We all have tried a variety of salads and you know that most of the vegetable salads are incomplete without the cucumber. This is one of the vegetables which most people like to eat. Have you heard about a cucumber which is present in the sea? Fascinating, right? Yes, there is a cucumber who lives in the sea, it is known as sea cucumber (Cucumaria). It is not a vegetable, but it is an invertebrate animal which is a member of the phylum Echinodermata. Sea cucumbers got this name because their shape resembles a cucumber. Sea cucumber is also used as a food item in Chinese cuisine.
Fig: Sea cucumber
This sea cucumber and starfish are included in the same phyla. Can you find any similarities between sea cucumber and starfish? It is really interesting right? So let’s find out why these organisms are included in the same phyla. To find the answer for this question we have to know more about the phylum Echinodermata. So now let’s discuss more about it in this article.
Table of contents:
The word Echinodermata has been derived from two words ‘echinos’ meaning spiny and ‘derma’ meaning skin or covering. Echinoderms have spiny skin like porcupines. Now let’s discuss some general characteristics of phylum Echinodermata.
General characteristics of Phylum Echinodermata
The following are the major characteristics of the phylum Echinodermata:
They are well-adapted to marine environments and it is the only phyla with many stenohaline members. Stenohaline means they cannot tolerate a wide range of saline conditions. They are also benthic animals, which means they live at the bottom of the sea.
Echinoderms possess calcareous ossicles as their endoskeleton.
Larvae of echinoderms and adult echinoderms have different symmetries in their body. It is known as biradial symmetry. Biradial symmetry is present in organisms that have both bilateral and radial symmetry in their morphological traits (internal or exterior).
The larvae of echinoderms are bilaterally symmetrical (the body can be divided into two equal and identical halves, i.e., right half and left half, through only one plane called the median sagittal plane.
Fig: Bilateral symmetry in larva of echinoderms
Adult Echinoderms show radial symmetry, which means the body can be divided into two equal halves through any plane passing through the centre of the body.
Fig: Radial symmetry in adult echinoderms
Level of organisation
They show the organ system level of body organisation where division of labour can be seen between different organ systems.
Fig: Organ system level of organisation in echinoderms
Germ layer organisation
They have triploblastic cells which means the cells are arranged in three embryonic layers i.e., outer ectoderm, inner endoderm and middle mesoderm.
Fig: Triploblastic organisation in echinoderms
The body wall is made up of a ciliated epidermis over a dermis with a calcareous endoskeleton. Many echinoderms have pedicellariae, little pincer-like appendages that protrude through the skin.
They have a coelom, hence are called coelomates. True coelom is found in the body cavity. It has a ciliated peritoneum lining. It is an enterocoelom which contains a fluid with amoeboid cells called amoebocytes. It is a true coelom developed as a lateral pouch from the gut of the embryo. A section of this coelom is transformed into a water vascular system.
Fig: Coelom in echinoderms
Organ systems in phylum Echinodermata
The following are the major organ systems in Echinodermata.
Digestive system in the phylum Echinodermata
Echinoderms have a complete digestive system, i.e., they have two different openings for ingestion (mouth) and egestion (anus). Digestive system possesses the mouth, stomach, intestine and anus. The mouth is located ventrally on the lower side while the anus is located dorsally on the upper side. The majority of echinoderms are carnivorous, eating bivalves and other molluscs. They consume the digested prey, as they are able to push their stomachs outside the body and insert it into its prey. This allows them to digest the food externally.
Fig: Top side and bottom side of starfish
Water vascular system in echinoderms
This is a feature unique to echinoderms. Main functions of the water vascular system are respiration, locomotion and the capture and transport of food. A section of the coelom is transformed into a unique ambulacral or water vascular system, complete with a network of radiating channels and tube feet. A permeable plate called mandreporite allows water to enter in this system. From here water enters into the stone canal, ring canal, radial canal and finally reaches the tube feet. The madreporite operates like a trap door through which water can move in and out in a regulated manner.
