Phylum Echinodermata – Definition, Classification, Characteristics, Examples

What is Phylum Echinodermata?

• Derived from the Greek terminology, “Ekhinos” signifying “spiny” and “derm” denoting “skin”, the Echinodermata phylum encompasses organisms characterized by their spiny integument. This distinctive feature is attributed to their endoskeleton, composed of calciferous ossicles, rendering them the designation of Echinoderms.
• Exhibiting an organ system level of organization, these multicellular entities possess intricate and well-developed organ systems. Members of the Echinodermata phylum, colloquially termed Echinoderms, are predominantly marine entities.
• Remarkably, no evidence suggests the existence of either terrestrial or freshwater representatives within this phylum. Distributed extensively across marine habitats, Echinoderms inhabit the vast expanse from intertidal zones to the profound depths of abyssal zones.
• With a repertoire of approximately 7,000 species, this phylum stands as the second most extensive, surpassed only by Phylum Chordata. Echinoderms exhibit a unique morphological trait: while their larval stage displays bilateral symmetry, the adult form transitions to a pentamerous radial symmetry.
• This radial symmetry is complemented by their calcareous endoskeleton, an assembly of overlapping minute plates that not only provide protection but also bestow structural integrity. Integral to their physiology is the water vascular system, a specialized coelomic derivative. This system facilitates various vital functions, from gaseous exchange and nutrient circulation to waste excretion.
• Furthermore, it operates the podia, small fleshy protrusions controlled by the Echinoderm’s nervous system, which function through a combination of water pressure and muscular activity. Echinoderms, with their vibrant hues and distinct morphologies, contribute significantly both ecologically and geologically.
• Notable representatives of this phylum encompass marine stars, sea urchins, and starfish, among others. Their absence in freshwater and terrestrial habitats underscores their exclusive marine lineage.
• In summation, Phylum Echinodermata represents a diverse group of marine organisms, distinguished by their calcareous endoskeleton, pentamerous radial symmetry, and a sophisticated water vascular system. Their ecological and geological significance, coupled with their unique physiological attributes, underscores their pivotal role in marine ecosystems.

Phylum Echinodermata Definition

Phylum Echinodermata encompasses marine organisms characterized by a calcareous endoskeleton, pentamerous radial symmetry in adulthood, and a specialized water vascular system, with notable members including starfish, sea urchins, and sea cucumbers.

Characteristics of Phylum Echinodermata 

1. Marine Habitat: Echinoderms are strictly marine organisms, populating every marine environment from the intertidal zones to the profound oceanic depths. Their distribution is both extensive and diverse, making them one of the most widespread marine taxa.
2. Body Organization: These organisms exhibit an organ system level of organization, with a triploblastic origin, meaning they develop from three primary embryonic layers: ectoderm, mesoderm, and endoderm.
3. Symmetry: Adult echinoderms predominantly display radial symmetry, typically pentamerous, where body parts are arranged in sets of five or multiples thereof. However, their larval stages exhibit bilateral symmetry.
4. Body Structure: Echinoderms possess varied body morphologies, ranging from star-shaped (as in starfish) to cylindrical (as seen in sea cucumbers) or even spherical (like sea urchins). Notably, they lack a distinct head, and their body is often adorned with spines, providing both protection and a unique appearance.
5. Endoskeleton: Their internal skeleton is composed of calcium carbonate, forming either closely fitted plates known as theca or test, or separate ossicles.
6. Water Vascular System: A hallmark of echinoderms is their water vascular system, also termed the ambulacral system. This intricate network, including tube feet or podia, facilitates locomotion, respiration, and the transportation of nutrients and waste.
7. Digestive System: Echinoderms possess a complete digestive system, with a ventrally located mouth and a dorsally situated anus. However, exceptions exist, such as in brittle stars, where the digestive tract is incomplete.
8. Circulatory and Respiratory Systems: These organisms have an open circulatory system. Respiration is facilitated through specialized structures like gills or the cloacal respiratory tree.
9. Nervous System: Echinoderms have a simple radial nervous system, devoid of a brain, but with a circumoral ring and radial nerves.
10. Excretory System: Notably absent in echinoderms, the elimination of nitrogenous waste, primarily ammonia, occurs through diffusion across respiratory surfaces, such as tube feet.
11. Reproduction: Echinoderms can reproduce both sexually, with typically separate sexes and external fertilization, and asexually through processes like fragmentation. Their development is indirect, involving free-swimming larvae that undergo metamorphosis.
12. Regeneration: A remarkable ability of many echinoderms is regeneration, allowing them to replace lost or damaged body parts. For instance, starfish can regenerate lost arms.
13. Sensory Organs: While echinoderms possess rudimentary sense organs, they include structures like chemoreceptors, tactile organs, and terminal tentacles, aiding in environmental interactions.

In essence, Phylum Echinodermata represents a diverse group of marine organisms, distinguished by their unique water vascular system, radial symmetry, and regenerative capabilities, among other characteristics. Their ecological significance and adaptability underscore their evolutionary success in marine habitats.

Phylum Echinodermata Classification

The classification is based on Hyman, L.H. (1995). Only orders and living classes have been mentioned.

