Echinoderms Are Secondarily Radially Symmetric Deuterostomes Whose Ancestors Were Bilaterally

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Echinoderms are secondarily radially symmetric deuterostomes whose ancestors were bilaterally symmetric. The adult radial symmetry is pentamerous with body parts occurring in fives or multiples thereof. Echinoderms have strong affinities with the ancestral trimeric deuterostomes especially in the tripartite organization of the coelomic cavities. Echinoderm larvae have the coelom divided into three regions, as is typical of the early coelomates, and these regions have important adult derivatives. All echinoderms are marine and benthic. About 6000 Recent species are known but the fossil record includes 13,000 extinct species.

An important echinoderm characteristic is the water vascular system that in most groups functions in support of locomotory tube feet but is also important in gas exchange, excretion, and feeding. The body wall includes a thick connective tissue dermis in which calcareous ossicles (little bones) are almost always an important component. 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 and are alluded to by the name echinoderm (= spiny skin). The connective tissue is mutable and its consistency is under nervous control.

Excretion in echinoderms is accomplished 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 through transport system that delivers nutrients from the gut to these compartments for local distribution. The nervous system consists of two central intraepidermal nerve rings from which arise radial nerves to the periphery. Echinoderms are gonochoric and fertilization is usually external.

This week in laboratory, you will start your comparative study of echinoderms. One individual should read the text to a partner who trys to find the structures described. Be sure to switch duties often. Next week you will be expected to be familiar with the structures you will be looking at this week and do more videotaping of echinoderms. So please take the time to find and learn the basic morphology of echinoderms this week. Your photographs should be good enough to serve as comparisions to those you will obtain next week.

Exercise one to three: Asteroidea

Asteroids are the sea stars, which are the best known echinoderms. Sea stars 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 with the tube feet and papulae. Each arm has an eyespot at its tip. A pair of large pyloric ceca and a pair of gonads are present in each arm. Gastric hemal tufts are present. About 1500 Recent species are known.

Exercise one: Try to film feeding in one of the species available of sea stars Obtain a specimen of one of the species available, place in a small dish and add a few (one maybe two) fish flakes to the dish near the animals for the small species and one small fish for the larger species. You may have to place the larger species on the fish to encourage feeding. Wait till the animals start to feed and then start your observations. Once they start actively feeding, turn the animal upside down and see if you can see exactly how the animal feeds. The larger species may invert their stomach while feeding.. If the large species is uncooperative, we may have some smaller species that will feed upside down on fish food.

Exercise two: External Anatomy

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 structures on this side are said to be oral. The dark upper side is the aboral surface and structures on this side are aboral. Structures remote from the axis are said to be peripheral whereas those near the axis are central. In radially symmetrical animals anterior-posterior, dorsal-ventral, right-left are irrelevant and have no meaning.

Aboral Surface

Find the calcareous, usually orange or brown madreporite on the aboral surface of the disk Examine it with the high power of the dissecting microscope and note its grooved surface. Numerous microscopic pores in the bottoms of the grooves open into canals (stone canal and axial canal) of the internal water vascular system .

Orient the star with the aboral side up and with the madreporite close to you. The arm on the left of the madreporite is arm I, arm II is to the right of the madreporite, and the remaining arms are numbered sequentially moving counterclockwise around the star .

Aboral view of Asterias.

The surface is covered by a monociliated epidermis. Can you see evidence of such under the stereoscope?

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.

You may also be able to see circle of short-stemmed, white pedicellariae (singular: pedicellaria), around the spine.. Pedicellariae have an endoskeleton of ossicles . These can move and in some species have jaws that open and close. Can you photograph or film the pedicellaria found in the species available. Do they have movable jaws?

See if you can see soft, thin-walled, translucent, fingerlike papulae between the spines as well. Papulae are thin-walled diverticula of the coelom through the body wall and are its respiratory organs. The ciliated peritoneum generates a bidirectional flow of fluid into and out of the papulae. The papulae are muscular and can be retracted into the surface of the body wall. They may be retracted and inconspicuous in somespecimens. Touch a papula with the microneedle and observe its response.

The anus is located near the center of the aboral surface but is almost impossible to demonstrate externally. It is surrounded by a palisade of tiny ossicles, much smaller than the spines that stud the surface of the disk and is in an area free of papulae.

