Phylum Echinodermata: Definition, Characteristics, Classification

Phylum Echinodermata: Have you ever stepped accidentally on a starfish while walking around a beach? It may or may not give you a painful sting. Have you ever dived into the sea and seen some star-like radiating creatures? These animals are invertebrates and come under Phylum Echinodermata. An interesting fact of this phylum is that all animals belonging to Echinodermata are marine.

Starfish is a beautiful creature that is a bottom dweller and sometimes comes to the beach with a water current. If not at the beach, you must have seen organisms belonging to the Phylum Echinodermata in the science lab! Some of the echinoderms are star-shaped, while some are cylindrical or oval-shaped. Let us find out more about this beautiful exclusively marine creature in this article.

Phylum Echinodermata Definition

Animals that are spiny skinned, exclusively marine and have a unique water vascular system come under phylum Echinodermata. In \(1738,\) Jacob Klein introduced the name “Echinodermata”. “Echinos”– spines, “derma”-skin. Starfish, Sea urchin, Sea cucumbers are examples of this phylum. Around \(6000\) species are known.

History of Phylum Echinodermata

Echinoderms are known from ancient times. Jacob Klein coined the term “Echinodermata” in \(1738.\) Linnaeus researched many echinoderms like Echinus, Asterias and Holothuria but placed them in the phylum Mollusca. Huxley, in \(1875\) gave a new name Deutrostomata and kept bilater­ally symmetrical coelomates in this group. Metchnikoff, in \(1881\) studied Echinoderm larvae and Tornaria larva of Hemichordata and found they have resemblance with each other, so he kept Echinoderms and Hemichordates under Ambulacraria.

General Characteristics of Phylum Echinodermata

1. This phylum includes spiny skinned animals.
2. Habitat: Exclusively Marine and benthic animals, i.e. found at the sea bottom.
3. Bottom dwellers, few of its species are attached to the substratum while others are-swimming.
4. Symmetry: Radially (pentamerous) symmetrical-adults
   Bilaterally symmetrical-larva
5. Body form: Star-shaped, cylindrical or spherical. The body is unsegmented and lacks a head.
6. Body cover: It is covered by the epidermis, which is single layered, delicate and ciliated. Some have calcareous plates called ossicles in the dermis, beneath the skin, which forms the endoskeleton of the body and is mesodermal in origin.
7. The body bears spines and pincer-like pedicellariae. Pedicellariae help in cleaning the debris off the surface of the body. The function of spines is to protect the body.
8. The body is divided into oral and aboral surfaces.
9. They are called deuterostomes, which means anus form first from the blastopore during embryonic development in Echinoderms and chordates.
10. Body cavity: It has a true coelom, which is lined by ciliated peritoneum.
11. Water Vascular System (Ambulatory system): It is the most distinctive feature of the echinoderms. It comprises tube feet which helps in locomotion, the capture of food, respiration. Madreporite, a perforated plate, is present in which numerous pores are there. These pores allow water to pass through the system. This system is of coelomic origin and is used for locomotion, respiration, and attachment to the substratum.
12. The digestive system is simple, complete with mouth and anus. Incomplete digestive systems are found in Brittle stars.
13. It has an open type of circulatory system called haemal and perihaemal system of coelomic origin. The respiratory pigment is absent in blood. The heart is absent.
14. Respiratory organs: It is poorly developed. They use simple gills called peristomial gills like sea urchins, tube feet in starfishes, genital bursae in brittle stars to exchange gases. The exchange of gases occurs by the process of simple diffusion.
15. Nervous system: Two nerve rings are present. One ring is called an oral ring, while the other ring is called an aboral ring.
16. Reproduction is indirect, i.e. there will be a larval stage. Most common larva is bipinnaria larva. Sexual reproduction takes place.
17. Echinoderms have the power of autonomy. It means cutting its arm in self-defence. The cut arm keeps on wiggling to draw the predator’s attention, and in the meantime, the starfish would run away.
    In some animals like sea cucumbers, evisceration takes place, in which the animal throws out part of the alimentary tract like the stomach, etc., for self-defence.
18. Excretory Organs: Specialized excretory organs are absent. They are ammonotelic.

Classes Under Phylum Echinodermata

Phylum Echinodermata is divided into five classes:

1. Asteroidea– (Gk. aster–star, eidos– form).
   a. The body is star-like.
   b. Five arms radiate from the body, which is not sharply marked off.
   c. Larval forms are bipinnaria and brachiolaria.
   d. Examples: Asterias (starfish), Pentacerous (starfish).
2. Ophiuroidea– (Gk. Ophis– snake, Oura– tail, eidos– form).
   a. The central disc of the body is a pentamerous, flat disc.
   b. The body is star-like as that of starfish in which arms sharply point out.
   c. Larva is Ophiopluteus.
   d. Examples: Ophiothrix (brittle star), Ophioderma (brittle star).
3. Echinoidea– (Gk. echinos– hedgehog, eidos– form).
   a. The body is globular or disc-like.
   b. The unique feature of this class is that they have Aristotle’s Lantern, which is a chewing and biting apparatus with teeth.
   c. Larval forms are Pluteus and Echinopluteus.
   d. Examples: Echinus (Sea urchin), Clypeaster.

4. Holothuroidea– (Gk. Holothurian– sea cucumber, eidos- body form).
   a. The body is cylindrical.
   b. The oral end has tentacles that surround the mouth.
   c. Larval forms are Auricularia and Doliolaria.
   d. Examples: Holothuria (Sea cucumber), Cucumaria.

5. Crinoidea– (Gk. Crinon– lily, eidos– form).
   a. Arms are branched and.
   b. The body has a central disc that is attached to the substratum.
   c. Larval form is Doliolaria.
   d. Example: Antedon (feather star), Sea lilies.

