Echinodermata-Definition, Characteristics, Classification, Exampales, Facts

Echinodermata: Definition, Characteristics, Classification, Exampales, Facts

Edited By Irshad Anwar | Updated on Jul 02, 2025 06:07 PM IST

Phylum Echinodermata includes a unique group of exclusively marine animals. The term “echinoderma” comes from the Greek word ‘echinos’ meaning spiny, and the Latin word ‘derma’ meaning skin, which means spiny skin. Phylum Echinodermata organisms are found in a variety of ocean habitats. The marine animals of starfish, sea urchins, sand dollars, brittle stars, and sea cucumbers belong to the phylum Echinodermata

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Definition of Echinodermata
Scientific Classification of Phylum Echinodermata
Characteristics of Echinoderms
Classes Under Echinoderms
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Echinodermata: Definition, Characteristics, Classification, Exampales, Facts

Phylum Echinodermata has characteristics such as radial symmetry, a calcareous endoskeleton, and a water vascular system. These all belong to the general phylum Echinodermata characteristic that distinguishes them from all other marine organisms. The family members of Echinodermata are mainly aquatic animals. This is one of the important topics of biology from the chapter Animal Kingdom.

Definition of Echinodermata

Echinoderms are marine animals that constitute the phylum Echinodermata. The family members include starfish, sea urchins, sand dollars, and sea cucumbers. Some of the general characteristics of the phylum Echinodermata include the radial symmetry of adults, possession of a calcareous endoskeleton that comprises calcium carbonate, and the presence of the water vascular system. Echinoderms have other features, such as tube feet, which assist in movement, attachment, and food capture. The types of echinoderms are divided into five main classes:

Asteroidea: Starfish
Echinoidea: Sea urchins and sand dollars
Holothuroidea: Sea cucumbers
Ophiuroidea: Brittle stars
Crinoidea: Feather stars and sea lilies

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Echinoderms are very important to marine ecosystems. By feeding on algae, preying on other invertebrates, and bioturbating the seabed, they establish balance within the environment. They are often involved in biogeochemical processes, keep diversity in the ocean, and make benthic habitats healthy. The general characteristics of echinodermata make them crucial for the maintenance of health in the aquatic ecosystem.

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Scientific Classification of Phylum Echinodermata

The phylum Echinodermata includes exclusively marine animals such as sea stars, sea urchins, and sea cucumbers. These echinoderm animals are classified under the domain Eukaryota and belong to the kingdom Animalia.

Domain	Eukaryota
Kingdom	Animalia
Subkingdom	Eumetazoa
Superphylum	Deuterostomia
Phylum	Echinodermata

Characteristics of Echinoderms

Characteristics of Echinodermata such as radial symmetry, endoskeleton, water vascular system, pedicellariae, and regeneration, give these Echinodermata organisms a distinct place as unique and important marine organisms. Some basic characteristics of Echinoderms are discussed below:

Radial Symmetry

Echinoderms are marine animals in the phylum Echinodermata that exhibit radial symmetry, usually pentameral. It allows for an equal distribution of sensory and feeding organs that enables interaction with the environment from all sides.

Endoskeleton Composition

The main characteristics of echinodermata include an endoskeleton that is composed of calcareous ossicles, which are mainly composed of calcium carbonate. The ossicles allow rigidity with flexibility, enabling the body to be supported in several forms.

Water Vascular System

The characteristic water vascular system is fluid-filled canals. It aids in movement, feeding, and respiration. Echinoderms move with tube feet and also use hydraulic pressure in handling the food.

Pedicellariae and Spines

On the calcareous plates the surface of echinoderms, there is the presence of pedicellariae, or spine-like organs and spines. These help in defence, cleaning, and sometimes trapping food. Such is one of the salient characteristics of Echinodermata.

Regeneration Abilities

Echinoderms are remarkable for their remarkable regenerative capabilities. They can regenerate body parts lost, which is a life-saving feature, especially after an injury or attack. This is an important survival trait in general characters of Echinodermata.

