Phylum Echinodermata "Spiny Skin"

Sea Stars, Sea Urchins, Sand Dollars, Sea Cucumbers, Sea Lillies 


Five Fold Radial Symmetry

Limb Regeneration Abilities

Water Vascular System

Calcium Carbonate Endoskeleton

Mutable Collagenous Tissue



Water Vascular System – Movement is accomplished using hydraulics powered by muscles that expand reservoir bulbs called ampullae. Ampullae power the suction tube feet.    




Calcium Carbonate Endoskeleton – formed from a biomineral matrix composed of calcium carbonate and protein. The calcium carbonate is deposited as numerous tiny crystaline plates (ossicles) with a sponge-like microstructure that is unique to the phylum. It is termed an endoskeleton because it is covered on the outside by a thin layer of epidermis. Functionally, in the majority of classes, the structure acts more like an exoskeleton, enclosing other tissues in a tough covering.




Mutable Collagenous Tissue - Ligaments composed predominantly of collagen protein interconnect the ossicles of echinoderms. The material properties of this ligamentous connective tissue are able to morph on a short timescale (seconds to minutes).  The change, between stiff and flexible forms is under neuronal control.  Ligaments are normally "locked" (rigid), but can be temporarily "unlocked" (loosened). This provides important mechanical advantages, including the ability to maintain a variety of postures with no muscular effort. This allows starfish to cling to rocks in surf zones. 


Reproduction - Echinoderms are typically dioecious (separate sexes) with no visible sexual dimorphism.  External fertilization is the usual. Some species brood their eggs.  Some reproduce by fission.  

Consumption – 

Digestive System – systems vary by class – filter feeding, external digestion, & crushing "teeth" 

Gas Exchange – many lack a defined respiratory system.  In-pouching and out-pouching of the body wall aids gas exchange. 



Hemolymph / Excretory System - linked into an "axial complex."  It is composed of a thickened vessel (the "heart"); a region where ultrafiltration occurs via podocytes; a closed circulatory system; and an opening to the sea called the madreporite.

Growth - life cycle is typically complex, with free-living larvae that feed on plankton. Larval forms show bilateral symmetry - Metamorphosis occurs during settlement.

Environmental Response - Decentralized Nervous System

The system is poorly understood.  No known echinoderm contains anything that could be called a brain, although ganglia are present along the radial nerves in some. Echinoderms lack any trace of cephalization and have no identifiable specialized sense organs. Sensory neurons are located within the ectoderm of podia, and send axons to the radial nerves. Even so, many echinoderms show clear evidence that they sense and react to their environments in a variety of ways.



Class Asteroidea – Sea Stars (Starfish)

Star-shaped body plan consisting of a central disc and multiple (typically 5) radiating arms. True starfish have no sharp demarcation between arms and central body. Sea stars are common along shorelines.  Many gather on rocks but some live on muddy or sandy bottoms or among coral reefs. They range from a centimetre in size to over a meter. Many asteroids can regenerate limbs, and in some cases, entire bodies. Some require the central body to be intact to regenerate, but a few species can grow an entirely new sea star just from a portion of a severed limb.

Motion - Locomotion by asteroids is accomplished by means of the podia (tubefeet) powered by the water vascular system.  Each arm contains rows of tiny, flexible tube feet that are operated by a hydraulic system. Intricate canals draw in seawater and channel it to the feet, enabling them to extend. Suction cups on the bottom help the sea star move with a grip-and-pull action. The tube feet have suckers, which chemically adhere to the substrate. A chemical bond is secreted to release the tube feet.

Reproduction - Asteroids can regenerate arms and some can reproduce asexually when the central disc divides. In sexual reproduction, asteroids are mainly dioecious (having separate sexes), but a few are hermaphroditic. Asteroids usually have two gonads in each arm and a gonopore opening to the oral surface. Most asteroids are free spawners, releasing sperm and eggs into the water. A female starfish sheds several million eggs in just a few hours. A few species brood their young.

Consumption - A sea star's mouth is located on its underside, right in the middle of its central disk. Most sea stars are predators, feeding on sessile or slow-moving prey such as molluscs and barnacles. The aptly named crown-of-thorns, Acanthaster, specializes in corals, and may do considerable damage to coral reefs. Sea stars can consume prey outside their bodies. Using tiny, suction-cupped tube feet, they pry open clams or oysters, then their sack-like cardiac stomach emerges from their mouth and oozes inside the shell. The stomach envelops the prey to digest it, and finally withdraws back into the body.






