Phylum Echinodermata: Characteristics And Classification

The oceans of our planet provide a large habitat for a wide variety of organisms.

From thousands of species of fish to tiny bacteria, marine life is just as varied as life on land.

The organisms that belong to Phylum Echinodermata are also part of this oceanic wildlife.

This species lives solely in saltwater and there is no evidence that Echinoderms can be found in freshwater.

In this article, we explore the individual characteristics of different Phylum Echinodermata, and their different classifications.

What Is Phylum Echinodermata?

Phylum Echinodermata is a group of ocean organisms that comprises a number of different spiny skinned animals.

The name “Echinodermata” originates from the Greek words “Echinos”, which means hedgehog, and “Derman” meaning skin.

This hints very much at the appearance of these animals, and there is a vast number of species that are covered by Phylum Echinodermata.

Research found that there are roughly 7,000 different species of these animals that can be found across our oceans.

In addition, paleontologists discovered that 13,000 species of Echinodermata have gone extinct over the course of life on earth. 

When researchers found the first animals classified under this group, they initially only discovered animals with a star shape, such as star fish.

However, over the years, scientists added a number of different organisms with different sizes and shapes.

While these animals may look different, they have a number of characteristics in common, such as their circular symmetry and the absence of a distinct head shape.

General Characteristics Of Echinoderms

With around 7,000 different living species, Echinodermata covers a large variety of animals. 

Habitat

The majority of them live in the world’s oceans.

However, scientists also have discovered some species that live in brackish water where salinity levels vary.

Echinoderms living in the ocean live in a range of different depths, from the ocean floor, all the way to shallow waters.

Appearance

Adult Echinodermata often have a radial symmetry. They can grow anywhere from 1cm up to 2m in width, depending on the individual species.

Body

Echinoderms all have an internal skeleton which contains ossicles made of proteins and calcium carbonate.

These small bones provide structural support to the animal but they do not make the organism more solid.

This means that these animals have a structure which resembles a sponge. This is called a “stereom”.

In addition to this sponge-like porous structure, all organisms under the classification of Echinodermata also have a water vascular system.

This system is made up of a tiny, radial canal network. This system is vital to the animal as it allows the organism to feed and slowly move itself forward.

Classification Of Phylum Echinodermata

Let’s take a closer look at the classification details of Phylum Echidermata.

Kingdom

All members of this species are classed as animals. This means that they are multicellular and heterotrophic.

In addition, these animals can also move by themselves. 

Domain

All animals that are covered under Phylum Echinodermata belong to the domain of Eukarya. This indicates that they are classed as eukaryotic organisms which means that their cells are made of membrane-bound organelles.

Phylum

Members of Echinodermata have an internal skeleton which consists of little spiny plates, tube-like feet and an effective water vascular system.

Class

Phylum Echinodermata are divided into five main classes. You can find more details on these classes and their characteristics below.

Some well-known Phylum Echinodermata include Anasterias antarctica, Stichopus chloronotus and Odontaster validus.

Echinodermata Characteristics Of Major Classes

While Echinoderms all have an internal skeleton, a water system and tube-like feet in common, there are a lot more characteristics unique to the individual classes.

Let’s take a look at the main characteristics of the five major classes of Echinodermata.

Class Asteroidea

Echinoderms that fall into the class of Asteroidea are well known under their common name, sea star.

There are around 1,900 species of sea stars distributed throughout all oceans of our planet, including the cold Antarctic zones.

Marine biologists found Asteroidea at a range of different depths, from the sea floor to intertidal zones.

These animals typically live on rocks or surfaces with soft sediment.

Morphology 

As many of us know, star fish are typically relatively small in size.

The members of this class can range from smaller than an inch up to forty inches in width.

For example, the species Pyconopodia can grow over one meter wide, whereas species such as Granaster nutrix only grow around o.4 inches in diameter. 

Just as their size varies, members of the Asteroidea class also vary in color. Some species are bright red, while others are orange, yellow and even blue or gray.

Species such as Perknaster can be a vibrant red, while Neosmilaster georgianus are more yellow and orange.

The majority of the species have a pentaradial symmetry with five arms which radiate out from the center.

