a small region at the end opposite the mouth (Fig. 3.85 D). Most of the long body is covered by the oral surface, with tube feet projecting in five rows. Many of the tube feet around the mouth take the form of long tentacles, used for gathering food. The body lies on its side, giving the appearance of bilateral symmetry. The tube feet touching the bottom usually bear suction cups and are used for locomotion. The tube feet on the “upper” part of the body are often simple pointed structures.


All echinoderms also lack any kind of central nervous system or brain, but have a nerve ring. Echinoderms also have calcium carbonate endoskeletons, ranging from microscopic spicules in sea cucumbers to visible plates in sea stars and urchins. Most echinoderms have a complete digestive system and a large coelom. They have separate sexes, usually with gonads in sets of five, showing internal pentaradial symmetry. All echinoderm species live in the ocean; there are no freshwater or terrestrial echinoderms.


Class Echinoidea

<p><strong>Fig. 3.87.</strong> (<strong>A</strong>) Long sharp spines on a long-spined sea urchin (<em>Diadema antillarum</em>)</p><br />
<p><strong>Fig. 3.87.</strong> (<strong>B</strong>) Short sharp spines on a reef urchin (<em>Echinometra viridis</em>)</p><br />
<p><strong>Fig. 3.87.</strong> (<strong>C</strong>) Blunt spines on a red slate pencil urchin (<em>Heterocentrotus mammillatus</em>)</p><br />

<p><strong>Fig. 3.87.</strong> (<strong>D</strong>) Flat plate spines on a <em>ha‘uke‘uke kaupali</em> or shingle urchin (<em>Colobocentrotus atratus</em>), Kewalo Basin, Oʻahu, Hawaiʻi</p><br />
<p><strong>Fig. 3.87.</strong> (<strong>E</strong>) Velvet-textured spines on a sand dollar</p><br />

Sea urchins belong to the class Echinoidea, named for the movable spines projecting from their body like a hedgehog’s spines (from the Greek word echinoid meaning like a hedgehog). Sea urchins (Fig. 3.83 A) are common around the world, from the ocean’s shoreline to great depths and from tropical waters to polar waters. Sea urchins are relatively small; most species could fit in the palm of your hand. The spines are adaptations that protect the urchins from predators. Spines and tube feet help urchins move and get food. The long, thin, sharp spines of some sea urchins easily penetrate flesh and in some species, toxic chemicals on the tissue covering the sharp spines make its stab extremely painful (Fig. 3.87 A and B). Other species, with short, thick, or blunt spines are safe to handle (Fig. 3.87 C and D). A few species that have adapted to live in the wave surge zone of rocky coastlines have flattened spines (Fig. 3.87 D). Flat, broad plate spines give these urchins a low profile and prevent them from getting swept away by powerful waves. Sand dollars have fine velvet-textured spines that help these animals burrow into sand (Fig. 3.87 E). Pedicellariae are small jaw-like pincher appendages found on many species of sea urchins and sea stars (Fig. 3.88). They are typically attached to the echinoderm body at the base of the spines. The name pedicellaria comes from the Latin root words ped- meaning foot and -icellus meaning little. A pedicellaria snaps open if something touches its outer surface; it snaps shut if it is touched on its inner surface. Some pedicellariae are toxic, containing a small poison gland. Others have powerful jaws that can crush small organisms.

<p><strong>Fig. 3.88.</strong> Generalized diagram of pedicellariae found on many species of (<strong>A</strong>) sea stars and (<strong>B</strong>) sea urchins</p><br />
<p><strong>Fig. 3.88. </strong>(<strong>C</strong>) Pedicellariae on the aboral surface of a Forbes sea star (<em>Asterias forbesi</em>)</p>


The soft inner organs of sea urchins are protected by a hard structure called a test. An urchin test is a hard internal skeleton composed of calcium carbonate (CaCO3) plates (Fig. 3.89 A). The plates interlock in a tight geometric pattern that makes the skeleton rigid. Because the test is covered by very thin skin or epidermis, it is considered to be an endoskeleton. Most of the plates have tiny pores through which internal organs of respiration protrude into the seawater. The spines attach to the plates on tubercles, ball-and-socket joints with muscles attached around the base that support and move the spines. The mouthparts of urchins are called Aristotle’s lantern (Fig. 3.89 B and Fig. 3.89 C). Most sea urchins are herbivores and scrape algae from hard substrates with five tooth-like structures in the mouth on the lower surface of the body. Small bits of food move through a long digestive tube to be digested and absorbed. Indigestible material passes out through the anus, opposite the mouth (Figs. 3.90 A and C).