Exercise two a: Photograph the aboral surface or in large specimen the area aroung madreporite. Label the madreportie and spines.

Oral Surface

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 . Each groove lies on an ambulacral axis. 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 is the tips of the arms. Several long, narrow sensory tube feet extend from the tip of each arm. These are easily seen in living specimens but contract and become inconspicuous in preserved material. They have chemo- and mechanoreceptors. 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.

Exercise three: a. Take a photograph of one of the arms at high power and label what your can. Turn the animal over and watch it move, which it will want to do and quickly away from you. B. Describe the movement in your journal.

Exercise three and four: Ophiuroidea

Also known as brittle stars or serpent stars, ophiuroids have long sinuous arms sharply demarcated from the central disk. The arms contain vertebra-like ossicles and are very flexible, exhibiting a motion reminiscent of that of snakes (ophio = snake). About 2000 living species are known.

The ophiuroid gut ends blindly, there being no anus or intestine. Digestion and absorption occur in the stomach and pyloric ceca are absent. The digestive system is confined to the central disk with none of it extending into the slender arms. The madreporite is oral and the tube feet lack suckers. The gonads open into invaginations of the integument which also serve as respiratory organs.

Brittle stars are not as cooperative as sea stars and are delicate animals. Our specimens are already showing signs of stress. Try never to expose them to air as you transfer specimens from one container into another. Also try to be very gentle with them, ao both classes have good experiences with the specimens we have. Use the handles, instead of the tips of forceps or teasing needle to poke and prode them.

Exercise three, movement: Treat a brittle star as the sea star a. Watch the animals move and compare their movements with that of a sea star in your journal. If your animal is reluctant to move freely, shine a bright light on the animal. Brittle stars are noctornal and avoid light. Next give your specimen a rock to hide and explore. b. How do movements over a rock compare with a brittle star moving freely in water and away from you? Record your observations in your notebook.

Exericise 4. External Anatomy

The dark upper side of Ophioderma is the aboral surface and the pale side opposite it is the oral surface. Note the well defined central disk (Fig below) from which extend five long snakelike arms (ophio = snake.) The aboral-oral axis runs through the center of the disk perpendicular to the surfaces and is the axis of radial symmetry. Structures remote from the axis are said to be peripheral whereas those near the axis are central. In radially symmetrical animals anterior-posterior, dorsal-ventral, right-left are not applicable.

Aboral Disk Surface

a. Take a photograph of the animal’s central disk. The following description of the aboral surface should help you locate important structures.

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Examine the aboral surface of the disk and arms undre low and high magnification. Ophiuroids have no papulae, no pedicellariae, and no paxillae such as are common in asteroids. The epidermis is a thin, non-ciliated syncytium and most of the body wall is a connective tissue dermis, which contains abundant calcareous ossicles of many sizes and shapes and are often known as shields.

The aboral disk surface is covered by small, spherical, calcareous dermal granules, which are themselves covered by epidermis In many genera the aboral disk bears larger, distinct, scalelike ossicles. The granules are best seen at about 40X magnification. At the base of each arm, but on the disc, are two large oval radial shields . Radial shields are an obvious feature of the aboral disks of most ophiuroids but soma are covered by dermal granules and cannot be seen. There are no distinct muscle layers in the body wall of ophiuroids

Aboral surface of the arm of Ophioderma brevispina.

Most of the large dermal ossicles, or shields, covering the aboral surface of the disk are hidden by dermal granules but the shields of the arms are exposed and easily observed. Most important are the four rows of overlapping dermal shields on each arm. A row of aboral arm shields runs along the top of the arm and similar oral arm shields extend along the bottom. Each side of the arm bears a row of lateral arm shields. Thus the arm is encircled by a linear series of rings, each consisting of four shields. The name "ophioderma" means snake skin. Is the name apt?

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b. Take a photograph of the oral surface and label as many structures as you can. Label at least the mouth and tube feet. The diagram and descriptions below are provided to help you. Try to see as much as possible in the leg region. Brittle stars are not as cooperative as sea stars and may quickly turn over. Again be patient with these beautiful animals, they break off legs easily and often develop fatal infections if they do. The following description should help you locate important structures.

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Oral Disk Surface

Turn the animal over and examine the oral surface of the disk with magnification. Unlike that of the aboral disc, the epidermis of the oral disk is ciliated.