Subphylum 1. Pelmatozoa (Gr., pelmatos=stalk+ zoon=animals)

Subphylum Pelmatozoa, derived from the Greek words “pelmatos,” meaning stalk, and “zoon,” meaning animals, represents a group of echinoderms, primarily known for their sedentary nature.

1. General Characteristics:
   • Extinct Predominance: The majority of the Pelmatozoa are extinct, with only a few extant representatives.
   • Sedentary Nature: These echinoderms are typically stationary, anchored to substrates by an aboral stalk.
   • Body Orientation: The mouth and anal aperture are oriented on the oral surface, facing upwards, while the viscera are protected within a calcareous test.
   • Absence of Suckers: Unlike some other echinoderms, Pelmatozoa lack suckers.
   • Function of Tube Feet: Their tube feet or podia are primarily adapted for capturing food.
   • Nervous System: The main nervous system is situated on the aboral side.
   • Taxonomic Representation: This subphylum comprises only one extant class.
2. Class Crinoidea:
   • Common Names: Often referred to as Sea Lilies or Feather Stars.
   • Temporal Distribution: Crinoidea includes both extinct and living forms.
   • Morphological Distinction: Living representatives are typically free-moving and lack a stalk, whereas the extinct forms were stalked.
   • Body Structure: The body is divided into an aboral cup, termed the calyx, and an oral cover or roof known as the tegmen. The structure is strongly pentamerous.
   • Oral Features: Both the mouth and anus are located on the oral surface.
   • Arms: These are movable structures, typically numbering 5 or 10, which may be simple or branched. They may also possess pinnules.
   • Adaptations: The tube feet lack suckers, and there’s an absence of madreporite, spines, and pedicellariae. The ambulacral grooves are open and extend along the arms and pinnules to their extremities.
   • Reproduction: Crinoidea have separate sexes, and their larval form is known as doliolaria.
3. Order Articulata:
   • Temporal Distribution: This order encompasses both extinct and living crinoids.
   • Mobility: Members of this order are non-sessile and exhibit free-swimming behavior.
   • Body Features: The calyx is pentamerous and flexible, incorporating the lower arm ossicles, while the tegmen is leathery, embedded with calcareous particles or small plates. The mouth and ambulacral grooves are exposed.
   • Representative Species: Notable examples include Antedon (commonly known as sea lily), Rhizocrinus, and Metacrinus.

In summary, Subphylum Pelmatozoa offers a glimpse into the evolutionary history of echinoderms, with its predominantly extinct members and unique sedentary adaptations. The Crinoidea class, with its sea lilies and feather stars, stands as a testament to the adaptability and resilience of this group within marine ecosystems.

Subphylum 2. Eleutherozoa (Gr., eleutheros=free+ zoon= animals)

Subphylum Eleutherozoa, derived from the Greek terms “eleutheros,” meaning free, and “zoon,” signifying animals, encompasses a group of echinoderms characterized by their free-living nature.

1. General Characteristics:
   • Free-living Nature: Eleutherozoa members are predominantly free-living echinoderms, unencumbered by attachment structures.
   • Absence of Stalk: Unlike some echinoderms, these organisms lack a stem or stalk, further emphasizing their free-living nature.
   • Pentamerous Symmetry: Their body structure adheres to a strict pentamerous symmetry, a common trait among echinoderms.
   • Oral Surface Orientation: The oral surface, which houses the mouth, is oriented downward or is positioned laterally, allowing these organisms to interact directly with the substrate.
   • Anus Positioning: Typically, the anus is located on the aboral surface, opposite the mouth.
   • Ambulacral Grooves: These grooves, characteristic of echinoderms, are not primarily utilized for food gathering in Eleutherozoa.
   • Locomotory Adaptations: The tube feet, equipped with suckers, primarily serve as locomotory organs, facilitating movement across various substrates.
   • Nervous System: The primary nervous system of these organisms is oriented orally, reflecting their unique body orientation and interactions with their environment.

In essence, Subphylum Eleutherozoa represents a distinct group of echinoderms, characterized by their free-living lifestyle and specific anatomical adaptations. Their absence of a stalk, combined with specialized locomotory organs and an oral nervous system, underscores their evolutionary adaptations to a mobile existence within marine ecosystems.

Class 1. Holothuroidea (Gr., holothurion=water polyp+ eidos=form)

Class Holothuroidea, derived from the Greek terms “holothurion,” meaning water polyp, and “eidos,” signifying form, represents a unique group of echinoderms commonly known as sea cucumbers.