Fig: Water vascular system in Echinoderms
Respiratory system in echinoderms
Dermal branchiae, which are tiny gills and tube feet, help in this process. Cloacal respiratory trees also can function as respiratory organs in rare cases.
Circulatory system in echinoderms
It is referred to as the haemal system since it is reduced and open in nature. The heart is absent. The circulatory system plays a minor role in gas transport.
Excretory system in echinoderms
Excretory system is absent in echinoderms. Nitrogenous wastes are expelled partly through dermal branchiae or papulae and partly through the body surface.
It consists of a circum-oral ring present in the centre and radial nerves. These radial nerves enter into each arm. Sense organs are a simple type. They are sensitive to touch, changes in temperature, light intensity, orientation, and the water surrounding them.
Mode of reproduction and development of echinoderms
They show sexual reproduction with indirect development. They also show asexual reproduction.
Echinoderms are dioecious in nature, i.e., they have separate sexes. But they don’t show sexual dimorphism.
They usually follows a sexual reproduction with external fertilisation, characterised by the fusion of gametes in an external medium such as water.
Type of development
Indirectly through intervening larval stages which are free-swimming. Larvae will be ciliated and bilaterally symmetrical. They are usually free swimming and undergo metamorphosis. For example, bipinnaria larvae are present in starfish.
Fig: Free swimming echinoderm larvae
Some Echinoderms have the ability to reproduce asexually by fragmentation. Here the body is divided into two or more parts and each part develops into a new organism. Regeneration of missing body parts is also seen in them.
It can be observed in this phyla. It is the ability of echinoderms to release a part of the body that has been captured by a predator or an external agent.
The animals in this phylum are divided into five classes. They are as follows:
The body form is flat and star- like. They have five arms which are thick and not sharp with a central disc. They have larval forms like bipinnaria or brachiolaria in their development. Examples include Asterias (Star fish).
It is usually referred to as a starfish. It is a bottom-dwelling, slow-creeping, nocturnal animal that eats carnivores. It has a star-shaped body with short spines all around it that range from orange to purple. Each arm has a tentacle and an eye at the end. It has a strong autotomy and regeneration ability. Calcium-rich fertilisers are made from dried and powdered starfish.
The body form is flat and star-like here. They have five arms which are thin, long, jointed and brittle. Anus is absent for them and egestion happens through the mouth. Their larval stage is called ophiopluteus larvae. Examples include Ophiothrix (Brittle star).
Brittle star is the common name for it. It can be found in both shallow and deep sea water. It comprises a small core disc with five slender flexible arms that sprout from it. With its arms, it swims like a snake.
The body form is globular, disc-like. They do not have arms. Their spines are long and movable, which are used for locomotion. Aristotle’s lantern (biting and chewing apparatus) is present in the mouth. It is used for biting and chewing. Larval stage is called echinopluteus. Examples include Echinus (Sea-urchin).
It is seen in the tide pools of rocky shores. It has a spherical body with long movable spines. Heterocentrotus mammillatus is commonly called the slate pencil urchin as the detached spines from the body can be used as slate pencil.
Fig: Echinus- Sea urchin
The body form is long and cylindrical. Arms are absent in these animals. Mouth contains branched and unbranched tentacles. Larval form is auricularis. Examples include Holothuria (Sea cucumber).
It's also known as sea cucumber. It can be found in the sea's shallow waters. It eats plankton and bottom trash. In self-defence, it can lose its viscera and regenerate new ones later. It is used as an edible animal by Chinese people.
Fig: Cucumaria- Sea cucumber
The body form is plant-like, fixed to the substratum.They have several arms that sprout off of a calcareous plate. Each arm has lateral pinnules and bifurcates. They don't have any spines. Doliolaria larva are a part of the development process. Examples include Antedon (Feather star).