However, some members of this class can have even up to fifty arms/rays, together with a large number of small tube feet.

These tube feet connected to the central disk of the animal allow it to move forward.

They extend to attach themselves to the ground they are on, and then contract, propelling the organism slowly forward.

While for many animals of this class, the tube-feet extend from the tube-foot groove, this is not the case for all.

Instead, they make an oar-like movement with their arms which helps them to slowly move around.

The central disk also has the oral surface which contains the oral opening which the animal uses to feed. 

This opening is surrounded by tiny spines which rise from the animal’s internal skeleton.

The mouth part of every sea star is connected to a relatively large stomach through a short esophagus.

The organism’s stomach is divided into the aboral stomach and the cardiac stomach. When the animal is feeding, the stomach extends outwards which then allows it to swallow any prey or sediments.

The sucker tube feet help to hold the starfish’s prey in place, so that it has enough time to swallow the prey with its extended stomach.

Members of the Asteroidea class feed on a variety of crustaceans, mollusks and krill.

Some species, such as Stegnaster inflatus, are specialized predators that hunt organisms, such as cnidarians and worms.

Other species in this class are much more opportunistic feeders, such as Patiriella regularis, that hunt any prey which comes along.

Just like all Echinoderms, Asteroidea species also have a water-vascular system which is made up of a lateral canal, a stone canal, the madrepories as well as a ring and radial canal.

The radial canals are a network that lead to the ring canal which connects to each arm.

Through the stone canals which are made of madrepories, radial canals are open to the body cavity.

You can also find other important body structures in this species, such as radial nerves, gonads, pedicellaria and ampulla.

Reproduction

Similar to other sea animals, most Asteroidea species are separate sexes, also called dioecious. 

Researchers found that the gonads on each arm of the animal do get enlarged when star fish reproduce.

Starfish then release reproductive cells, also known as gametes, into the surrounding area for fertilization which happens externally.

A few species of Asteroidea also carry fertilized eggs or juveniles in their stomach, mouth or other regions of their bodies.

Once fertilization has happened, the produced embryos are the size of plankton and they can only use cilia to propel themselves forward.

As the small organisms grow, they slowly turn into bilaterally symmetrical larvae that hunt protists nearby.

This helps them to grow further and develop into juvenile Echinoderms that can then hunt in a similar way to their adult counterparts.

Other Characteristics Of Sea Stars

Sea fish also have a large number of other characteristics that make them unique.   

Just like other animals, members of Asteroidea also have a way to excrete waste and exchange gas. This is achieved through diffusion.

Scientists found that collated cells around the body are used to diffuse nutrients, gases and other waste products.

These products are then excreted through tube feet, dermal brachiae or other membranous parts.

Materials that cannot be digested are excreted through the mouth part. Star fish have a very poorly developed anus, which is typically not used very often.

Asteroidea also have a relatively basic nervous system which is made up of only one nerve ring around an oral opening. Radial nerves connect the nerve network to each arm. 

This species of Echinoderms also have sensory receptors all over their body which help them to react to different environmental stimuli, such as chemicals or light.

One of the best known characteristics of sea stars is that they are able to regenerate themselves. This means that they can regrow lost body parts, such as arms.

Species Examples Of Asteroidea

Some of the best known examples of Asteroidea are Neosmilaster georgianus, Diplasterias brucei, Psilaster charcoti, Acodon-taster hodgsoni and Anasterias antarctica (which is also known as the Cinderella starfish).

Class Ophiuroidea

The species of the class Ophiuroidea are also commonly known as brittle stars or basket stars.

The name originates from the Greek “Orphis” (which means snake) and “Oura” (meaning tail).

This class covers an even larger number of species than Asteroidea, namely 2,000. With this large variety of species, it is one of the most diverse groups of marine animals in the world.

You can find these organisms in the sea and oceans where they sit on coral crevices or rocks.

They typically also cling to algae and other plant material. 

Just like Asteroidea, animals of this class can also regenerate their limbs.

Research has shown that when the arm of a brittle star is grasped too firmly, then these animals can contract certain muscles which can lead to the arm being severed.

However, the animal will be able to regenerate a perfectly new, working arm.