Starfish

Classes Under Echinoderms

The phylum Echinodermata is divided into five main classes based on their structure, movement, and habitat. Each class includes unique echinoderm animals that play a vital role in marine ecosystems. Some of the basic classification of Echinoderms is discussed below:

Class Asteroidea (Starfish)

Starfish, or sea stars, are marine animals falling in the group Echinodermata. They are star-shaped, with a stiff body, but some species have more than five arms. Their bodies contain a central disc and a disc-like series of arms covered with tube feet, which enable them to move and feed. Starfish are predators and carnivores and are important components in ocean food chains.

Class Ophiuroidea (Brittle Stars)

Brittle stars have long, slender arms separated from their central disk unlike starfish, they locomote by moving with a body covered in arms rather than using tube feet. Most brittle stars feed on detritus, consuming small amounts of organic materials mixed with minute organisms.

Class Echinoidea (Sea Urchins and Sand Dollars)

The sea urchins are spherical with long spines, but the sand dollars are flat and circular. Both are covered with a hard outer cover called a test. Some sea urchins, like butterfly urchins, feed on algae and thus help control the growth of it, whereas the sand dollars feed on sediment particles. These animals are important examples of characteristics of phylum echinodermata.

Class Holothuroidea (Sea Cucumbers)

Sea cucumbers are soft-bodied, elongated sea creatures that live on the sea floor. They play a large role in nutrient recycling and oxidation of organic matter by turning over sediment on the sea floor. Some species have a defence mechanism in which they expel their internal organs when threatened.

Class Crinoidea (Feather Stars and Sea Lilies)

Feather stars and sea lilies have branched arms that extend from the central disc, which they use in filter-feeding by capturing food particles in the water. Sea lilies are usually attached to the seabed using a stalk, but feather stars can move around freely. These are considered primitive members of the general characteristics of phylum Echinodermata, with its first appearance traced back to the Paleozoic era.

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Frequently Asked Questions (FAQs)

1. What are Echinodermata?

The Phylum Echinodermata includes sea stars, brittle stars, sea urchins, sand dollars and sea cucumbers and these are marine animals that possess radial symmetry, a water vascular system and calcium carbonate endoskeleton. It generally has five-rayed symmetry and locomotory and feeding organs in the form of tube feet.

2. Describe the water vascular system in Echinoderms.

The water vascular system is another system of Echinoderms that is relevant for movement, ingestion, and respiration. This comprises several of chambers filled with water with an opening to the tube feet and worked by muscular action and is governed by a ring canal.

3. How do Echinoderms reproduce and develop?

Feeding in echinoderms is mostly obtaining food by prehension and digestion Later developed sexual and asexual modes of reproduction. Sexual reproduction in fish has external fertilization in which sperms and eggs are released into the water and fertilizing occurs. In asexual reproduction, the organisms can be regenerated fully to form other individuals and body parts that were lost can grow into fully adult organisms. The process through which they evolve to adulthood mainly entails a period of a free-moving larval stage (for instance; bipinnaria, or doliolaria larva).

4. What are types of Echinodermata?

The types of echinoderms are divided into five main classes:

Asteroidea: Starfish
Echinoidea: Sea urchins and sand dollars
Holothuroidea: Sea cucumbers
Ophiuroidea: Brittle stars
Crinoidea: Feather stars and sea lilies

5. Discuss the economic importance of Echinoderms.

Some species, such as sea cucumbers and some types of sea stars, are being read as sources of fishery and have market value, being used in food preparations by some cultures. Another classification is based on the species’ uses; for example, some sea urchins have significant applications in biomedicine because the animals are capable of regenerating their tissues. Also, their calcium carbonate skeletons are utilised in the making of lime and fertilizer.

6. How do sea cucumbers defend themselves against predators?

Sea cucumbers have several unique defense mechanisms. Some species can expel their internal organs (evisceration) to distract predators and later regenerate them. Others have sticky threads (Cuvierian tubules) that entangle threats, while some release toxic chemicals that repel or harm predators.

7. What is the function of pedicellariae in echinoderms?

Pedicellariae are small, pincer-like structures found on the surface of many echinoderms, especially sea stars and sea urchins. They serve multiple functions, including defense against parasites and small predators, cleaning the body surface, and in some cases, capturing small prey or aiding in locomotion.

8. How do echinoderms maintain their internal calcium balance?

Echinoderms have specialized cells called podia that can absorb calcium from seawater. They use this calcium to build and maintain their calcium carbonate endoskeleton. Some species can also adjust the density of their skeleton by adding or removing calcium, which helps in buoyancy control.