Phylum Echinodermata Examples

Some of the examples of the phylum Echinodermata are as follows:-

1. Asterias (Starfish)
   a. The systematic position of Asterias is-
   Kingdom: Animalia
   Phylum: Echinodermata
   Class: Asteroidea
   Order: Forcipulatida
   Family: Asteriidae
   Genus: Asterias
   b. They have five radiating arms.
   c. Water Vascular System helps them to locomote, food capture and attachment.
   d. Forcep-like structures called pedicellariae help to clean debris on the surface of the body.
   e. The madreporite is present between two arms of the five arms, which helps draw water into the body.

2. Ophiura (Brittle star)
   a. The systematic position of Ophiura is-
   Kingdom: Animalia
   Phylum: Echinodermata
   Class: Ophiuroidea
   Order: Ophiurida
   Family: Ophiuridae
   Genus: Ophiura
   b. With the help of arms, it also swims like a snake.
   c. The anus is absent.
   d. Five sharply pointed arms come out of the pentamerous central disc.

3. Cucumaria (Sea cucumber)
   a. The systematic position of Cucumaria is-
   Kingdom: Animalia
   Phylum: Echinodermata
   Class: Holothuroidea
   Order: Dendrochirotida
   Family: Cucumariidae
   Genus: Cucumaria
   b. The mouth is surrounded by tentacles anteriorly.
   c. Respiratory trees in the cloaca perform respiration.

Phylum Echinodermata Ecological and Economic Importance

Animals coming under Echinodermata have the following ecological and economic importance-
a. Ecological Importance:
Some of the echinoderms, like sea cucumbers, burrow deep inside the sand, leading to more oxygen at greater depth, allowing more organisms to inhabit there.
Echinoderms serve as an important food source for some animals, so we can say that it helps carry ocean food chains.
Echinoderms check seaweed growth on the corals as they feed on seaweeds. Seaweeds destroy the entire coral reef.
b. Echinoderms as food:
It is used to cook delicious food like soup and stews in some countries like Japan, Peru, Spain, etc.
c. Echinoderms as medicine:
It is used to make medicines. The toxin of sea urchin helps in decreasing the growth of tumour cells. Calcareous scales of echinoderms are a rich source of calcium.

Common Symptoms of Phylum Echinodermata

Here are some of the common symptoms of the Phylum Echinodermata:

1. Habitat and Nature – The animals of the phylum Echinodermata are aquatic and marine animals. These animals live at the bottom of the sea and are of carnivorous nature. The animals of the phylum Echinodermata are sometimes independent or localised.

2. Body Shape – These animals are spherical, cylindrical and star-shaped, with 5 arms of equal size attached to the centre of their body, which is of undivided and divided type.

3. Segments – The body of the animals of the Phylum Echinodermata is unsegmented.

4. Symmetry – In these animals, the physical symmetry of adults is radial, but the symmetry of the larvae of those animals is an archipelago.

5. Body Wall – In these animals, the skin forms the body wall. In these, a plate made of calcium is found, which is formed from the mesoderm and from these plates develop thorn-like structures which are present on the outer surface of the skin.

6. Motion – The water vascular system in these animals develops from the embryonic coelom, due to which the work of circulation takes place. This mechanism is not found in other animals. Therefore it is the main characteristic of these animals. In these, structures like podia and tube feet act as circulation while some animals are also localized.

7. Nutrition – These animals are of carnivorous nature, which feeds on small organisms.

8. Respiratory System – In these animals, respiration is done through structures like gills, tube feet, respiration elder and bursae.

9. Nervous System – The nervous system of these animals is less developed, in which the brain is absent, circumpolar and radial nerves are found.

10. Sensory Organ – Animals have tactile sensory structures, chemical sensory structures, photosensitive structures, as well as balance bars and internal contacts.

Facts Related to Phylum Echinodermata

Here are some of the interesting facts about Phylum Echinodermata:

1. Asterias is called star fish. Its shape is like a five-rayed star.
2. Antidon is commonly known as Feather Star.
3. Ophiura is also known as Brittlestar and Echinus is known as sea urchin.

Summary

Phylum Echinodermata includes starfishes, sea cucumbers, etc. They are spiny skinned animals that are exclusively marine and benthic. Adults are radially (pentamerous) symmetrical, while the larva is bilaterally symmetrical. Its body can be star-shaped, cylindrical or spherical and bears calcareous plates below the epidermis.

It also bears pincer-like pedicellariae. Water Vascular System is the unique feature of the echinoderms and is used for locomotion, respiration, attachment, and food capture. The digestive system is simple and complete. They have poorly developed nervous systems and sense organs. Reproduction is indirect. Phylum Echinodermata are known to maintain the ecological balance of the sea.

FAQs

Q.1. What are the five characteristics of echinoderms?
Ans: \(5\) characteristics of echinoderms are:-
a. Exclusively marine.
b. Spiny-skinned animals
c. Adults are radially (pentamerous) symmetrical, and larva is bilaterally symmetrical.
d. They have special structures called water vascular systems for locomotion, respiration, food capture, etc.
e. Reproduction is indirect. 

Q.2. What are the five classes of phylum Echinodermata?
Ans: The five classes of phylum Echinodermata are:-
a. Asteroidea
b. Ophiuroidea
c. Echinoidea
d. Holothuroidea
e. Crinoidea

Q.3. Do echinoderms have a heart?
Ans: No, echinoderms do not have a heart. 

Q.4. How do echinoderms breathe?
Ans: They do not have proper respiratory organs. Instead, they breathe through the water vascular system and simple gills.

Q.5. What are the five examples of echinoderms?
Ans: Starfish, sea urchin, sea cucumber, feather star, Sea lilies are examples of echinoderms.

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