Growth - Fertilized eggs develop into bilaterally symmetrical planktonic larvae. The larva has ciliated bands running about the edges. After several weeks, the larva takes on a more elaborate form, with longer projecting arms.  After this phase, a large part of the larva degenerates and a rudimentary juvenile starfish develops. The whole process, from egg to miniature starfish, takes about two months, depending somewhat on water temperatures. A starfish is ready for reproduction after about one year.

Environmental Response - The non-centralized nervous system allows echinoderms to sense their environment from all sides. Sensory cells on the epidermis react to light, contact, chemicals and water currents. Higher densities of sensory cells are found in the tube feet and along feeding canal margins. Red pigmented eye spots are found on the end of each arm. These function as photoreceptors and are clusters of pigment-cup ocelli.

Class Ophiuroidea – Brittle Stars & Basket Stars

Brittle stars are often found in costal waters, hiding among rocks and coral reefs.  Unlike sea stars, whose arms are quite thick, the arms of the brittle stars are slender and distinct from the central disc.

Basket stars have a similar structure to brittle stars, although they are usually larger and less mobile. Their arms are very highly forked and branched, and even more flexible than those of brittle stars.

Motion - Brittle stars move with their entire arm, rather than pulling themselves along by tube feet.  They are quite agile but the arms are fragile. Brittle stars have the ability to throw off an arm that is being attacked, allowing the brittle star a chance to escape from a predator.

Reproduction - The majority are dioecious (each individual is either a male or female).  Most brittle stars are believed to be broadcast spawners that release their eggs and sperm through genital bursae into the water column to become fertilized. Some species also reproduce asexually by binary fission followed by each half regenerating the missing side.

Consumption - Most capture food by filter feeding – moving food particles up the arms to the mouth. The stomach is saclike; there is no intestine. The water-vascular, nervous and hemal systems resemble those of sea stars.

Growth – bilaterally symmetric larvae transform via multiple stages into juveniles. 

Environmental Response - All species of brittle stars are negatively phototaxic (aversion to light). It has been documented that brittle stars can quickly detect shadows of predators and safely escape although no type of photosensory organ had even been documented. Aizenberg et al. (2001) discovered that there were crystal clear bumps on top of each of the plates on the dorsal side of the disk and arms.  Because the entire dorsal surface is covered with spherical lenses, all collecting light from different angles, they form a compound "eye." These lenses have interested computer companies because the brittle star is able to make these lenses smaller and more precise than man-made lenses. Approximately ten thousand spherical crystals cover the five limbs and central body


Class Echinoidea – Sea Urchins & Sand Dollars

A distinguishing feature of the echinoids is that the calcitic ossicles making up the internal skeleton imbricate (overlap) and are fused into a globe or disk test (shell). This fused test is the familiar empty structure we find while beachcombing.  Generally, urchins have longer spines; sand dollars have short spines that give them a dark, fuzzy appearance. Regular echinoids are the sea urchins; they are generally found on rocky substrates. Irregular echinoids are the sand dollars, which are generally found on sandy or soft ground. Members of this class are important food for crabs, sea stars, fish, birds, otters, and other mammals.







Sea Urchins –

Motion - tubed feet end in a sucker. These suckers are used to grip the surface, The feet contract, pulling the animal. This locomotion is driven by a water vascular system.

Reproduction - Sexes are separate. Fertilization is external.

Consumption – Urchins are largely omnivorous, with a primary diet of algae. The mouth contains a powerful chewing apparatus called the Aristotle's lantern. The lantern is composed of five jaws and is capable of extending through the mouth of some urchins. The mouth leads to the intestine and anus, which is located in the center of the aboral (opposite the mouth) surface. Urchins have three kinds of projecting appendages: spine, tube feet and pedicellariae (pincers). The movable spines are the most conspicuous of the three.  Pincers located between spines are called pedicellariae. Some types of pedicellariae and specialized spines of urchins contain venom used in self-defense.

Growth - The free-swimming larvae live a planktonic existence for several months, and then metamorphose into young urchins. Some female urchins brood their young externally, within the protection of their spines or tube feet.

Environmental Response  - Touching the urchin's body with a sharp object causes all the spines to point to the region touched. The spines turn away if a blunt object is used, allowing the pedicellariae to be the primary mode of defense. They also show a shadow response, making rapid movements with the spines when a shadow suddenly appears. Sea urchins exhibit a generalized light sensitivity all over the body. Some urchins cover themselves daily with stones and shells held in place by the tube feet. This occurs a few hours after dawn, suggesting it is coordinated with light sensing abilities. Pedicellariae (small, pincerlike appendages) cover the body surface at the base of spines. These often have a poison sac with a hypodermic-like fang, used to deter predators.




