Phylum Echinodermata

Dr. Elizabeth A. Bergey and Dr. Eric G. Bright, University of Oklahoma

Modified, with permission, from Invertebrate Anatomy OnLine
copyright 2003 by Richard Fox (Lander University)

Echinodermata Overview

Echinoderms are secondarily radially symmetric - their ancestors were bilaterally symmetric.  The adult radial symmetry is pentamerous, with body parts occurring in fives or multiples of five.  All echinoderms are marine and benthic. About 6000 extant species are known, but the fossil record includes about 13,000 extinct species.

The water vascular system is an important echinoderm characteristic that in most groups controls the locomotory tube feet, but is also important in gas exchange, excretion, and feeding.  The body wall includes a thick connective tissue dermis with calcareous ossicles (‘little bones’).  These ossicles make up an endoskeleton which assumes different forms in different taxa.  In most echinoderms, calcareous spines of various sizes and shapes arise from the dermis and extend from the body surface; hence the name echinoderm (= ‘spiny skin’).  The connective tissue is changeable and its consistency is under nervous control.

Excretion in echinoderms is by simple diffusion of metabolic wastes (ammonia) across thin permeable regions of the body wall.  A variety of gas exchange structures, including the tube feet, is found in various echinoderms.  A hemal system is present but its role in transport is still poorly understood and the chief transport system is the circulating fluid of the various coelomic compartments.  The hemal system may be a transport system that delivers nutrients from the gut to the coelomic compartments for local distribution.  The nervous system consists of two central nerve rings with radial nerves to the periphery.  Echinoderms are dioecious and fertilization is usually external. 

Class Asteroidea (starfish; sea stars)

Starfish are the best known echinoderm group.  Starfish usually have five arms, but sometimes more, radiating from a central disk.  The ossicles of the body wall are rodlike and articulate via fibrous junctions to form a flexible grid.  Respiration is by the tube feet and skin gills. Each arm has an eyespot at its tip. A pair of large pyloric ceca and a pair of gonads are present in each arm. About 1500 living species are known.

The sea star, Asterias forbesi, is common in shallow water along the Atlantic Coast of North America from the Gulf of Maine to the Gulf of Mexico.

The body is divided into a central disk from which radiate five arms.  The principal body axis, and the axis of symmetry, is the short oral-aboral axis, which passes vertically through the center of the disk.  The animal's pale lower side is the oral surface and the dark upper side is the aboral surface. 

Find the calcareous, orange madreporite on the aboral surface of the disk.  Note its grooved surface (visible under high power with a dissecting microscope).  Numerous microscopic pores in the bottoms of the grooves open into canals (stone canal and axial canal) of the internal water vascular system.

If necessary, remove the congealed mucus and other debris from the aboral surface of the disk with strong jets of water from a squeeze bottle. On the aboral surface notice the numerous small fixed spines, so-called because they are fixed in position and cannot move.  These spines are extensions of the calcareous endoskeleton in the body wall.  Gently push one of the spines with the tip of a needle to see if it moves.  Look closely at the spines with the highest magnification of the dissecting microscope and confirm that they are indeed internal and are covered by a thin layer of living tissue, the epidermis.  Each spine is surrounded by a circle of short-stemmed, white pedicellariae (singular: pedicellaria).  Pedicellariae have an endoskeleton of ossicles. 

Aboral view of Asterias
Cut-away showing internal structure of Asterias

Remove several pedicellariae with your fine forceps and place them in a drop of bleach on a microscope slide.  Wait a few minutes for the organic tissue to be oxidized and then place a coverslip over the drop.  Examine it with the compound microscope and look for the jaw-like ossicles.  These pedicellariae contain three ossicles.  One is a short basal piece in the stalk, whereas the other two support the two jaws.  Tiny muscles extend between these ossicles to operate the jaws but these will have been removed by the bleach.

Between the spines are many soft, thin-walled, translucent, fingerlike skin gills.  Skin gills are thin-walled extensions of the coelom through the body wall and are respiratory structures.  The skin gills are muscular and can be retracted into the surface of the body wall and may be retracted and inconspicuous in preserved specimens.

