- Key People:
- Alexander Agassiz
Water-vascular system
The water-vascular system, which functions in the movement of tube feet, is a characteristic feature of echinoderms, and evidence of its existence has been found in even the oldest fossil forms. It comprises an internal hydraulic system of canals and reservoirs containing a watery fluid, the system consisting of a sieve plate, or madreporite, and a ring vessel, or water-vascular ring, that are connected by a frequently calcified vessel called the stone canal. Five radial water canals extend outward from the ring vessel and give rise to branches that end in the tube feet, which are in contact with the sea. The ring vessel in ophiuroids, asteroids, concentricycloids, and holothurians has bulbous cavities called Polian vesicles, which apparently maintain pressure in the system and hold reserve supplies of fluid; ophiuroids have four or more vesicles, asteroids five, holothurians from one to 50. Crinoids lack Polian vesicles, and echinoids have five structures known as either Polian vesicles or spongy bodies.
The madreporite, which is usually located externally, takes in water from outside the body; if internally located, as is the case in many holothurians, fluid is taken from the body cavity. The water or fluid passes from the madreporite to the ring vessel and along the radial canals to the tube feet. The tube feet are extended by contractions of localized muscle areas in the radial canals (ophiuroids) or by contractions of offshoots of the radial canals called ampullae (asteroids, concentricycloids, echinoids, and holothurians); the contractions force fluid into the tube feet, which then extend.
The structure of the system varies from group to group; asteroids frequently have more than one madreporite, and in holothurians, the madreporite is usually internal, hanging in the coelom. Radial canals may lie inward or outward from the skeleton. The tube feet may have well-developed suckers with great holding power, may taper to a point, or may be adapted for respiration, feeding, burrow building, mucus production, or sensory perception. Attachment of tube feet to hard substrates is achieved through a combination of suction and mucus production. The mucus contains adhesive and de-adhesive mucopolysaccharides. Respiratory tube feet have high oxygen uptake; they are usually located on parts of the body where water flow is unimpeded. Tube feet have been implicated in photoreception and chemoreception; the eyespots in the terminal tentacles of asteroids are the most conspicuous photoreceptors.
The tube feet of crinoids are arranged in clumps of three on the arms and on the pinnules. They secrete and spread a net of sticky mucus that traps small organisms. In ophiuroids the tube feet are used to gain a hold on a surface and to pass food to the mouth. The numerous tube feet of asteroids are used in locomotion; asteroids with suckered feet may use them to exert a continuous pull on the valves of shellfish (e.g., oysters, mussels) until muscles holding the valves tire and open slightly, allowing the asteroid to insert its stomach. In sea daisies the ring of tube feet is probably used for attachment to substrates. Holothurians use tube feet for the same purpose. Tentacles around the mouth of holothurians are modified tube feet used to capture food; tentacles used to capture plankton are branched and sticky, while those used to scoop mud and shovel it into the mouth have a simpler structure.
The tube feet of echinoids serve a variety of functions. The mouth of regular echinoids is surrounded by sensory tube feet, and tube feet farther from the mouth are used in locomotion. On the upper side of the body near the anus, the tube feet have respiratory and sensory functions. The tube feet of irregular echinoids, which burrow, are modified in various ways for feeding, burrow construction, and sensory and respiratory functions.
Body wall and body cavity
The outer body wall (epidermis) contains hairlike projections (cilia) in most echinoderms except ophiuroids; the body wall of crinoids has relatively few. The cilia produce a waving motion that carries food particles toward the mouth or removes unwanted particles from the body. The epidermis also contains glandular and sensory cells. The epidermis of skeletal elements such as spines and pedicellariae, which project from the body surface, often is worn away. The next layer, the dermis, includes the calcareous skeleton and connective tissues. Internal to the dermis are circular and longitudinal muscle layers. The extensive body cavity (coelom) is modified to form several specialized regions. Two subdivisions of the coelom are the perivisceral coelom and the water-vascular system. The perivisceral coelom is a large, fluid-filled cavity in which the major organs, particularly the digestive tube and sex organs, are suspended. Other regions of the coelom include the axial sinus (absent from adult holothurians and all echinoids), the madreporic vesicle, and the hyponeural sinus (often called the perihemal system).