1. General Characteristics:
   • Morphology: Sea cucumbers possess a bilaterally symmetrical body, typically elongated along the oral-aboral axis. The anterior end houses the mouth, while the posterior end typically contains the anus.
   • Body Surface: Their body surface is coarse, often devoid of the spiny structures seen in other echinoderms.
   • Endoskeleton: The endoskeleton in Holothuroidea is minimal, reduced to microscopic spicules or plates embedded within the body wall.
   • Oral Structures: The anterior mouth is encircled by tentacles connected to the water vascular system, facilitating feeding.
   • Ambulacral Grooves: These grooves, characteristic of many echinoderms, are concealed in sea cucumbers.
   • Locomotion: Podia or tube feet, when present, primarily aid in movement.
   • Digestive System: The alimentary canal is elongated and coiled, with some species possessing a cloaca equipped with respiratory trees for gaseous exchange.
   • Reproduction: Sea cucumbers have separate sexes, with gonads manifesting as single or paired tufts or tubules.
2. Taxonomic Orders:
   • Order Aspidochirota: Characterized by numerous podia, often forming a distinct sole. The mouth is encircled by 10-30, typically 20, peltate or branched oral tentacles. Notable examples include Holothuria, Stichopus, and Mesothuria.
   • Order Elasipoda: These sea cucumbers possess numerous podia, with tentacles being leaf-like in structure. Unique to this order, the tube feet are webbed, forming fin-like structures. They are primarily deep-sea dwellers, with species like Deima and Benthodytes.
   • Order Dendrochirota: This order is marked by numerous podia, either on a sole or covering the entire ambulacral surface. Tentacles are irregularly branched, and examples include Thyone, Cucumaria, and Phyllophorus.
   • Order Molpadonia: A distinguishing feature of this order is the absence of podia, except as anal papillae. The posterior end is tail-like, and species include Molpadia and Paracaudina.
   • Order Apoda: Representing worm-like sea cucumbers, members of this order lack both tube feet and respiratory trees. Their vermiform body may have a smooth or warty surface, with tentacles ranging from 10-20 in number. Notable species are Synapta and Chiridoata.

In summation, Class Holothuroidea offers a diverse array of sea cucumbers, each with unique morphological and ecological adaptations. Their distinct body structure, combined with specialized tentacles and reduced endoskeleton, underscores their evolutionary niche within marine ecosystems.

Class 2. Echinoidea (Gr., echinos=hedgehog+eidos=form)

Class Echinoidea, derived from the Greek terms “echinos,” meaning hedgehog, and “eidos,” signifying form, represents a group of echinoderms commonly recognized as sea urchins and sand dollars.

1. General Characteristics:
   • Morphology: Echinoidea members exhibit varied body shapes, ranging from spherical, disc-like, oval, to heart-shaped.
   • Endoskeletal Shell: The body is encased in a test, an endoskeletal shell formed from closely fitted calcareous plates. This test is adorned with movable spines.
   • Calcareous Plates: The outer calcareous plates are organized into 5 alternating ambulacral and 5 inter-ambulacral areas.
   • Locomotion: Podia or tube feet, which protrude from the pores of ambulacral plates, primarily function in movement.
   • Oral Structures: The mouth is centrally situated on the oral surface, encircled by a membranous peristome. A unique chewing apparatus, known as Aristotle’s lantern, equipped with teeth, is present.
   • Ambulacral Grooves: These grooves are shielded by ossicles, with tube feet equipped with suckers.
   • Anus Positioning: The anus is positioned at the aboral pole, encircled by a membranous periproct.
   • Pedicellariae: These are stalked structures with three jaws.
   • Reproduction: The sexes are separate, with gonads typically numbering five or fewer. Development involves a free-swimming echinopluteus larva.
2. Taxonomic Subclasses and Orders:
   • Subclass Bothriocidaroida: This subclass comprises primarily extinct echinoids, with each inter-ambulacral having a single row of plates. Notable example: Bothriocidaris.
   • Subclass Regularia: Members have a globular body, predominantly circular or occasionally oval. The mouth is centrally located on the oral surface, surrounded by the peristome. Orders within this subclass include:
     • Order Lepidocentroida: Characterized by a flexible test with overlapping plates. Example: Palaeodiscus.
     • Order Cidaroidea: Features a globular and rigid test. Examples: Cidaris and Notocidaris.
     • Order Aulodonta: The test is symmetrical and globular. Examples: Diadema and Astropyga.
     • Order Camarodonta: The test is rigid, occasionally oval. Examples: Echinus and Strongylocentrotus.
   • Subclass Irregularia: Members possess an oval or circular body, flattened along the oral-aboral axis. Orders within this subclass include:
     • Order Clypeastroida: Recognized by a flattened test with an oval or rounded shape. Examples: Clypeaster and Echinarachinus.
     • Order Spatangoida: The test is either oval or heart-shaped. Examples: Spatangus and Echinocardium.

In summary, Class Echinoidea presents a diverse array of echinoderms, each with unique morphological and ecological adaptations. From the movable spines of sea urchins to the flattened bodies of sand dollars, this class showcases the evolutionary versatility and adaptability of echinoderms within marine ecosystems.

Class 3. Asteroidea (Gr., aster=star+ eidos= form)

Class Asteroidea, derived from the Greek terms “aster,” meaning star, and “eidos,” signifying form, represents a distinctive group of echinoderms commonly identified as starfishes or sea stars.