Morphology

The majority of the species has a banded or mottled pattern. Colors vary widely but most of these animals are brightly colored.

The species in this class can vary widely in size.

Their main part, a central disk in the middle of their bodies, can vary from 0.5cm to 10cm in diameter, and their arms can be almost twenty times the size of the central disc.

Although it is difficult to distinguish between female and female individuals of this species, scientists found that typically male Ophiuroids are smaller than their female counterparts.

Similar to Asteroidea, this species also feature a dorso-ventrally body which is flattened with a pentagonal or round center disk.

Unlike some other classes however, brittle stars, such as Ophiopholis aculeta and Ophionotus victoriae, have much longer and sharply demarcated arms.

Some species even have rays that branch out from the central disc in a fork-like manner.

One of these species is Astrophyton muricatum.

As the arms of this species are so long, they can move much more flexibly, and this is where the Ophiuroidea class gets its name from: the arms of these animals move as quickly as a snake.

The members of this class also have tube feet but they do not have suction disks that could be used to hold their prey.

However, these tube-like feet still act similar to a muscle.

They even contract, so these organisms can use their feet to crawl along a surface.

While these animals also have a water vascular system, it is not used for movement.

Just like other Echinoderms, this species also has a large amount of ossicles that are part of the internal, modified skeleton.

They can be found on both the oral and lateral surfaces of the arm.

Thanks to the slight modification of this internal skeleton, the arms of these animals can move freely.

Similar to Asteroidea, species of Ophiuroidea also have an oral opening similar to a mouth which is located on the bottom of the central disc.

This mouth opening has five triangular structures (not dissimilar to teeth) which are used for feeding and chewing.

Once the animal catches its prey, it feeds it through the mouth which leads directly into the stomach inside the coelom.

It’s important to note here that the stomach is just a sac-like structure, which sits together with the gonads in the central disc area. These animals also do not have intestines.

Any prey or plankton that has been caught in the tube feet of the organism will be passed from one foot to another all the way to the animal’s mouth where the prey is ingested.

As the stomach of Ophiuroidea organisms sits within a coelom, there are coelomocytes that break down the ingested material.

They help to distribute important nutrients throughout the body, and they also expel any waste material.

The release of waste products happens across the bursae and tube feet through diffusion.

This species can also move water in and out of the bursae thanks to small slits in the oral disk.

Reproduction

Just like Asteroidea, Ophiuroids has individual male and female animals to allow for reproduction.

When this species reproduces, there are two ways in which this can be done.

Either eggs are released into the surrounding water, and they get fertilized externally, or the eggs remain in the bursa where they can get fertilized. 

The advantage of keeping the eggs inside the bursa is that they are not exposed to the external environment as they grow.

Over time, these little embryos grow into larva, called Ophiopluteus, which are around the size of plankton.

These larvae transform into benthic adults that can then feed and breed just like their parents.

Species Examples Of Ophiuroids

Some well-known members of this class include Amphiophiura, Astroniwa, Ophionereis, Cryptopelta and Ophiurolepis

Class Echinoidea

Also better known by their common name of sea urchin, this class also covers sand dollars and heart urchins.

Compared to Asteroidea and Ophiuroids, Echinoidea only covers around 1,000 living species which can be found in a large variety of marine environments.

Some species, such as regular sea urchins, live on rocky shores, while other animals of this class, such as heart urchins and sand dollars, prefer life at the bottom of the ocean.

They can live in a depth of up to 6,000m.

Similar to many other species of Echinoderms, sea urchins can also regenerate some parts of their bodies, including the pedicellaerie and their spines.

Morphology

Most Echinoids are relatively small. Ranging from three to ten centimeters in diameter, some can grow even up to 15cm.

Some animals of this species feature very bright colors, while others have a little more subdued appearance.

Although many members of Phylum Echinodermata are dorso-ventrally flattened and have long arms, the species of Echinoidea are mostly armless.

They are also more likely to have a discoidal or spherical shape, although a few animals of this class are flat.

The endoskeleton of this species also differs to many other Echinoderms as it is made of fitting plates which interlock at the edges.

You will also find that these plates arch between the aboral and oral ends.