Alimentary and blood systems
The digestive canal consists of a tube, which is almost straight (asteroids and ophiuroids), coiled in a clockwise direction (crinoids and holothurians), or coiled first clockwise, then counterclockwise (echinoids). The tube may be divided into esophagus, stomach, intestine, and rectum. Specialized branches of the digestive tube enlarge the digestive surface and may serve other functions; e.g., digestive glands of asteroids, diverticula of echinoids and crinoids, siphons in echinoids, and respiratory trees in holothurians. The anus, absent in ophiuroids and a few asteroids, is present in most groups. The mouth is near the centre of the oral surface, at the point of convergence of the areas containing the tube feet.
The blood system is a complex system of spaces that are neither part of the coelom nor true vessels. A hemal ring and five radial hemal canals surround the esophagus and radial canals of the water-vascular system. A sixth hemal space arises from the hemal ring and enters the axial organ. In addition, a complex network of hemal spaces is associated with the alimentary canal and gonads.
Axial organ
The axial organ, a complex and elongated mass of tissue found in all echinoderms except holothurians, represents the common junction of the perivisceral coelom, the water-vascular system, and the hemal system. Although its functions are not yet well understood, the axial organ plays a part in defense against invading organisms, can contract, is responsible for a circulation of fluids, and may have excretory and secretory activity.
Nervous system and sense organs
The echinoderm nervous system is complex. In all groups, a nerve plexus lies within and below the skin. In addition, the esophagus is surrounded by one to several nerve rings, from which run radial nerves often in parallel with branches of the water-vascular system. Ring and radial nerves coordinate righting activity.
Although echinoderms have few well-defined sense organs, they are sensitive to touch and to changes in light intensity, temperature, orientation, and the surrounding water. The tube feet, spines, pedicellariae, and skin respond to touch, and light-sensitive organs have been found in echinoids, holothurians, and asteroids.
Reproductive system
The masses of sex cells that compose the gonads of crinoids fill special cavities in the arms or pinnules. Crinoids are the only echinoderms with gonads outside the main body cavity, probably because its volume is reduced. Asteroids typically have 10 gonads, two in each arm, which are located near the arm base, appearing as a feathery tuft or a mass of tubules resembling a bunch of grapes. The gonads of some species are arranged in rows along each arm. Concentricycloids have five pairs of saclike gonads. Ophiuroids have gonads attached to sacs that hang into the body cavity; the sacs, which open outside the body at the bases of the arms, may have one gonad or as many as 1,000.
Regular echinoids typically have five gonads attached to the interambulacra. A duct from each gonad opens to the exterior near the anus. Most irregular echinoids have four gonads, some have three or five, a few have two; the ducts are on the upper surface of the body. Holothurians differ from all other living echinoderms in having a single gonad, which consists of branching or unbranched tubules; the tubules open into a single duct, which opens to the exterior near the front end of the body. Since many early fossil echinoderms have a single genital opening, or gonopore, it is assumed that these forms also had only one gonad; the condition in holothurians thus is regarded as primitive.
Paleontology and evolution
Extinct echinoderms
Because the phylum Echinodermata was already well diversified by the Lower Cambrian Period, a considerable amount of Precambrian evolution must have taken place. A Precambrian fossil from Australia has triradiate symmetry and a superficial resemblance to an edrioasteroid; it has been suggested that the triradiate condition may have been a precursor of pentamerous symmetry, and that this fossil is a “pre-echinoderm.” Scientists speculate that the lack of Precambrian fossil echinoderms indicates that while the earliest echinoderms may have possessed a water-vascular system, they lacked a calcite skeleton and thus did not fossilize. While the fossil record of echinoderms is extensive, there are many gaps, and many questions remain concerning the early evolution of the group. Ancient echinoderms exhibited an extraordinary variety of bizarre body forms; the earliest classes seemed to be “experimenting” with body shapes and feeding mechanisms; most were relatively short-lived. Early echinoderms were adapted to life on the surface of hard or soft seafloors, though the burrowing habit may have been acquired relatively early by sea cucumbers.
Extant echinoderms
Relationships among the living classes of echinoderms have been the subject of debate for many decades. Some scientists believe that larval stages reflect the interrelationships of the groups; thus, because sea urchins and brittle stars have pluteus larvae, they form a natural group, and starfishes and sea cucumbers form another for the same reason. Some biochemical studies support this scheme. On the other hand, comparative anatomy and some paleontology studies suggest that brittle stars and starfishes may have originated from a crinoidlike ancestor and should be placed together, and their general star shape would support this. Modern sea cucumbers and sea urchins share a globoid body but little else; however, some fossil sea urchins with overlapping skeletal plates share several features with some sea cucumbers.