1. General Characteristics:
   • Morphology: Asteroidea members typically exhibit a flattened body, which can be pentagonal or star-shaped.
   • Body Surfaces: The oral and aboral surfaces are clearly demarcated. The oral surface faces downward, while the aboral surface is oriented upward.
   • Arms: These organisms possess five or more arms, which seamlessly extend from the central disc.
   • Oral Structures: The mouth is centrally located on the oral surface and is encircled by a membranous peristome.
   • Anus Positioning: The anus is small, often inconspicuous, and is situated eccentrically on the aboral surface.
   • Locomotion: Tube feet, located in ambulacral grooves on the oral surface, are equipped with suckers aiding in movement.
   • Endoskeleton: The endoskeleton is flexible, composed of individual ossicles.
   • Pedicellariae: These are small, movable, spine-like structures consistently present in Asteroidea.
   • Respiration: Respiration is facilitated by papulae.
   • Reproduction: The sexes are separate, with gonads arranged radially. Development typically involves either a bipinnaria or branchiolaria larva.
2. Taxonomic Orders:
   • Order Phanerozonia: This order is characterized by a body with marginal plates, usually with papulae on the aboral surface. The arms have two rows of prominent marginal plates. Examples include Luidia, Astropecten, Archaster, and Pentaceros.
   • Order Spinulosa: Members of this order generally lack prominent marginal plates on their arms. The aboral skeleton is either imbricated or reticulated, adorned with single or grouped spines. Pedicellariae are infrequent in this order. Notable examples are Aesterina, Echinaster, Hymenaster, and Solaster.
   • Order Forcipulata: This order lacks conspicuous marginal plates. The aboral skeleton is primarily reticulated, featuring prominent spines. Pedicellariae in this order are of the pedunculate type with a basal piece. Examples include Brisingaster, Heliaster, Zoraster, and Asterias.

In summation, Class Asteroidea showcases a diverse array of star-shaped echinoderms, each with unique morphological and ecological adaptations. From the iconic starfishes found on many coastlines to the more obscure species, this class underscores the evolutionary diversity and adaptability of echinoderms within marine ecosystems.

Class 4. Ophiuroidea (Gr., ophis=serpent+ oura=tail+ eidos= form)

Class Ophiuroidea, derived from the Greek words “ophis” meaning serpent, “oura” denoting tail, and “eidos” signifying form, represents a distinctive group of echinoderms commonly referred to as brittle-stars and their allies.

1. General Characteristics:
   • Morphology: Ophiuroidea members typically exhibit a flattened body, either pentamerous or rounded, centralized around a disc.
   • Body Surfaces: The organisms possess clearly demarcated oral and aboral surfaces.
   • Arms: These are star-like and distinctly separated from the central disc.
   • Pedicellariae: These structures are notably absent in Ophiuroidea.
   • Ambulacral Grooves: These are either absent or concealed by ossicles.
   • Digestive System: The class is characterized by a sac-like stomach, with the absence of both an anus and intestine.
   • Locomotion: Tube feet are present but lack suckers.
   • Madreporite: This structure is located on the oral surface.
   • Reproduction: The sexes are separate, with a pentamerous gonad arrangement. Development typically involves a free-swimming pluteus larva.
2. Taxonomic Orders:
   • Order Ophiurae: This order encompasses brittle and serpent stars. The arms, predominantly numbering five, move primarily in the transverse plane and are articulated by pits and projections. The discs and arms are typically shielded by distinct scales. Lateral spines on the arms point outwards and towards the arm tips. Notable examples include Ophioderma, Ophioscolex, Ophiothrix, and Ophiolepie.
   • Order Euryalae: Members of this order possess arms that are either simple or branched. These arms are long, flexible, and can coil around objects or roll up vertically. The arm ossicles are articulated in a streptospondylus manner. The discs and arms are enveloped by soft skin, with downward-pointing spines that often form hooks or spiny clubs. Representative species include Asteronyx and Gorgonocephalus (basket stars).

In conclusion, Class Ophiuroidea presents a fascinating array of echinoderms, distinguished by their serpent-like arms and unique anatomical features. Their adaptive morphology and ecological roles further underscore the evolutionary diversity of echinoderms within marine ecosystems.

Evolution of Body Plan

The body plan of echinoderms, a unique group of marine organisms, showcases a series of evolutionary adaptations that have contributed to their success in diverse marine habitats. This article delves into the key features of the echinoderm body plan and their evolutionary significance.

1. Deuterostome Development:
   • Ancestral Traits: Echinoderms exhibit deuterostome development, a characteristic where the blastopore, formed during embryonic development, evolves into the anus. This developmental pattern is a distinguishing feature of deuterostomes.
   • Taxonomic Significance: Among the deuterostomes, echinoderms represent the largest group outside of the Chordates, highlighting their evolutionary prominence.
2. Water Vascular System:
   • Unique Adaptation: The water vascular system is a specialized network of fluid-filled canals and tubes exclusive to echinoderms. This system aids in various physiological functions, including feeding, locomotion, and respiration.
   • Key Components:
     • Tube Feet: Equipped with ampullae, these structures can fill with water, allowing the foot to extend and adhere to surfaces or prey.
     • Sieve Plate (Madreporite): This structure serves as the entry point for water into the water vascular system.
     • Ring Canal: Positioned centrally, this canal acts as the primary hub from which all other canals radiate.
3. Endoskeletal Features:
   • Composition: The endoskeleton of echinoderms is composed of calcium carbonate, providing structural support.
   • Adaptability: Depending on the species, the endoskeletal components can either be loosely connected, facilitating movement as seen in sea stars, or fused together to form a rigid structure, as observed in sea urchins.
4. Radial Symmetry:
   • Pentaradial Symmetry: Echinoderms exhibit a unique form of radial symmetry known as pentaradial symmetry, characterized by five symmetrical sections around a central axis.
   • Evolutionary Transition: Intriguingly, echinoderms evolved from bilaterally symmetrical ancestors. This transition from bilateral to radial symmetry is a secondary evolutionary adaptation.
   • Evidence of Bilateral Ancestry: The bilateral symmetry of echinoderm larvae provides compelling evidence of their bilaterally symmetrical ancestors.