The anus is located near the center of the aboral surface but is almost impossible to demonstrate externally.

Turn the animal over and study the oral surface. Find the large mouth in the center of the disk, surrounded by the thin peristomial membrane.  The yellowish-orange curtain-like folds of the cardiac stomach may be visible inside the mouth.

Five deep ambulacral grooves radiate outward from the mouth, one along the midline of the oral surface of each arm.  The numerous soft, tubular structures projecting into the groove from either side are the tube feet, or podia.  Two rows of tube feet are present on each side of the groove.  The tube feet of Asterias bear suckers at their distal ends.  Note the rows of long, flattened movable spines on each side of the ambulacral groove.  The word ambulacrum is Latin for "covered way," an apt name as these spines are used to cover the groove to protect the tube feet.

Look at the tip of one of the arms.  As is usual in radially symmetrical animals, the sensory structures are arrayed around the periphery, which in sea stars are the tips of the arms.  Several long, narrow sensory tube feet (with chemo- and mechanoreceptors) extend from the tip of each arm.  These are easily seen in living specimens but contract and become inconspicuous in preserved material. At the tip of the arm is a small circle of short, blunt movable spines that are not associated with pedicellariae.  These spines surround a small, pale red or yellow eyespot.  The eyespot is on the oral surface of the arm, almost at the tip.

Starfish development

Slides of early starfish developmental stages may be available in the laboratory.  These may include all early developmental stages on a single side or on separate slides. Many of the stages are spheres which must be distinguished from each other using various clues.   Keep in mind as you interpret the embryos that they are whole mounts, not sections, and their appearance will vary depending on the level of your plane of focus (optical section).

First in the developmental series is the unfertilized female gamete, the egg, or ovum.  It is a large sphere recognized by the presence of a conspicuous female pronucleus (= germinal vesicle) which contains a distinct nucleolus. The plasma membrane is the outer boundary of the cell and, since the egg has not been fertilized, there is no fertilization membrane. 

Once fertilized, the egg becomes a zygote. This stage is also a sphere but a wrinkled, transparent fertilization membrane is present around the cell. The fertilization membrane is produced immediately after fertilization and prevents penetration by additional sperm. The zygote is the same size as the ovum.

Starfish early development. A. Ovum, B. Zygote, C. 2-celled embryo, D. Polar view of 4-celled embryo, E. Polar view of 8-celled embryo, F. Early blastula, G. Late blastula, H. Early gastrula.

An early bipinnaria larva with a single ciliary band.

Search the side for examples early cleavage stages, namely 2-cell, 4-cell, and 8-cell embryos, each enclosed in a fertilization membrane. Pay attention to the orientation of the blastomeres with respect to each other, especially those of the 8-cell stage.  Observe an 8-cell embryo and verify that the cell arrangement is typical of radially cleaving embryos.  One of the tasks accomplished by early development is conversion of the enormous ovum to a multitude of much smaller cells, closer in size to normal somatic cells.  The embryo does not grow during this period so, since cells are dividing, they must be getting smaller.

Subsequent divisions produce ever smaller cells, which are arrayed in a hollow ball known as a blastula. The blastula is ciliated and if these embryos were alive you could see them rotating inside their fertilization membranes.  As divisions continue the cells in the blastula wall get smaller and smaller until they can no longer be distinguished from each other.  You can, however, still see the blastocoel in the center and the wall of cells enclosing it. The embryo is still about the same size as the original ovum but its cells are much smaller.

Eventually one end of the blastula thickens in preparation for gastrulation.  This end then invaginates to form a double walled embryo known as a gastrula. The early gastrula has only a short invagination but it will rapidly increase in size and you should be able to find examples of several ages.  The blastocoel remains but there is now a second cavity, the archenteron, or embryonic gut. The archenteron opens to the exterior by an opening, the blastopore.

The asteroid gastrula develops into a series of two larvae, the first of which is the bipinnaria larva, followed by the brachiolaria larva. The larvae are ciliated and feed on diatoms in the plankton. Once the embryo begins feeding it can start to grow.  The larvae are bilaterally symmetrical. 