Fig: Antedon (Sea lily)
Q 1. Which of the following statements is incorrect?
a. Pila, sometimes known as apple snail, is a Mollusca species
b. Echinodermata includes Antedon, sometimes known as sea lily
c. Aplysia is a sea hare that belongs to the Echinodermata family
d. Brittle star is the popular name for Ophiura, which belongs to the Echinodermata family
Answer: Pila globosa is a freshwater gastropod mollusc with an operculum that belongs to the Ampullariidae family of apple snails. Antedon is a genus of stemless, free-swimming crinoids, which is commonly called sea lily. Brittle stars, serpent stars, and ophiuroids are echinoderms that belong to the Ophiuroidea class of echinoderms, which are closely related to starfish. Aplysia belongs to phylum Mollusca. Hence the correct option is c.
Q 2. Blastopore develops into the anus in _________________.
Answer: The blastopore is the opening of the archenteron in the gastrula stage of the embryo. The blastopore develops into the mouth in protostomes. Later on, anus appears. These animals are known as protostomes because their mouths are developed first. The words ‘proto’ and ‘stoma’ imply ‘first’ and ‘mouth’ respectively. Annelids, arthropods, and molluscs are protostomes. The blastopore develops into the anus in deuterostomes, and the mouth develops later. These animals are known as deuterostomes because their mouths are created as secondary openings. The Greek word 'deutero' means secondary, while the Greek word 'stoma' denotes mouth. Echinoderms, hemichordates, and chordates are all deuterostomes. Hence the correct option is b.
Q 3. Identify the correct statement.
a. Molluscs are triploblastic coelomate animals having an incomplete digestive system
b. The notochord is a structure found in echinoderms
c. Hemichordates are worm-like invertebrates that live in freshwater
d. With a free-swimming larva, echinoderm development is indirect
Answer: Molluscs are triploblastic (meaning they have all three germ layers) coelomate (mesoderm-lined body cavity) animals with a complete digestive system that includes a mouth and anus. Echinoderms lack a notochord and are non-chordates. Hemichordates are worm-like animals that live in the sea. In echinoderms, development is indirect and includes a free-swimming larva. Hence the correct option is d.
Q 4. Explain the water vascular system of echinoderms with a beautiful diagram?
Answer: Water vascular system is a feature unique to echinoderms. Main functions of the water vascular system are respiration, locomotion and the capture and transport of food. A section of the coelom is transformed into a unique ambulacral or water vascular system, complete with a network of radiating channels and tube feet. A permeable plate called mandreporite allows water to enter in this system. From here water enters into the stone canal, ring canal, radial canal and finally reaches the tube feet. The madreporite operates like a trap door through which water can move in and out in a regulated manner.
Fig: Water vascular system in echinoderms
Q 1. How do Echinoderms defend themselves?
Answer: Echinoderms use the mechanism called autotomy for defence. For example, when a starfish is captured by an arm, they exploit their regeneration ability as a defence mechanism. These animals will just drop an arm and move away. The remaining part of the animal walks away and grows a new limb. Here the unfortunate attacker is left with a twitching arm.
Q 2. How do Echinoderms contribute to the ecosystem by living at the bottom of the sea?
Answer: Echinoderms play a variety of responsibilities in the environment. Sea cucumbers and sand dollars are able to burrow through the sand. This helps in supplying more oxygen to the seabed at the higher depths. This makes it possible for more organisms to live there. Starfish prevents the growth of algae on coral reefs.
Q 3. What is the role of echinoderms in agriculture?
Answer: Farmers in some locations where limestone is scarce, use the hard skeletons of echinoderms as a supply of lime. Lime is used in soil to help plants absorb more nutrients.
Q 4. How are Echinoderms involved in the production of medicines?
Answer: Echinoderms are also used in scientific study and medicine. For example, some sea cucumber toxins slow the development rate of tumour cells, therefore they can be used in cancer research.
YOUTUBE LINK: https://www.youtube.com/watch?v=2V-RxvwschU&t=280s
Basis of Classification: Levels of organisation, Practice Problems and FAQs
Basis of Classification: Symmetry, Germ layer organisation, Coelom, Practice Problems and FAQs
Basis of Classification: Body plan, Segmentation, Notochord, Broad classification of Kingdom Animalia, Practice Problems and FAQs
Phylum Mollusca, Practice Problems and FAQs
Phylum Hemichordata, Practice Problems and FAQs