In conclusion, the echinoderm body plan is a testament to the intricate processes of evolution. From their unique water vascular system to their transition from bilateral to radial symmetry, echinoderms offer valuable insights into the adaptive strategies that have shaped marine biodiversity over millennia.

Feeding and Excretion Mechanisms in Phylum Echinodermata

Echinoderms, a diverse group of marine organisms, have evolved specialized mechanisms for feeding and excretion that contribute to their ecological success. This article elucidates the intricacies of these processes in the phylum Echinodermata.

1. Feeding Mechanisms:

• Digestive System Configuration: Echinoderms possess a one-way digestive system, ensuring a streamlined flow of food and waste.
• Dual Stomach System:
  • Cardiac Stomach: Remarkably, the cardiac stomach of echinoderms can be everted, or turned inside out, allowing it to extend outside the body. This unique adaptation facilitates the external commencement of digestion, particularly beneficial when handling larger prey.
  • Pyloric Stomach: Following the initial external digestion, the prey is further processed in the pyloric stomach, where it undergoes complete digestion.
• Digestive Enzymes: Echinoderms secrete potent enzymes that liquefy their prey, ensuring efficient nutrient absorption and minimal waste production.

2. Excretion Mechanisms:

• Digestive Waste Elimination: Post-digestion, the residual waste is expelled from the body through the anus, which is strategically positioned opposite the mouth. This ensures a clear separation between the intake and expulsion sites, optimizing the flow of materials.
• Nitrogenous Waste Management: Unlike many other organisms, echinoderms lack specialized excretory organs. Instead, they have evolved a system where nitrogenous waste is efficiently eliminated. This waste is primarily removed from the water vascular system, a unique feature of echinoderms. The process occurs via two primary pathways:
  • Diffusion through Tube Feet: The tube feet, integral components of the water vascular system, facilitate the diffusion of nitrogenous waste into the surrounding water.
  • Concurrent with Digestive Waste: Some nitrogenous waste is expelled along with the digestive waste, ensuring efficient waste management.

In summary, the phylum Echinodermata showcases a blend of evolutionary adaptations in its feeding and excretion mechanisms. The ability to initiate digestion externally and the efficient management of waste, even in the absence of specialized excretory organs, underscore the evolutionary finesse of these marine organisms.

Respiration and Circulation of Phylum Echinodermata

The phylum Echinodermata, comprising marine organisms such as starfish and sea urchins, exhibits unique physiological adaptations for respiration and circulation. These mechanisms are intricately intertwined, ensuring the efficient exchange of gases and the movement of nutrients throughout the organism.

1. Respiration:

• Water Vascular System: Central to the respiratory processes of echinoderms is their water vascular system. This hydraulic system not only aids in locomotion and feeding but also plays a pivotal role in respiration.
• Gas Exchange Mechanisms:
  • Dermal Gills: Echinoderms possess simple gills, known as dermal gills or papulae, which facilitate the direct exchange of gases with the surrounding water. These structures increase the surface area available for gas exchange.
  • Diffusion through Tube Feet: The tube feet, which are extensions of the water vascular system, further assist in gas exchange. Oxygen from the surrounding water diffuses into the tube feet, while carbon dioxide diffuses out, ensuring efficient oxygenation of internal tissues.

2. Circulation:

• Reduced Circulatory System: Unlike many other organisms, echinoderms have a rudimentary circulatory system. The absence of specialized circulatory structures is compensated by the multifunctional water vascular system.
• Fluid Movement: The “blood” or coelomic fluid in echinoderms lacks pigmentation, rendering it colorless. This fluid circulates throughout the water vascular system, distributing nutrients and removing waste products.
• Absence of a Heart: Notably, echinoderms do not possess a heart. Instead, the movement of fluid within the water vascular system, driven by hydraulic forces, ensures the distribution of nutrients and gases throughout the organism.

In conclusion, the phylum Echinodermata showcases a harmonious integration of its water vascular system in both respiratory and circulatory functions. The ability to efficiently exchange gases and circulate nutrients without the need for complex circulatory structures underscores the evolutionary adaptability of these marine organisms.

Reproduction of Phylum Echinodermata

The reproductive strategies of the phylum Echinodermata, which encompasses marine organisms such as starfish, sea urchins, and sea cucumbers, are multifaceted and diverse. These strategies can be broadly categorized into sexual and asexual reproduction, each with its own set of characteristics and processes.