Sand Dollars -

Motion - Unlike urchins that use tube feet for locomotion, sand dollars use their spines to move along the sand, or to drive edgewise into the sand. This locomotion is driven by a water vascular system. In quiet waters, these flattened animals stand on end, partially buried in the sand. When waters are rough, sand dollars hold their ground by lying flat—or burrowing under. In fast-moving waters, adults also fight the currents by growing heavier skeletons. Young sand dollars swallow heavy sand grains to weigh themselves down.

Reproduction - Sexes are separate. Fertilization is external.

Consumption – The sand dollar's mouth has a jaw with five teeth-like sections to grind up tiny plants and animals. Sand dollars use their fuzzy spines, aided by tiny hairs, to ferry food particles along their bodies to a central mouth on their bottom side. They capture plankton with spines and pincers (pedicellariae) on their body surfaces. The anus is either posterior or on the oral surface of irregular echinoids.

On the upper half of the sand dollar's body, spines serve as gills, extracting oxygen from sea water.

Growth – larvae swim as part of the plankton at various levels in open water, feeding as they slowly grow. After six weeks, the larvae reach a size of perhaps a three-hundredths of an inch and then settle to the ocean floor to finish developing to adulthood.

Environmental Response  - When exposed to fish mucus, larvae developed clone buds that ultimately detached and formed new larvae that were much smaller than the original organism. This is believed to be a survival response.  They sense their environment but do not react in ways we understand. 



Class Holothuroidea – Sea Cucumbers

Holothurians are common in the oceans of today; in certain parts of the deep sea, the ocean floor swarms with vast herds. They are also found in shallow-water habitats such as tidepools. Sea cucumbers are elongated in the oral-aboral axis.  They retain the skeleton of echinoderms, but in most species the skeletal plates are reduced to microscopic spicules, often shaped like wheels, bars, or anchors.

Motion Sedentary by nature, they may not move more than a dozen feet in one day. Some species crawl on the ocean bottom, others are found under rocks or buried in burrows. Some-sea cucumbers have tiny tube feet along the length of their body. Those species that lack tube feet creep about the sea floor by muscular movements of the body or remain in burrows.

Reproduction - Sexes are separate – a few are hermaphroditic.  Fertilization is external and produces free-swimming larvae. A few brood their young inside the body or on the body surface. During spawning, sea cucumbers travel to the top of reef structures and release their gametes into the surrounding currents.

Consumption - The tentacles filter seawater or sand for plankton and other organic matter. Some trap particles on the mucus of their tentacles, ingesting food particles in their pharynx. Others graze the sea bottom with their tentacles. The tentacles also serve for respiration and excretion; gas exchange can occur through the skin and tube feet. Some eat mud, using their tentacles to burrow and push mud through their gut.

Growth - The feeding larva of sea cucumbers swims for about 10-40 days before settling on the bottom and metamorphosing into a baby sea cucumber.

Environmental Response - When disturbed by an enemy such as a fish or a crab, the sea cucumber will often eject long, sticky threads from its body to entangle its enemy. Some species of sea cucumbers will violently contract their bodies and cast out a large part of their internal organs. The internal organs contain a toxin that is deadly to fish and other marine animals (but not to humans). These organs grow back in about six weeks.


Class Crinoidea – Sea Lillies and Feather Stars

Sea lilies have a flower-shaped body at the tip of a stalk and are sessile in the adult stage. Feather stars have long, many-branched arms; adults can be free moving or sessile. Many crinoids are deep-water species; feather stars can be found in more shallow water.  Five arms branch to form more arms, each with lateral pinnules - as in a feather. 

Motion - Modern stalked crinoids live almost entirely in waters deeper than 200 m.  Feather stars shed their stalks retaining only the topmost stalk segment, which usually bears numerous hooks for anchoring the animal.  Feather stars are found throughout modern seas from the intertidal zone to abyssal depths.

Reproduciton - Sexes are separate; gonads are merely masses of cells in the genital cavity of the arms and pinnules. Gametes escape through ruptures in the pinnule wall; some brood their eggs.

Consumption All crinoids are passive suspension feeders. They produce no feeding/respiratory current and rely on water currents. Diets include a variety of protists (e.g., diatoms and other unicellular algae, foraminiferans, actinopods), invertebrate larvae, and small crustaceans. The upper surface has a mouth that opens into an esophagus and intestine; it then exits at the anus. Tube feet and mucous nets allow it to feed on small organisms in the ambulacral (arm) groove. Once a food particle is captured by a crinoid, the tube foot wraps it in mucous secretions; ciliary tracts on the groove floor then transport it toward the mouth.

Growth - Larvae are free-swimming before they become attached and metamorphose.

Environmental Response - The major nervous system consists of a nerve ring around the mouth and a major concentration of nerve cells opposite the mouth, from where nerves run through the ossicles in the arms