<p><strong>Fig. 3.89.</strong> (<strong>A</strong>) Top, bottom, and side view images of a <em>ha‘uke‘uke kaupali</em> or shingle sea urchin (<em>Colobocentrotus atratus</em>) test</p><br />
<p><strong>Fig. 3.89.</strong> (<strong>B</strong>) Cutaway diagram of sea urchin anatomy</p><br />
<p><strong>Fig. 3.89.</strong> (<strong>C</strong>) Oral surface of a purple sea urchin (<em>Strongylocentrotus purpuratus</em>) showing the five tooth-like structures of the Aristotle’s lantern mouthparts, tube feet, and spines.</p><br />

Class Asteroidea

Sea stars belong to the class Asteroidea (from the Greek word asteroid meaning like a star; Fig. 3.90). Like sea urchins, sea stars inhabit the oceans worldwide, from nearshore tide pools to deep ocean seafloors. Sea stars come in a range of sizes, reaching up to one meter (m) in length, but most are much smaller. Sea stars may be red, blue, or many other colors. Most sea stars have a central disk with five radial arms; some species have 15 to 40 arms. A few species have arms so short that they barely protrude. Their bodies look like pin cushions.

<p><strong>Fig. 3.90.</strong> (<strong>A</strong>) Aboral surface of a sunflower sea star (<em>Pycnopodia helianthoides</em>)</p><br />
<p><strong>Fig. 3.90.</strong> (<strong>B</strong>) Oral surface of a sunflower sea star <em>(Pycnopodia helianthoides</em>)</p><br />

<p><strong>Fig. 3.90.</strong> (<strong>C</strong>) Aboral surface of a cushion star (<em>Culcita novaeguineae</em>)</p><br />
<p><strong>Fig. 3.90.</strong> (<strong>D</strong>) Oral surface of a cushion star (<em>Culcita novaeguineae</em>) with ambulacral grooves visible</p><br />

The sea star’s skeleton, like the sea urchin’s, is an endoskeleton consisting of small plates of calcium carbonate embedded in the epidermis. These plates, called ossicles, are much smaller than those of sea urchins. The sea star’s ossicles are connected by muscles and connective tissue to form a network that lets the arms bend and twist into shapes to fit rocky contours. Some sea stars have spines extending from the ossicles, to help defend them from predators. Sea stars have a water vascular system and tube feet much like those of the sea urchins. Ambulacral grooves (from the Latin root ambul meaning walk) are narrow channels in the oral surface of a sea star filled with tube feet. The tube feet are used mainly for grabbing and locomotion. In some sand-dwelling sea stars the tips of tube feet are paddle-shaped, making them efficient for “walking” and burrowing. Sea stars have remarkable powers of regeneration. Many species can regenerate a whole arm that breaks off (Fig. 3.91 A). These regenerated pieces are called comets (Fig. 3.91 B).