1. Sexual Reproduction:

• Gonochorism: Echinoderms typically exhibit gonochorism, where individuals are distinctly male or female. This separation of sexes ensures genetic diversity during reproduction.
• External Fertilization: A hallmark of echinoderm reproduction is external fertilization, predominantly achieved through broadcast spawning. During this process, both males and females release their gametes into the water column, where fertilization occurs.
• Prodigious Egg Production: Female echinoderms are prolific egg producers, with the capability to release up to 100 million eggs in a single spawning event. This vast number increases the likelihood of successful fertilization in the vast marine environment.
• Developmental Transition: Post-fertilization, the zygote undergoes embryonic development to form a bilaterally symmetrical, planktonic larva. Over a span of approximately two months, these larvae undergo metamorphosis, transitioning to a radially symmetrical adult form. Subsequently, they settle on the ocean floor, marking the completion of their life cycle.

2. Asexual Reproduction:

• Fragmentation and Fission: Some echinoderms possess the ability to reproduce asexually through processes like fragmentation or fission. Although less common than sexual reproduction, this method allows for rapid population expansion.
• Regenerative Capabilities: A defining trait of echinoderms is their remarkable regenerative abilities. If an echinoderm is fragmented, ensuring that a portion of its central disk remains intact, it can regenerate into a complete organism. This is facilitated by the repetitive arrangement of organs in each segment and the presence of a nerve disk.

In summation, echinoderms exhibit a blend of reproductive strategies, ranging from the vast release of gametes in sexual reproduction to the remarkable regenerative capabilities in asexual reproduction. These strategies, evolved over millions of years, ensure the survival and propagation of these marine organisms in their diverse habitats.

Importance of Phylum Echinodermata

Echinoderms, a distinctive group of marine organisms, hold substantial importance across various sectors, ranging from culinary practices to scientific research. This article elucidates the multifaceted significance of members of the Phylum Echinodermata.

1. Culinary Importance:
   • Seafood: In 2019, echinoderms, particularly sea cucumbers and sea urchins, were predominantly harvested for consumption. Their unique flavors and textures make them a delicacy in various cuisines.
   • Cultural Preferences: In regions like China, sea cucumbers are an integral ingredient in gelatinous soups. Meanwhile, the gonads of sea urchins are relished in countries like Japan and France, highlighting their global culinary appeal.
2. Medicinal Value:
   • Traditional Medicine: Beyond their culinary applications, sea urchins and sea cucumbers hold medicinal significance, especially in Traditional Chinese Medicine. Their therapeutic properties have been acknowledged and utilized for centuries in such practices.
3. Research Applications:
   • Developmental Biology: The robust larvae of sea urchins have made them a preferred model organism in developmental biology and embryological studies. Their transparent embryos offer insights into various developmental processes.
   • Fertilization Studies: The sperm of sea urchins provides a valuable tool for researchers studying the intricate processes of ovum fertilization.
   • Neurological Research: The arms of brittle stars are employed in research focused on neurodegenerative disorders, offering potential insights into the mechanisms of such diseases.
4. Miscellaneous Uses:
   • Agricultural Application: The calcareous shells of echinoderms serve as a lime source for farmers, aiding in soil neutralization and enrichment.
   • Ecological Role: Starfish, with their predatory nature, are a food source for various marine organisms, playing a pivotal role in maintaining ecological balance.
   • Artistic and Educational Value: Starfish are not only appreciated for their aesthetic appeal in art but also serve as dried specimens for educational purposes, aiding in the understanding of marine biology.

In essence, echinoderms, with their diverse roles and applications, underscore the interconnectedness of nature, culture, and science. Their significance spans across domains, emphasizing the importance of conserving and understanding these unique marine entities.

Examples of Echinoderm

Echinoderms, characterized by their unique water vascular system and radial symmetry, encompass a diverse range of marine organisms. This article delves into some prominent examples of echinoderms, elucidating their distinct characteristics and ecological roles.

1. Sea Stars (Class Asteroidea):
   • Morphology and Behavior: Sea stars, commonly referred to as starfish, are among the most mobile echinoderms. They employ numerous podia to traverse various terrains.
   • Feeding Mechanism: Predominantly predatory, sea stars consume invertebrates and other echinoderms, such as sea urchins. They envelop their prey, extruding their stomach over it. Digestive enzymes initiate the breakdown of the prey, and once mostly dissolved, the starfish retracts its stomach, absorbing the nutrients.
   • Movement: While they may seem sluggish to the human eye, time-lapse observations reveal their active pursuit of prey over extended periods.
2. Sea Urchins (Class Echinoidea):
   • Morphology: Sea urchins possess a rigid test, or exoskeleton, enveloping their body. This test is overlaid with a thin epidermis from which numerous spines and tube feet protrude, facilitating protection and movement.
   • Feeding Mechanism: Equipped with an intricate mouth structure termed Aristotle’s lantern, sea urchins primarily feed on algae and bacteria. They scrape these nutrients from rocks, which often serve as their habitat.
3. Sea Cucumbers (Class Holothuroidea):
   • Morphology and Behavior: Sea cucumbers, despite their deviation from the typical pentamerous symmetry seen in echinoderms, are closely related to starfish. Their endoskeleton comprises calcareous ossicles, which are widely spaced and interconnected by muscles and connective tissues. This arrangement grants sea cucumbers their flexibility.
   • Feeding Mechanism: Sea cucumbers release sticky filaments that trap food particles. These filaments are subsequently retracted into the mouth, where the food is ingested. As a result, many sea cucumbers lead a sedentary, filter-feeding existence on the ocean floor.