Several examples of bipinnaria larvae should be present on the slide.  The bipinnaria larva has one or two locomotor ciliary bands. Young larvae have only one band but older larvae have two.

With age, the bipinnaria becomes a juvenile starfish. These juveniles are not on the composite slide.  The juvenile starfish has five pairs of ciliated (for swimming) larval arms.  It is the will settle out of the plankton onto a (hopefully) suitable substrate and metamorphose into the pentaradial adult.

Class Echinoidea (sea urchins, sand dollars and kin)

Echinoidea includes about 950 living species of sea urchins, sand dollars, sea biscuits, heart urchins, and their relatives. The dermal ossicles are thin plates fused to form a rigid, more or less spherical, endoskeletal test.  The test is covered by an abundance of movable spinesTube feet are the major respiratory organs and the madreporite is aboral.

Sea Urchins

Urchins may be regular or irregular.  Regular urchins are the sea urchins, with pentaradial symmetry; globose, nearly spherical bodies; and long spines.  Most are benthic surface-dwellers (= epifaunal habitat).  Irregular urchins are sand dollars, sea biscuits, and keyhole, heart, and cake urchins.  These are usually infaunal in soft sediments and have a superficial bilateral symmetry superimposed on their radial symmetry.  The body is usually flattened and the spines short.

Epifaunal species (regular urchins) possess a feeding apparatus known as Aristotle's lantern, equipped with five strong teeth, used for scraping food from hard substrates.  The lantern is reduced in infaunal species (irregular urchins) because most are deposit feeders.

Examine a preserved regular urchin. Note the generally spheroid shape of the urchin.  The lower oral pole is flattened whereas the opposite aboral pole is more rounded. Why?  The surface of the body is firm and rigid due to the underlying test of fused calcareous ossicles.  The test is hollow and most of the animal's soft parts are inside it. The test is an endoskeleton, however, and is located in the connective tissue dermis of the body wall and covered by a thin, inconspicuous, ciliated epidermis.  Because the epidermis is not readily apparent, the test appears to be external.

Most of the surface bears articulated, movable spines, which are also part of the connective tissue skeleton and are also covered by a thin epidermis.  Spine length and structure vary among species. Some species have smaller secondary spines. The basal end of each spine bears a socket that articulates with a ball, or tubercle, on the test.  A ball and socket joint permits motion in a wide range of directions.  An outer ring of muscles and an inner ring of collagen fibers extend from the spine to the test.  Selective contraction of muscles in the outer sheath move the spine in any desired direction atop its tubercle.  The collagen fibers of the inner sheath can be locked so that the spine becomes rigidly fixed. So effective is this locking mechanism that the spines cannot be moved without breaking them. 

The 10 rows of long slender tube feet, or podia, extend between the oral and aboral poles.  The 10 rows of tube feet are paired into five ambulacra, each consisting of two rows of tube feet. The ambulacra are separated by five zones without tube feet.  This arrangement of tube feet is only clear when you study a cleaned test.  The tube feet of regular urchins are normally much longer than the spines but are contracted in preserved animals.

The test is composed of rows of thin calcareous ossicles, or plates.  The rows are easiest to see from the inside.  Hold the test against a white background. The plates should be obvious.  The plates are homologous to the dermal ossicles of other echinoderms and differ from them primarily in that they are thin, plate-like, and fused together.  They form a rigid endoskeketon whose articulations are fixed and immovable. Because the test is rigid, the rest of the body wall is poorly developed and lacks the thick layer of connective tissue and muscles characteristic of other echinoderms. The ambulacral plates are easily recognized because each one bears several pairs of pores (= podial pores) for the tube feet.  Each pair of pores serves one tube foot.  The interambulacral plates do not bear pores.

Aboral poles of a cleaned tests of two different taxa of sea urchin taxa. 

Most tube feet end in wide suckers used to hold the animal firmly to hard substrates.  The tube feet are used for locomotion and respiration, and some urchins use them to hold bits of shell or vegetation above the body, presumably for camouflage or protection from UV light in shallow water.  Tube feet may also be sensory. 