9. How do sea stars digest food outside their body?

Sea stars use a unique method of external digestion. They evert their stomach through their mouth onto their prey, releasing digestive enzymes that break down the prey's tissues. The liquefied nutrients are then absorbed by the sea star's stomach before it retracts back into the body.

10. How do echinoderms sense their environment without a centralized brain?

Echinoderms have a decentralized nervous system with a nerve ring around the mouth and radial nerves extending into each arm or body section. They use various sensory structures, including photoreceptors for light detection, chemoreceptors for detecting chemicals, and mechanoreceptors for touch and vibration, distributed throughout their body.

11. What does "Echinodermata" mean, and why is this name appropriate for this phylum?

"Echinodermata" comes from the Greek words "echinos" meaning spiny and "derma" meaning skin. This name is appropriate because most echinoderms have a spiny or bumpy outer surface, which is a key characteristic of the phylum.

12. How do echinoderms contribute to bioturbation in marine sediments?

Echinoderms, particularly sea cucumbers and some sea stars, play a significant role in bioturbation - the reworking of sediments. As they move through or feed on sediments, they mix and oxygenate the seafloor, influencing nutrient cycling and creating microhabitats for other organisms. This process is crucial for maintaining healthy marine ecosystems.

13. How do echinoderms differ from other animal phyla in terms of body symmetry?

Echinoderms are unique among animals because they exhibit pentaradial symmetry as adults, meaning their body parts are arranged in fives or multiples of five. This is different from the bilateral symmetry found in most other animal phyla.

14. What is the water vascular system, and why is it crucial for echinoderms?

The water vascular system is a network of fluid-filled canals unique to echinoderms. It's crucial for locomotion, feeding, and respiration. This system operates hydraulically to extend and retract tube feet, which are used for movement and capturing food.

15. What role does the madreporite play in echinoderm physiology?

The madreporite is a sieve-like structure on the surface of echinoderms that acts as an entry point for seawater into the water vascular system. It filters the incoming water, maintaining the proper fluid balance and pressure within the system.

16. How do the endoskeletons of echinoderms differ from those of vertebrates?

Echinoderm endoskeletons consist of small, interlocking calcium carbonate plates called ossicles, which are embedded in the skin. This differs from the continuous, jointed internal skeletons of vertebrates. The echinoderm skeleton is more flexible and allows for growth without molting.

17. How do echinoderms cope with the high salinity of seawater?

Echinoderms are osmoconformers, meaning their internal salt concentration matches that of their environment. They have specialized cells in their body wall that actively pump out excess salts, maintaining proper osmotic balance without the need for complex excretory organs.

18. How do echinoderms reproduce, and what is unique about their larval stage?

Most echinoderms reproduce sexually through broadcast spawning, releasing eggs and sperm into the water. Their larval stage, called a pluteus or bipinnaria, is bilaterally symmetrical and free-swimming, unlike the pentaradial adult form. This dramatic metamorphosis is unique among animals.

19. What is the significance of the blastopore in echinoderm development?

In echinoderm development, the blastopore (the opening of the gastrula) becomes the anus, while the mouth forms as a new opening. This developmental pattern, called deuterostomy, is a key characteristic that echinoderms share with chordates, providing evidence for their evolutionary relationship.

20. How do echinoderms regenerate lost body parts?

Echinoderms have remarkable regenerative abilities due to their decentralized nervous system and the presence of stem cells throughout their bodies. When a part is lost, these stem cells can differentiate into the necessary cell types to regrow the missing part, sometimes even regenerating entire bodies from a small fragment.

21. What is the evolutionary significance of echinoderms in relation to chordates?

Echinoderms and chordates (including vertebrates) are both deuterostomes, sharing a common ancestor. The study of echinoderms provides insights into the evolution of complex body plans and the development of features like internal skeletons, which are important in understanding chordate evolution.

22. How do echinoderms contribute to bioerosion in coral reef ecosystems?

Echinoderms, particularly sea urchins, play a significant role in bioerosion of coral reefs. They scrape algae off coral surfaces using their teeth or spines, which can also erode the coral skeleton. While this process can damage corals, it also creates space for new coral growth and helps maintain reef health by controlling algae populations.