The skeleton also has allowed for openings where tube feet can fit through.

Although these animals do not have any arms, they have hollow, sharp spines which is the most common way to identify this class of Phylum Echinodermata.

At the bottom of each of these sharp spines is a concave socket with different muscles which move with the spine. Some species also have venomous spines.

All Echinoid species used their tube feet and spines for locomotion.

The tube feet are responsible for pulling the animal forward, and the spines push against the floor to support the feet’s pulling movement.

Some species also use their spines to burrow into the ground or other surfaces.

When you look at an Echinoid closely, you will also notice pincer-like structures, called pedicellariae.

They are small claw-like structures that help to protect the animal from prey or remove debris from the body’s surface.

Similar to the spines, some sea urchins also have been found to have venom in their pedicellariae. This helps them to protect themselves against predators.

In addition to the defense mechanisms, Echinoids also have an oral opening with a chewing apparatus on their underside.

This chewing apparatus, also called Aristotle’s lantern, consists of a range of muscles and jaws made of ossicles.

This clever apparatus does not only function as a way to feed the animal, but some species also use it for burrowing.

Unlike Asteroids, Echinoid species have a long, coiled intestine which is located inside the coelom and it winds its way through the esophagus and pharynx.

When an Echinoid catches prey, such as dead animals, algae or coral polyps, it first chews the material, and then passes it to the pharynx and esophagus.

The digestion process has inside the intestine, and once digested, the waste products are passed through the animal’s anus at the aboral side.

Echinoids also have a water-vascular system which is made of radial canals that run throughout the animal’s body.

These organisms “breathe” through gas exchanged which happens through diffusion in the rube feet and the epithelium.

Reproduction

Just like many other Echinoderms, Echinoids are also dioecious which means that this species has both male and female individuals.

Researchers found that animals can increase vastly in size when they reproduce.

First, gametes are released into the surrounding water, and fertilization happens externally.

As the embryos develop over time, they turn into small larvae which spend a few months eating plankton.

Then they transform into adults through metamorphosis.

Species Examples Of Echinoidea

Some popular species of sea urchins and Echinoidea include Fellaster zelandiae, Kermabrissoides siculum, Evechinus chloroticus, Echinocardium cordatum and Apatopygus  recens.

Class Holothuroidea

Also commonly known as sea cucumber, Holothuroidea covers around 1,500 species.

They are typically found at different sea depths, where they either crawl over different rocks or burrow into the soft sea floor.

Sea cucumbers vary in length, ranging from 10cm to 30cm.

This species typically has an elongated body with a cylindrical shape. Their body wall is thick and very muscular.

However, their ventral side is flattened as they lie on this side.

Just like many other Echinoderms, sea cucumbers can also regenerate lost body parts.

What makes this type of Echinoderm more unusual is that this species does not have any arms, and it also has a reduced skeleton which consists of calcareous plates.

They have an oral opening like a mouth, tube feet and the water-vascular system, typical for all Echinoderms.

The oral opening is enclosed with tube feet that work like tentacles as they are often used to catch prey or other food material.

However, Holothuroidea also use their tube-like feet to move their body.

The feet can stick to a surface and detach as the organism moves forward.

The mouth of this organism is connected to a coiled intestine which runs through the esophagus, and it ends in the cloacae where waste material is expelled.

The water vascular system of a sea cucumber contains coelomic fluid which helps to break down food and excrete any waste products.

The ring canal of the system is essential for the sea cucumber as it is not just used for digestion, but it is also the place where the tentacles originate.

Beneath the epidermis of a sea cucumber, you will find a thick layer of tissue which is connected to varied ossicles.

Most species of sea cucumbers also produce various toxins which help them to defend themselves.

Respiration

A sea cucumber has a large coelom. This helps the cilia in this area to circulate all fluids inside the body.

This is essential for the distribution of gases, fluids and other materials. 

In addition, the body of a sea cucumber also has a pair of respiratory trees (somewhat similar to lungs) that emerge out of the rectum. 

As the rectum performs a pumping action, water is then circulated through these tubes, and the fluid is only expelled once the tubes themselves contract.

Reproduction

Although the majority of species of Holothuroidea are known to be having both male and female individuals, some sea cucumber species are hermaphrodites which can self-fertilize.