<p><strong>Fig. 3.91.</strong> (<strong>A</strong>) Arm and body regeneration in sea stars</p><br />
<p><strong>Fig. 3.91.</strong>&nbsp;(<strong>B</strong>) A spotted linckia (<em>Linckia multiflora</em>) in “comet” form is regenerating its body from a detatched arm.</p><br />


Sea stars are voracious predators, crawling over the ocean bottom in search of prey. They feed not only on sessile molluscs such as clams, oysters, and mussels, but also on dead organisms lying on the bottom. The crown-of-thorns starfish (Acanthaster planci) consumes so much live coral that it is considered a significant threat to coral reefs in the tropical Indo-Pacific region. The mouth of a sea star opens into the stomach in the central disc. The anus is on the upper surface (Fig. 3.90 A). Most sea stars are carnivores. Although a sea star has no teeth, it can eat coral polyps and molluscs by pushing its stomach out of the body, spreading it over its prey, and digesting it. To eat a clam, the sea star grasps the bivalve in its arms, attaches its suction cups to both shells, pulls steadily until the shells open slightly, and extends its stomach into the clam. In this way it preys on clams whose shells are open as little as 0.1 mm. After the sea star digests and absorbs the tissue of its prey, it sucks its stomach back into its own body.


Class Ophiurodea

<p><strong>Fig. 3.92.</strong> Brittle star showing an enlarged cross-section of one arm</p><br />

Brittle stars are the most abundant echinoderms. About 2,000 species inhabit the ocean floor worldwide, from the shoreline to great depths. In some areas, clusters of millions of brittle stars thickly carpet the bottom. This group is active only at night, hiding under rocks and in crevices during the day. Brittle stars have long, flexible arms attached to a small central disc (Figs. 3.83 C and 3.92). Skeletal ossicles form a series of scaly plates along the arms, and a series of large cylindrical ossicles runs through the center of each arm. These ossicles look somewhat like the row of vertebrae in a fish skeleton. They are connected by muscles that contract, producing a snakelike action. This characteristic movement gives the class its name, Ophiuroidea (from the Greek root words ophio- meaning snake and -uroid meaning tail-like). It is also the basis for another common name, serpent star. Moving brittle stars can appear to be dangerous, but they are harmless to humans. A row of movable spines projecting from the sides of the arms helps the animal move along the bottom. Although the arms appear to be radial, one or two of them usually lead in pulling the animal along while the others trail (Fig. 3.93). These animals got the name brittle star because an arm often breaks off if they are captured. The broken arm is left wiggling as the rest of the brittle star scoots away. The missing arm regenerates quickly. Most brittle stars are small with a central disc diameter less than three centimeters, but the arms may be up to ten centimeters long.

<p><strong>Fig. 3.93.</strong> Movement of a brittle star with one arm leading</p><br />

Brittle stars feed on detritus—small particles of food—on the bottom. Some brittle stars curve their arms up to collect food particles suspended in the water. The tube feet, shaped like pointed tentacles, are used mainly for collecting food. One tube foot passes particles to another toward the mouth. The food then passes into the stomach, where it is digested. Unlike the other echinoderms, brittle stars have no anus; they eject undigested material through the mouth.


Class Holothuroidea

The class Holothruoidea is better known by as the sea cucumbers. Sea cucumbers are cylindrical echinoderm animals with feathery tentacles at the mouth end of their bodies. They are often mistakenly called worms. Some species resembles fat pickles a few centimeters long (Fig. 3.94 A). Others are like thin tubes over a meter long (Fig. 3.94 B). These animals are common residents of reefs and rocky shorelines worldwide. A few species swim constantly in the water, seldom touching the bottom; they are the only members of this phylum to do so. Some Pacific islanders collect sea cucumbers, remove their intestines, and dry the muscular body wall, making a food eaten in many countries.

<p><strong>Fig. 3.94.</strong> (<strong>A</strong>) Filter-feeding sea cucumber</p><br />
<p><strong>Fig. 3.94.</strong> (<strong>B</strong>) The Lion’s paw or sticky snake sea cucumber (<em>Euapta godeffroyi</em>) uses its tentacles to collect food particles from the sediment surface.</p><br />

<p><strong>Fig. 3.94.</strong> (<strong>C</strong>) The pineapple sea cucumber (<em>Thelenota ananas</em>) is internationally recognized as an endangered species due to overfishing.</p><br />
<p><strong>Fig. 3.94.</strong> (<strong>D</strong>) A black cotton-spinner sea cucumber (<em>Holothruia forskali</em>) ejects its Cuverian tubules in self-defense.</p><br />


Unlike other groups of echinoderms, sea cucumbers have no large plates or ossicles forming a rigid skeleton. Their skeletal structures are microscopic spicules embedded in the animal’s skin. Because the spicules differ by species, they are useful in identification. Muscles in the body wall of many sea cucumbers are developed enough to aid in locomotion. When the muscles contract, the body becomes firm and rigid. In some species the muscles are so thin that the internal organs show through the body wall. When these animals are taken from the water, the body wall collapses like thin plastic tubing.