23. What is the ecological importance of echinoderms in marine ecosystems?

Echinoderms play crucial roles in marine ecosystems as grazers, predators, and detritivores. They help control algae populations, recycle nutrients, and serve as food for other organisms. Some species, like sea urchins, are considered keystone species due to their significant impact on community structure.

24. How do crinoids (sea lilies and feather stars) differ from other echinoderms in terms of lifestyle?

Crinoids are unique among echinoderms as many species are sessile (attached) as adults, particularly sea lilies. They have a flower-like appearance with arms radiating from a central body, and they filter feed by capturing plankton with their feather-like arms, unlike the more mobile, predatory, or grazing lifestyles of other echinoderms.

25. How do brittle stars move differently from other echinoderms?

Brittle stars move by using their long, flexible arms to pull or push themselves along the sea floor, rather than primarily using tube feet like other echinoderms. Their arms can coil and uncoil, allowing for rapid, snake-like locomotion that is unique among echinoderms.

26. What is the function of the Aristotle's lantern in sea urchins?

Aristotle's lantern is a complex, five-sided structure in sea urchins used for feeding. It consists of five hard, calcareous teeth that can be moved to scrape algae off rocks or crush food. This specialized feeding apparatus allows sea urchins to consume a variety of food sources.

27. What is the significance of the ambulacral grooves in sea stars?

Ambulacral grooves are channels on the underside of sea star arms that house rows of tube feet. These grooves are essential for the sea star's movement, feeding, and respiration, as they contain extensions of the water vascular system and allow for the coordinated movement of tube feet.

28. How do crown-of-thorns starfish impact coral reef ecosystems?

Crown-of-thorns starfish are voracious predators of coral polyps. In normal populations, they contribute to coral reef diversity by preventing any one coral species from dominating. However, during population outbreaks, they can consume coral faster than it can grow, causing significant damage to reef ecosystems.

29. What is the evolutionary advantage of pentaradial symmetry in echinoderms?

Pentaradial symmetry in echinoderms is thought to provide several advantages. It allows for efficient radial movement in any direction, which is beneficial for bottom-dwelling organisms. It also enables even distribution of sensory and feeding structures around the body, maximizing interaction with the environment in all directions.

30. How do echinoderms cope with changes in ocean pH due to climate change?

Echinoderms are particularly vulnerable to ocean acidification because their calcium carbonate skeletons are sensitive to changes in pH. They cope by increasing energy allocation to maintain their skeletons, which can come at the cost of other functions like growth and reproduction. Some species may also alter their skeletal structure or density in response to pH changes.

31. How do echinoderms maintain their salt and water balance in marine environments?

Echinoderms are osmoconformers, meaning their internal salt concentration matches that of the surrounding seawater. They have specialized cells in their body wall that actively pump out excess salts, maintaining proper osmotic balance. The water vascular system also plays a role in regulating internal fluid pressure and composition.

32. What is the significance of the bipinnaria larva in echinoderm development?

The bipinnaria larva is a free-swimming, bilaterally symmetrical larval stage in the development of sea stars and some other echinoderms. It's significant because it represents a dramatic difference from the adult form, demonstrating the complex life cycle of echinoderms. This larval stage allows for dispersal and plays a crucial role in the distribution of echinoderm populations.

33. What is the role of the axial organ in echinoderms?

The axial organ is a structure unique to echinoderms that plays a role in the immune system. It produces coelomocytes, which are cells involved in immune responses and wound healing. The axial organ also helps regulate the fluid in the water vascular system.

34. What is the function of the stone canal in the echinoderm water vascular system?

The stone canal is a calcareous tube that connects the madreporite to the ring canal of the water vascular system. It acts as a conduit for filtered seawater to enter the system and helps maintain proper pressure within the water vascular system, which is crucial for the functioning of tube feet.

35. What is the function of the siphon in sea cucumbers?

The siphon in sea cucumbers is a specialized structure used for respiration and waste removal. It draws water into the respiratory trees (modified tentacles inside the body cavity) where gas exchange occurs, and then expels the deoxygenated water and waste products.

36. How do sand dollars maintain their flat shape in the face of water currents?

Sand dollars maintain their flat shape through a combination of their rigid, disc-like endoskeleton and tiny, densely packed spines that cover their body. These spines can be moved to help the sand dollar burrow into the sand, reducing water resistance and helping it stay in place despite currents.