Classification
Distinguishing taxonomic features
The classification of the echinoderms underwent a great upheaval during the 1970s and 1980s, and much disagreement remains. The five subphyla presented here are based upon combinations of characters: Homalozoa are asymmetrical; Blastozoa are stalked, with simple feeding apparatus; Crinozoa are stalked, with complex feeding apparatus; Asterozoa are star-shaped; Echinozoa are globoid to discoid. Below the subphylum level, the criteria for classification vary, but the skeleton is the most important; most groups can be characterized on the basis of skeletal characters alone.
Annotated classification
The echinoderms once were divided into two great groups, the Pelmatozoa and the Eleutherozoa, the names referring to living habits; pelmatozoans were attached to the seafloor for at least part of their life cycle while eleutherozoans were unattached animals capable of moving freely over the seafloor. It has been argued that such a separation is confusing, because each group contains a mixture of subgroups bearing no relationship to the evolutionary history of the phylum. The terms pelmatozoan and eleutherozoan are often used to describe the life habits of echinoderms. Some sea cucumbers, for example, have adopted a pelmatozoan habit, attaching themselves to rocks and feeding on plankton; others are eleutherozoan, moving about the seafloor while feeding, or even actively swimming.
The classification presented here is based upon current research by paleontologists and zoologists. Totally extinct classes, marked with a dagger (†), are known only as fossils.
- Phylum Echinodermata (echinoderms)
- Marine invertebrates worldwide in distribution; skeleton composed of calcium carbonate in the form of calcite; most fossils and all living representatives with 5-part body symmetry (pentamerous); part of body cavity (coelom) comprises a water-vascular system. Cambrian to Recent. About 6,000 extant species, about 13,000 extinct species described.
- †Subphylum Homalozoa (carpoids)
- Middle Cambrian to Middle Devonian about 365,000,000–570,000,000 years ago; without 5-part symmetry; with fundamentally asymmetrical flattened body.
- †Class Stylophora
- Middle Cambrian to Upper Ordovician about 460,000,000–540,000,000 years ago; with unique single feeding arm sometimes interpreted as a stem.
- †Class Homostelea
- Middle Cambrian about 540,000,000 years ago; no feeding arm, but with stem of essentially 2 series of plates.
- †Class Homoiostelea
- Upper Cambrian to Lower Devonian about 400,000,000–510,000,000 years ago; with a feeding arm and a complex stem composed in part of more than 2 series of plates.
- †Class Ctenocystoidea
- Middle Cambrian about 540,000,000 years ago; no feeding arm and no stem, but with unique feeding apparatus consisting of a grill-like array of movable plates around mouth.
- †Subphylum Blastozoa (blastozoans)
- Cambrian to Permian about 280,000,000–540,000,000 years ago. Stalked echinoderms with soft parts enclosed in a globular theca (chamber) equipped with simple, erect food-gathering appendages (brachioles).
- †Class Eocrinoidea
- Lower Cambrian to Silurian about 430,000,000–570,000,000 years ago; body usually consisting of stem, theca, and feeding brachioles.
- †Class Blastoidea
- Silurian to Permian about 280,000,000–430,000,000 years ago; stem, theca with 18–21 plates arranged in 4 rings; numerous feeding brachioles; distinctive infoldings of theca (hydrospires) well developed.
- †Class Paracrinoidea
- Middle Ordovician about 460,000,000 years ago; with stem, theca, and arms with barblike structures (pinnules); plates of theca with pore system of unique type.
- †Class Parablastoidea
- Lower to Middle Ordovician about 460,000,000–500,000,000 years ago; resemble Blastoidea but differ in structure of ambulacra and in numbers of thecal plates.
- †Class Rhombifera
- Lower Ordovician to Upper Devonian about 350,000,000–500,000,000 years ago; theca globular; respiratory structures rhomboid sets of folds or canals.
- †Class Diploporita
- Lower Ordovician to Lower Devonian about 400,000,000–500,000,000 years ago; theca globular; respiratory structures pairs of pores.
- Subphylum Crinozoa
- Both fossil and living forms (Lower Ordovician about 500,000,000 years ago to Recent); with 5-part symmetry; soft parts enclosed in theca, which gives rise to 5 or more complex feeding arms.