This means that some sea cucumbers can also reproduce through asexual reproduction just by using transverse fission.

Generally, the animals release gametes into the water during the reproduction process, and the eggs are then fertilized.

The embryo that develops is a tiny planktonic larva which quickly transforms into a juvenile and matures over a period of time.

However, some species do not fertilize externally but fertilization happens inside the body cavity where the young embryo can develop safely.

After some time of maturing, the juvenile then leaves the cavity by breaking through the body wall.

Species Examples Of Holothuroidea

Some well-known members of the sea cucumber class include Paracaudina chilensis, Trochodota dunedinensis, Australostichopus mollis and Protankyra brychia

Class Crinoidea

The species under the classification of Crinoidea are somewhat different to the other Echinoderms.

This class covers 630 living species of feather stars and sea lilies.

Crinoids can be found at different depths of oceans across the world. 

Feather stars crawl and swim across various surfaces, but sea lilies typically live at the bottom of the ocean where they can attach themselves to different substrates.

Morphology

In comparison to other members of Phylum Echinodermata, the species of Crinoidea are very simple in their body structure.

The bodies of feather stars as well as sea lilies are made up of a cup-like calyx.

This in turn consists of different plates, branched arms and membranous tegmen.

The arms of a Crinoid emerge out of the tegmen and the edge of the calyx.

Sea lilies typically attach themselves to a rock or other substrate.

They use their root-like extensions at the bottom of a slender stalk connected to the calyx to get a firm grip.

Similar to many other Echinodermata species, sea lilies also have ossicles. Their ossicles are part of the stalk which holds together connective tissue. 

Each arm of a sea lily also has the typical Echinodermal tube feet.

Feather stars are slightly different when it comes to morphology.

They do not have a stalk but instead have a number of cirri at the end of their calyx.

This means that they can move their arms up and down, allowing the organism to hold on to surfaces or swim freely.

They can also simply crawl over surfaces by using the tip of their arms to pull themselves wherever they want to go.

Unlike other Echinoderms, this species has its oral opening on the upper side, specifically on top of its calyx.

This area is covered with ambulacral grooves that move food or prey from the animal’s arms to its mouth opening.

Typically, Crinoids catch foot with their tube-like feet which are covered in sticky mucus.

The food is then carried by the grooves to oral opening.

This is the only way that Crinoid species can catch their food. As suspension feeders, they can only use their arms and trap food particles that pass by.

The base of the calyx also holds a number of nerves, which forms the organism’s rather basic nervous system.

These nerves also extend into the arms, so the animal can react to different stimuli.

Just like other Echinoderms, this species also has a very simple water vascular system, which uses the arms for respiratory functions.

Reproduction

Crinoids are generally separate female and male individuals.

Similar to other Echinodermata species, they release sex cells into the water around and fertilization occurs externally.

The small embryo develops first into a small larva, and then into a juvenile which will turn into a mature adult after a while.

Frequently Asked Questions

What Makes Phylum Echinodermata Unique?

There are a number of things that make Phylum Echinodermata species unique. 

One of the most important body functions of this species is the water vascular system which is made up of a central ring and various radial canals which stretch out into the long arms.

This system helps water to circulate through the entire body, but it is also used for gaseous exchanges as well as movement and feeding.

Why Do Echinoderms Live Underwater?

Echinoderms do not have a solid skeleton. They have a loose ossicle structure but they would not be able to hold themselves up outside of the water.

The rest of their body, including the tube feet, are also filled with water.

As they use a water vascular system for a lot of their body functions, including locomotion and feeding, they depend on water.

Do Echinoderms Have Eyes?

No, Echinoderms do not have eyes. They also do not have a head which could contain a brain, ears or other typical features.

Echinoderms do not even have anything that resembles human eyes.

However, they have small sensors on their skin which allows them to sense different stimuli and react to them.

Conclusion

Phylum Echinodermata cover a range of different species, from sea stars, sea urchins, sea cucumbers and sea lilies, as well as brittle stars and feather stars.

All in all, there are over 7,000 species of Phylum Echinodermata that live in different depths of the oceans across the world.

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