The center of the oral surface has a large hole in the test.  This region is the peristome (peri = around, stome = mouth) and the hole in the test is the peristomial aperture.  You cannot see the aperture in preserved specimens because it is covered by the soft peristomial membrane with the mouth at its center; it is apparent in cleaned tests.

The triangular white tips of the five teeth of Aristotle's lantern can be seen around the mouth.  The mouth opens into the pharynx, which is not externally visible. 

Numerous pedicellariae (singular = pedicellaria) of several types are present on the body surface and some can be seen in the vicinity of the mouth.  Sea urchin pedicellariae have three tiny jaws at the end of a pedicle.  Pedicellariae have a skeleton consisting of three ossicles in the jaws and a long slender ossicle in the pedicle.

Remove a pedicellaria and make a wet mount.  Examine the preparation with the compound microscope and find the calcareous jaws and the calcareous rod in the pedicle.

The periproct is a small region at the aboral pole surrounding the anus.  The periproct is much smaller than the peristome and is harder to see.  The periproct is at the exact center of the aboral surface but the anus is a little off center, near one side of the periproct.  The anus is a large opening on a slight elevation and is surrounded by an irregular array of small spines.  The elevation bears numerous tiny, spiny knobs. The large, triangular madreporite is visible externally and lies to one side of the periproct on an interambulacral axis.  Its surface bears obvious perforations and is covered by a ciliated epithelium.  The madreporite is penetrated by numerous ciliated channels continuous with the stone canal and axial canal of the water vascular system in the interior.

Sand Dollars

 Flattened urchins are known as sand dollars and sea biscuits, and are adapted for living and moving infaunally in soft sediments. The test is flattened and is modified from the radial symmetry of echinoderms to be bilaterally symmetric. Jaws and a simplified Aristotle’s lantern are present.  Respiratory tube feet are arranged in petalloids.

Sand dollars are named because some of them have a flattened disk-like shape that resembles a silver dollar.  Most are infaunal deposit feeders with numerous short spines that are used to burrow through soft sediments.  As an adaptation to facilitate movement through sediment, irregular urchins tend to be slightly elongate and streamlined along an axis perpendicular to the oral-aboral axis.  This destroys the typical echinoderm radial symmetry and replaces it with a slight bilateral symmetry (which is why they are said to be “irregular”).  Notice that this is not the same as the bilaterality of sea cucumbers, in which the anteroposterior axis coincides with the oral-aboral axis.

The lower, oral surface of sand dollars is flattened or slightly concave.  The upper aboral surface is convex.  The antero-posterior axis is perpendicular to the oral-aboral axis.  Note the weak bilateral symmetry in addition to the still-evident radial symmetry.  The anterior margin of the disk is rounded whereas the posterior is truncate, slightly emarginate (indented), or rounded, depending on the species. 

Most of the soft anatomy of the sand dollars resembles that of the regular urchins and the major differences are in the test and respiratory podia.  Specialized features of the sand dollar test are usually studied using cleaned dried skeletons.

The test, or endoskeleton, like that of regular urchins, is made of fused, platelike, calcareous ossicles in the dermis of the body wall.  The lines of fusion may not be readily visible.  The aboral and oral ambulacral centers of the sand dollars are at the approximate center of the animals.  The anus, however, does not coincide with the aboral ambulacral center (it does in regular urchins) but is well off-center.  In contrast, the mouth remains associated with the oral ambulacral center, and for good reason.  The small tube feet of the oral ambulacra make up a set of radiating conveyor belts that transport food particles from the periphery to the central mouth.

As in other echinoids, five double rows of tube feet radiate from the center of the oral surface to the center of the aboral surface.  Each double row is an ambulacrum and the areas between ambulacra are interambulacra.  The arrangement of ambulacra is radially symmetrical.

On the aboral surface five conspicuous ambulacra are called petalloids, in reference to their resemblance to the petals of a flower. Under magnification you can see that each petalloid consists of two rows of paired pores which accommodate the ducts of tube feet as they pass through the test. The tube feet associated with the petalloids are respiratory and are long, low, flat vesicles that lie in the groove extending from one pore to the other. Thus, in irregular urchins some of the tube feet of the aboral surface are respiratory whereas those of the oral surface are used for feeding and/or locomotion. 