The digestive system has a mouth at one end, a digestive tube down the center, and an anus at the other end (Fig. 3.95). The mouth is ringed with tentacles that are modified tube feet. Some species use their tentacles to take in sediment particles rich in plant and animal matter (Fig. 3.94 B). Other sea cucumbers extend their tentacles to snatch passing food particles (detritus and plankton) (Fig. 3.94 A). This behavior makes them look somewhat like sea anemones, and so this class is named Holothuroidea (from the Greek root word holothuroid meaning like a polyp). The digestive tube has a stomach and a long, thin, coiled intestine where food is digested. Indigestible sand and other particles are expelled through the anus. Much the same happens in earthworms, which literally eat their way through soil.


<p><strong>Fig. 3.95.</strong> Internal anatomy of a sea cucumber</p><br />

The respiratory system of sea cucumbers is unusual in its arrangement. They breathe through an internal structure called a respiratory tree, which is attached to the intestine (Fig. 3.95). Seawater taken in through the anus fills this branching structure, where body fluids absorb the oxygen. The water is then “exhaled” through the anus. Because the anus is often open during this respiratory process, other organisms—small crabs and fish among them—sometimes enter and take up residence in the lower digestive tract and respiratory tree (Fig. 3.72 A). A few species of sea cucumbers have a set of tooth-like projections around the anus to ward off invaders.


Some sea cucumbers have another bizarre way of protecting themselves. Cuverian tubules are branches of sea cucumber respiratory trees in the form of long, slender threads (Fig. 3.95). These Cuverian tubules contain both sticky and toxic chemicals. When these sea cucumbers are disturbed, they can eject these sticky threads out the anus, thoroughly entangling any attacking predator (Fig. 3.94 D). The ejected tubules look like strands of limp spaghetti but stick like cobwebs. Under favorable conditions, these internal organs soon regenerate.


Class Crinoidea

The sea lilies and feather stars reside within the class Crinoidea (from the Greek root word crino meaning lily). Sea lilies are sessile organisms attached to the substrate by a flexible stalk (Figs. 3.96 A and B). The digestive organs are in a bud at the top of the stalk called the calyx. The arms of the crinoid extend out from the calyx. These arms are made up of the calcareous plates seen in other echinoderms. Like the brittle stars, they are jointed for flexibility. Each arm has am ambulacral groove containing tube feet in the center and is lined on each side with tubular extensions called pinnules. The feathery arms are used to collect food from the water, thus crinoids are filter feeders. Feather stars are similar in body form to sea lilies (Fig. 3.96 C and D). Rather than an attached stalk, feather stars have small flexible appendages called cirri at the base of the calyx. These appendages allow feather stars to move around. Some feather star species can even use their arms to actively swim (Fig. 3.96 E).

<p><strong>Fig. 3.96.</strong> (<strong>A</strong>) Deep sea crinoid attached by stalk</p><br />
<p><strong>Fig. 3.96.</strong> (<strong>B</strong>) Stalked crinoid or sea lily in the Mariana Trench</p><br />

<p><strong>Fig. 3.96.</strong> (<strong>C</strong>) Feather star</p><br />
<p><strong>Fig. 3.96.</strong> (<strong>D</strong>) <em>Lamprometra</em> sp. feather star with retracted arms, East Timor</p><br />
<p><strong>Fig. 3.96. </strong>(<strong>E</strong>) Feather star using its arms to swim</p><br />


Activity: Comparing Echinoderms

Use investigative skills to compare different groups within the phylum Echinodermata.


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Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.