- Class Crinoidea (sea lilies and feather stars)
- Lower Ordovician about 500,000,000 years ago to Recent; with, or secondarily without, a stem; theca reduced to small, cup-carrying hollow, usually branching, feeding arms with numerous small pinnules; includes fossil subclasses Camerata, Inadunata, and Flexibilia; living subclass Articulata, which includes stalked sea lilies and unstalked feather stars; about 700 living species.
- Subphylum Asterozoa
- Fossil and living forms (Lower Ordovician about 500,000,000 years ago to Recent); radially symmetrical with more or less star-shaped body resulting from growth of arms in 1 plane along 5 divergent axes; central mouth; 5 arms; dorsal tube feet and mouth.
- Class Stelleroidea
- Features as subphylum above.
- †Class Somasteroidea
- Lower Ordovician to Upper Devonian about 350,000,000 years ago. Superficially like Asteroidea, without a groove for tube feet.
- Class Asteroidea(starfishes or sea stars)
- Fossil and living forms (Middle Ordovician about 460,000,000 years ago to Recent); about 1,800 living species; arms broad, hollow; pinnate structure or arrangement of arms disrupted by dominant longitudinal growth axes; tube feet numerous, carried in grooves on the oral surface of the body; tube feet pointed or equipped with terminal suckers; respiration often by interradial gills on aboral surface of body; includes living orders Platyasterida, Paxillosida, Valvatida, Spinulosida, Forcipulatida, Notomyotida, and Brisingida.
- Class Ophiuroidea(brittle stars or serpent stars)
- Fossil and living forms (Ordovician about 460,000,000 years ago to Recent); disk sharply distinct from long, slender, solid arms; no furrow for tube feet; no suctorial tube feet; no anus; no pedicellariae; respiration by interradial gills on oral surface of body; includes living orders Oegophiurida, Phrynophiurida, and Ophiurida; about 2,000 living species.
- Class Concentricycloidea(sea daisies)
- Body flattened, disk-shaped, without obvious arms; water-vascular system with tube feet on oral surface of body; water-vascular canals form double ring; includes order Peripodida; 2 living species.
- Subphylum Echinozoa
- Fossil and living forms (Lower Cambrian about 570,000,000 years ago to Recent); radially symmetrical with fundamentally globoid body secondarily cylindrical or discoid; outspread arms or brachioles totally absent.
- †Class Cyclocystoidea
- Middle Ordovician to Middle Devonian about 375,000,000–460,000,000 years ago; small, disk-shaped; theca composed of numerous plates; ambulacral system with multiple branching.
- †Class Edrioasteroidea
- Lower Cambrian to Lower Carboniferous about 340,000,000–570,000,000 years ago; discoid to cylindrical; 5 well-developed straight or curved ambulacral food grooves radiate from a central mouth.
- †Class Edrioblastoidea
- Middle Ordovician about 375,000,000 years ago; stalked form with spheroidal theca; 5 well-developed food grooves.
- †Class Helicoplacoidea
- Lower Cambrian about 570,000,000 years ago; pear-shaped or spindle-shaped body with many plates arranged spirally.
- †Class Ophiocistioidea
- Lower Ordovician to Upper Silurian about 395,000,000–500,000,000 years ago; dome-shaped body partly or completely covered by well-developed test; 5 ambulacral tracts carry plated tube feet relatively enormous in size.
- Class Holothuroidea (sea cucumbers)
- Fossil and living forms (Ordovician about 460,000,000 years ago to Recent); cylindrical body, elongated orally–aborally, with mouth at or near one end, anus at or near the other; mouth surrounded by conspicuous ring of feeding tentacles; no spines or pedicellariae; single interradial gonad; skeleton usually reduced to form microscopic spicules; includes living orders Dendrochirotida, Dactylochirotida, Aspidochirotida, Elasipodida, Molpadiida, and Apodida; 1,100 living species.
- Class Echinoidea (sea urchins, heart urchins, sand dollars)
- Fossil and living forms (Ordovician 460,000,000 years ago to Recent); globular, discoid, or oval in shape, with complete skeleton (test) of interlocking plates bearing movable spines and pedicellariae; mouth directed downward; anus present; 5 or fewer interradial gonads. Includes subclass Perischoechinoidea with living order Cidaroida, and subclass Euechinoidea with living superorders Diadematacea and Echinacea (comprising the “regular” echinoids), and Gnathostomata and Atelostomata (comprising the “irregular” echinoids); 900 living species.