Aboral surface of the keyhole urchin
One of the five petalloids of the aboral surface

The test is perforated by five (six in some species, none in others) slots, or lunules that pass entirely through it.

The madreporite is a star-shaped area at the aboral ambulacral center.  Its surface is perforated with tiny madreporic pores.  Nine pores penetrate the test around the periphery of the madreporite. A gonopore is situated at the apex of four of the five points of the madreporite.  The gonopores are interambulacral.  The posterior interambulacrum lacks a gonopore. 

Examine the oral surface with your unaided eye.  The lunules are, of course, evident on this side also because they pass completely through the body.  In contrast, the mouth and anus penetrate only one wall, as do podial pores, gonopores, and tentacular pores.  
The center of the oral surface is the relatively small peristome. The peristomial aperture, which in life would have the mouth at its center, is a circular opening located a little anterior to the center of the oral surface.  It is covered by the peristomial membrane in living dollars.  The five narrow, incised food grooves, or ambulacral furrows, arise singly from the margin of the peristome but each quickly branches to form two food grooves that run beside the ambulacral axes, on either side of the lunules.

Study the oral and aboral surfaces with high power of the dissecting microscope.  Most of the surface is covered with tubercles located in shallow pits.  In life, each tubercle articulates with a movable spine.

Oral surface of a sand dollar

, be cross-linked with each other to form a rigid skeleton that is hard to cut through.

The gut is a simple tube extending from mouth to anus.  Gas excratherhange is via diverticula of the posterior gut (= the respiratory tree).  Large branched tube feet encircle the mouth and are used for feeding.  The hemal system is better developed than is typical in echinoderms and the coelom is large and spacious.  The madreporite opens into the perivisceral coelom, than to the outside.  Sea cucumbers are dioecious and have a single gonad opening to the exterior v

Class Holothuroidea (sea cucumbers)

Sea cucumbers are echinoderms in which the body is elongated along the aboral/oral axis.  The body wall is thick and well developed.  It consists of a nonciliated epidermis, connective tissue dermis, circular and longitudinal muscles, and a ciliated peritoneum.  It is unusual in that its longitudinal muscles are in five longitudinal bands and the ossicles are microscopic plates scattered in the dermis.  Abundant collagen fibers in the mutable (changeable) dermis can, under nervous controlia a gonopore at the oral end. 

The body is elongate with the mouth at the oral end and the anus at the opposite, aboral, end. A weak bilateral symmetry is superimposed on the underlying echinoderm radial symmetry. The aboral-oral axis is the long axis of the body and is the axis of symmetry, both bilateral and radial. Sea cucumbers are radially symmetrical but also possess a superficial bilateral symmetry.

The anterior end bears the mouth and a circle of ten branched tentacles. It is an introvert that can be fully retracted into the body by a set of powerful retractor muscles.  Threatened animals or those preserved without adequate relaxation may have retracted introverts, in which case the tentacles and mouth will not be apparent externally.

The five ambulacra are indicated externally by five longitudinal rows of small, inconspicuous tube feet.  The areas between the ambulacra are the interambulacra. The longitudinal body wall muscles of cucumbers do not occur in a continuous sheet rather are concentrated in five strong longitudinal muscle bands.  These bands are ambulacral in position and can sometimes be seen externally. 

Most cucumbers creep over the surface of the substrate or burrow into it.  A (very) few swim.  When creeping, they keep the ventral surface in contact with the bottom and this surface is specialized for the purpose.  The flat ventral surface has better developed tube feet that usually have suckers.

Find the mouth at the center of the anterior, oral end surrounded by a narrow peristomial membrane.  The ten tentacles are large, branched tube feet, one pair of which is associated with each ambulacrum.  A single small genital papilla lies between the trunks of the two dorsal podia and bears the gonopore. The opposite, aboral end of the cucumber has the anus at its center.

© Copyright by Elizabeth Bergey and Eric Bright 2016

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