37. What is the purpose of the tube feet in sea urchins, and how do they differ from those in sea stars?

Tube feet in sea urchins serve multiple functions, including locomotion, attachment to surfaces, and food capture. Unlike sea stars, which primarily use tube feet for movement and prey capture, sea urchin tube feet are also involved in respiration and sensing the environment. They are often longer and more flexible than those of sea stars.

38. What is the function of the hemal system in echinoderms?

The hemal system in echinoderms is a network of channels and sinuses that complements the water vascular system. It's involved in the transport of nutrients throughout the body and may play a role in gas exchange. However, its exact function is still not fully understood and is a subject of ongoing research.

39. What is the role of the ampullae in the echinoderm water vascular system?

Ampullae are small, muscular sacs connected to the tube feet in echinoderms. They contract to force water into the tube feet, causing them to extend. When the ampullae relax, the tube feet retract. This hydraulic system allows for precise control of tube foot movement, essential for locomotion and feeding.

40. How do sea stars detect and locate their prey?

Sea stars use chemoreceptors located on their tube feet and the tips of their arms to detect chemical cues in the water. These receptors can sense amino acids and other molecules released by prey. Once prey is detected, the sea star can follow the chemical gradient to locate its food source.

41. What is the function of the respiratory trees in sea cucumbers?

Respiratory trees are highly branched, thin-walled tubules in the body cavity of sea cucumbers. They are the primary organs for gas exchange, taking in oxygenated water through the cloaca and expelling deoxygenated water. These structures greatly increase the surface area for efficient oxygen absorption.

42. How do feather stars attach to and detach from substrates?

Feather stars use specialized appendages called cirri to attach to substrates. These cirri are located on the underside of the central body and can grip surfaces. Feather stars can release their hold and swim to new locations using coordinated movements of their arms, a unique ability among adult echinoderms.

43. What is the function of the peristomial membrane in sea urchins?

The peristomial membrane is a flexible tissue surrounding the mouth of sea urchins. It protects the soft tissues around the mouth and allows for movement of the Aristotle's lantern (feeding apparatus). The membrane can expand or contract, enabling the sea urchin to adjust its feeding posture and protect its mouth when not feeding.

44. What is the purpose of the apical system in echinoderms?

The apical system is a set of plates located at the top of echinoderms, particularly evident in sea urchins. It includes the madreporite and genital plates. This system is involved in reproduction (containing gonopores for gamete release) and in regulating the water vascular system through the madreporite.

45. How do brittle stars reproduce asexually?

Some species of brittle stars can reproduce asexually through a process called fission. The central disc splits in half, and each half regenerates the missing parts, resulting in two individuals. This ability allows for rapid population growth and is an effective strategy for colonizing new areas.

46. What is the role of the axial complex in echinoderms?

The axial complex is a group of interconnected organs in echinoderms that includes the axial organ, stone canal, and axial sinus. It plays a role in the formation of coelomocytes (immune cells), helps regulate the water vascular system, and may be involved in excretion. The exact functions of this complex are still being studied.

47. How do sea cucumbers use their Cuvierian tubules for defense?

Cuvierian tubules are sticky, thread-like structures in some sea cucumbers. When threatened, the sea cucumber can expel these tubules through its anus. Upon contact with seawater, the tubules rapidly elongate and become sticky, entangling potential predators. This defense mechanism is highly effective and the tubules can be regenerated.

48. What is the function of the test in sea urchins?

The test is the hard, spherical endoskeleton of sea urchins. It provides protection for internal organs, supports the body structure, and serves as an attachment point for spines and tube feet. The test is composed of interlocking calcium carbonate plates that allow for growth without molting.

49. How do echinoderms maintain their mineral balance in calcium-poor environments?

In calcium-poor environments, echinoderms can actively concentrate calcium from seawater using specialized cells. They may also adjust the density of their skeleton, reducing calcification in low-calcium conditions. Some species can even resorb calcium from their own skeletons during times of scarcity.

50. What is the significance of the ophiopluteus larva in brittle star development?

The ophiopluteus larva is the free-swimming, bilaterally symmetrical larval stage of brittle stars. It's significant because it represents a completely different body plan from the adult, demonstrating the dramatic metamorphosis in echinoderm development. This larval