Water-vascular system in Asteroidea

The water-vascular system in asteroids (commonly known as starfish) plays a crucial role in their locomotion, feeding, and sensory activities. It is a hydraulic system made up of various interconnected structures, working together to circulate seawater through the organism’s body. This system primarily facilitates movement through the tube feet and is essential for the starfish’s interaction with its environment. The following are the key structures of the water-vascular system in Asteroidea, along with their respective functions:

• Madreporite:
  • The water-vascular system begins at the madreporite, a round, calcareous plate located on the aboral surface (the side opposite the mouth) of the starfish.
  • It has furrows with numerous pores at the bottom, and these pores open into a series of pore canals.
  • Typically, around 200 pores and pore canals are present. These canals unite to form collecting canals, which lead to a sac-like structure called the madreporic ampulla.
  • The madreporite allows seawater to enter and exit the water-vascular system, serving as a crucial point of entry and exit for the hydraulic fluid.
• Stone Canal:
  • The madreporic ampulla extends into an ‘S’-shaped cylindrical tube called the stone canal, which is reinforced with calcareous rings, giving it the name “stone canal.”
  • These rings offer structural support, ensuring the canal maintains its shape.
  • The wall of the stone canal contains a ridge that bifurcates into two spirally coiled lamellae, which can occupy a large portion of the canal’s lumen.
  • In some species, the lumen of the stone canal becomes highly complex due to the extensive development of these lamellae.
  • The stone canal serves as a pump, actively driving the circulation of seawater through the system.
• Ring Canal:
  • Below the stone canal is a wide, pentagonal ring-like canal that encircles the mouth of the starfish.
  • The ring canal is a critical component, forming the central hub that connects the various radial canals extending into the arms.
• Polian Vesicles:
  • In some species of starfish, pear-shaped sacs called polian vesicles are found on the outer side of the ring canal.
  • Typically, there are ten polian vesicles in total, two in each inter-radius (space between the arms). However, the number of polian vesicles can vary across species, and in some species such as Asterias, they may be absent.
  • The function of the polian vesicles is to act as expansion chambers for the storage of the fluid used in the water-vascular system.
• Tiedmann’s Bodies:
  • Attached to the neck of each polian vesicle are small, spherical, yellowish glandular bodies known as Tiedmann’s bodies. These structures are situated on the inner wall of the ring canal.
  • In species where polian vesicles are absent, Tiedmann’s bodies are found directly along the ring canal. In Asterias, for example, the ring canal gives rise to nine such bodies.
  • The specific function of Tiedmann’s bodies is not fully understood, but they are believed to filter fluid from the water-vascular system into the body cavity and may play a role in producing the amoebocytes of the system.
• Radial Canals:
  • The ring canal gives off five radial canals, one for each arm of the starfish.
  • These canals run along the ambulacral grooves (the grooves on the underside of the arms) and extend to the tips of the arms.
  • At the tips of the arms, the radial canals end as the lumen of terminal tentacles.
• Lateral or Podial Canals:
  • Each radial canal gives rise to several paired small branches called lateral or podial canals.
  • These lateral canals are connected to the base of each tube-foot (also called podium) and contain valves to prevent backflow of water from the tube feet into the radial canal.
  • The valves regulate the flow of water from the lateral canal to the ampulla and tube-foot.
• Tube-Feet (Podia) and Ampulla:
  • The lateral canal divides at right angles at the ambulacral pore into two branches: one forms the lumen of the tube-foot, while the other forms the cavity of the ampulla.
  • The ampullae are muscular, sac-like structures located on the anterior side of the tube-feet. Typically, each tube-foot has a single ampulla, although in certain species like Astropecten irregularis, the ampulla may be bilobed with a constriction in the middle.
  • In Asterias, the ampullae are simple and undivided.
  • The tube-feet themselves are hollow, elastic, tube-like structures with a flattened, sucker-like portion at the tip, allowing for attachment and movement on surfaces.
  • The tube-feet project outward along the body’s surface, lying within the ambulacral grooves.
• Muscular and Connective Tissue of Tube-Feet:
  • The movement of the tube-feet operates through an intricate mechanism involving the antagonistic musculature of the ampulla and tube-foot.
  • The ampulla contains smooth circular muscles, called ampullary muscles, that help maintain the position of the tube-foot.
  • The tube-foot consists of retractor (longitudinal) muscles that aid in movement, allowing the tube-foot to elongate and contract.
  • Surrounding the retractor muscles is a collagenous connective tissue sheath, covered by a cuticle. The inner side of the tube-foot is lined by a ciliated epithelium that interacts with the lumen of the tube-foot.

Larval Forms of Echinoderms

Echinoderms exhibit a variety of larval forms, each of which plays a crucial role in their developmental process. These larval stages are essential for understanding the evolutionary and functional characteristics of different echinoderm classes. Despite the diversity of these forms, there are some common features that indicate a shared evolutionary ancestry, particularly in the bilaterally symmetrical larvae.

• Pluteus Larva:
  • The pluteus larva is characterized by five to six pairs of arms, which are supported by calcareous rods, and typically have pigmented tips.
  • It possesses four ciliated bands that form epaulettes at the base of the postoral and posterodorsal arms.
  • The arms are named based on their position relative to the body: preoral, anterolateral, anterodorsal, postoral, posterodorsal, and posterolateral.
  • Two types of pluteus larvae are observed:
    • Ophiopluteus Larva:
      • This free-swimming larva has four pairs of slender arms, supported by a calcareous skeleton.
      • The posterolateral arms are the longest and directed forward, giving the larva a V-shaped appearance.
      • Ciliated bands are found on the edges of the arms.
      • Its alimentary canal includes a mouth, esophagus, stomach, and intestine, all opening through the anus.
      • The ophiopluteus is the larva of the class Ophiuroidea, though in viviparous forms such as Amphiura vivipara, the pluteus stage is omitted.
      • In species like Ophionotus hexactis, development occurs within the ovary, and the pluteus larva may lack arms and an anus.
    • Echinopluteus Larva:
      • This free-swimming larva has five or six pairs of pigmented arms, supported by a calcareous skeleton.
      • The posterolateral arms are shorter and directed outward or backward.
      • The skeletal rods can be simple, thorny, fenestrated, or branched.
      • Like the ophiopluteus, its alimentary canal includes a mouth, esophagus, stomach, and intestine, with the anus as the exit.
      • The echinopluteus is the larva of the class Echinoidea.
• Auricularia and Doliolaria Larvae (Holothuroidea):
  • Auricularia Larva:
    • This free-swimming larva is barrel-shaped and bilaterally symmetrical.
    • It has a well-formed preoral lobe and a single winding ciliated band, which may develop into lobes.
    • The digestive system includes a mouth, sacciform stomach, hydrocoel, right and left stomocoels, and an anus.
    • The hydrocoel forms primary tentacles and communicates with the hydropore via a canal.
    • The calcareous rods typical of other echinoderm larvae are replaced by spheroid, star-shaped, or wheel-like bodies.
  • Doliolaria Larva:
    • The doliolaria larva is also free-swimming and barrel-shaped, with bilateral symmetry.
    • It has a well-developed preoral lobe.
    • The single ciliated band breaks into 3-5 transverse rings, which are flagellated.
    • The gut is divided into distinct zones, similar to the auricularia larva.
    • The doliolaria larva is similar to the one found in Crinoidea, and it eventually attaches to a substratum to metamorphose.
• Doliolaria Larva of Crinoidea:
  • The doliolaria larva in Crinoidea is free-swimming and features an elongate oval body, narrower at the posterior end.
  • It has four to five transverse ciliated bands that encircle the body.
  • The larva has an apical sensory plate with cilia at the anterior end.
  • An adhesive pit, located near the apical plate, allows the larva to attach to a substratum.
  • The larva’s gut has distinct zones, with a stomodaeum situated between the second and third ciliated bands.
  • After attachment, the internal organs rotate, and the larva develops a stalk, transforming into a cystidian larva, which later metamorphoses into a young crinoid.
• Homology and Phylogeny of Echinoderm Larvae:
  • While the larvae of different classes of echinoderms display a range of features, they share several common traits that point to a common ancestry:
    1. The presence of preoral and postoral loops.
    2. Ciliated bands that are often V-shaped.
    3. A digestive system with defined regions and openings.
    4. An enterocoelic coelom.
  • These shared features indicate that echinoderms, except for Crinoidea (which are sedentary as adults), have a common origin.

Quiz

Question 1: What is the primary characteristic feature of Phylum Echinodermata?

A) Bilateral symmetry
B) Radial symmetry
C) Pentaradial symmetry
D) Asymmetry

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Question 2: Which part of the echinoderm’s body is responsible for gas exchange in respiration?

A) Tube feet
B) Ambulacral grooves
C) Gills
D) Pedicellariae

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Question 3: What is the term for echinoderms’ unique system of fluid-filled canals and tubes used in feeding, movement, and respiration?

A) Vascular system
B) Hydraulic system
C) Respiratory system
D) Circulatory system

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Question 4: Which class of echinoderms includes sea cucumbers?

A) Asteroidea
B) Ophiuroidea
C) Holothuroidea
D) Echinoidea

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Question 5: What is the primary method of reproduction in echinoderms?

A) Budding
B) Internal fertilization
C) Fragmentation
D) External fertilization

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Question 6: What is the primary function of the water vascular system in echinoderms?

A) Circulation of nutrients
B) Gas exchange
C) Locomotion and feeding
D) Sensory perception

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Question 7: In echinoderms, what is the structure responsible for controlling the entry of water into the water vascular system?

A) Tube feet
B) Madreporite
C) Ambulacral groove
D) Pedicellariae

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Question 8: Which echinoderm class includes organisms commonly known as “brittle stars”?

A) Asteroidea
B) Ophiuroidea
C) Echinoidea
D) Holothuroidea

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Question 9: What is the primary function of the pedicellariae in echinoderms?

A) Gas exchange
B) Reproduction
C) Defense and cleaning
D) Locomotion

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Question 10: In which echinoderm class would you find organisms commonly known as “sea urchins”?

A) Asteroidea
B) Ophiuroidea
C) Echinoidea
D) Holothuroidea

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FAQ

References

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