Phylum Echinodermata
Phylum Echinodermata (Gr., echinus=spiny + derma = skin, ata = characterized by) includes exclusively marine invertebrates displaying pentamerous radial symmetry and an endoskeleton of calcareous plates and spines. Jacob Klein gave the name Echinodermata. This phylum is a collection of about 7,000 living species and constitute some of the most beautiful members of sea fauna, such as starfishes or sea stars, sea urchins, sea cucumber, sea lillies and sand dollars etc. The name ‘starfish’ is, however, misleading as these animals are not true fishes. A more suitable name suggested for them is ‘sea star’. Common genera of sea stars are Asterias, Pentaceros, Astropecten, Solaster, Heliaster, Luidia, Asterina etc. Genus Asterias includes about 150 species of which most common is A. Rubens.
The echinoderms are important both biologically and geologically. Biologically, there are few other groupings so abundant in the biotic desert of the deep sea, as well as shallower oceans. Echinoderms generally have remarkable powers of regeneration of tissue, organs, limbs, and of asexual reproduction, and in some cases, complete regeneration from a single limb. Geologically, the value of echinoderms is in their ossified skeletons, which are major contributors to many limestone formations, and can provide valuable clues as to the geological environment.
Echinoderms possess a simple digestive system which varies according to diet. In many species, the large cardiac stomach can be everted and digest food outside the body. Gonads are present in each arm. In echinoderms such as sea stars, every arm bear two rows of tube feet on the oral side which help in attachment to the substratum. Echinoderms are efficient scavengers of decaying matter on the seafloor. They prey upon a variety of small organisms, thereby helping to regulate their numbers. In addition, echinoderms produce vast numbers of larvae that provide food for other, planktonic organisms.
General characters: -
- Echinoderms are exclusively marine, free living and mostly bottom dwellers.
- Organ system grade of body organization.
- Body triploblastic, coelomate and symmetrical.
- Body unsegmented with globular, star-like, spherical, discoidal or elongated with radiating arms.
- Head absent. Body generally pentameric.
- Body surface with five radial areas, the ambulacra, with tube feet and five inter-radial areas, the inter-ambulacra.
- Endoskeleton is made up of spines and calcareous spicules.
- Coelom is a large cavity lined by ciliated peristomium, coelomic fluid with coelomaocytes.
- Water vascular system or ambulacral system present, usually with a madreporite.
- Nervous system without brain but with a circum-oral ring and radial nerves.
- Sense organs are poorly developed.
- Alimentary canal straight or coiled.
- Respiratory organs include dermal branchia, tube feet, respiratory tree and bursae.
- Circulatory or haemal or blood lacunar system is typically present.
- Excretory organs absent.
- Reproduction is usually sexual, but few reproduce asexually or by regeneration.
- Sexes are separate; fertilization is external, development indirect through free-swimming larval forms.
A few other Echinoderms: -
- Echinus: -
Echinus is commonly called sea-urchin. It is a species of marine invertebrate in the echinidae family. It is a deep-sea animal found from intertidal zone to a depth of 5000 meters. In the North Sea, the species is common in all areas with hard substrates. It is found off the coasts of Portugal, Spain, France, Belgium, the Netherlands, Denmark, Norway, Sweden, the United Kingdom and Ireland. Sea urchins feed mainly on algae but can also feed on sea cucumbers and a wide range of invertebrates, such as mussels, polychaetas, sponges, brittle stars, and crinoids.
It has a globe-shaped body enclosed within a shell or corona and having very long movable spines. The surface of corona is divided into five ambulacral regions alternating with five interambulacral regions. In each ambulacral region, there are two rows of tube feet. Mouth is present on oral surface and is surrounded by peristome. The chewing apparatus or Aristotle’s lantern projects from the mouth. Anus is surrounded by a periproct and is present in the center of the aboral surface. Water vascular system is well developed. Development is indirect and involves free swimming echinopluteus larva. Removal of the sea urchins results in the overgrowth of seaweeds and the devastation of the coral reef habitat. When present in large numbers, sea urchins can devastate seagrass beds in the tropics, adversely affecting the organisms dwelling within.
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| Fig: - Echinus |
- Ophioderma: -
Ophioderma is also known as brittle star. It occurs from the lower shore to depths of around 150m, living on hard substrates. The common brittle star is extremely variable in coloration, ranging from violet, purple or red to yellowish or pale grey, often spotted with red. The arms are usually white or grey with pink bands. Body is made up of a roughly pentagonal central disc and five freely movable arms arising from the base of the disc. Arms are covered with calcareous plates and on either side by spines. Each arm has two grooves called bursal slits. Brittle star moves by means of its tube feet. Development is indirect through free-swimming ophiopluteus larva. The common brittle star is a scavenger, feeding on dead organisms.
- Holothuria: -
Holothuria is a genus of marine animals of the family Holothuriidae. It is commonly known as sea-cucumber and is found in nearly every marine environment but is most diverse on tropical shallow-water coral reefs. They creep slowly while engulfing micro-organisms alongwith mud. The Holothuria, or sea cucumbers, are an abundant and diverse group of worm-like and usually soft-bodied echinoderms. Their habitat ranges from the intertidal, where they may be exposed briefly at low tide, to the floor of the deepest oceanic trenches.
The body is elongated, sausage-shaped, covered with leathery skin and having well-developed respiratory tree. The mouth and anus are at the opposite ends. Body bears numerous podia or feet, locomotory on the ventral surface and papilliate on the dorsal surface. Mouth is anteriorly placed, surrounded by 1-30 peltate tentacles. Sexes are separate. Reproduction is sexual. Development is indirect. It includes two larval stages-auricularia and doliolaria respectively. Several species can swim and there are even forms that live their entire lives as plankton, floating with the ocean currents. Sea cucumbers have great economic importance. Some species produce toxins (antimicrobial, anti-inflammatory agents and anticoagulants) that are of interest to pharmaceutical firms. They also form an important part of gourmet food industry in the orient.
- Pentaceros: -
Pentaceros is known as sea pentagon. Central disc is large and the five arms are short and tapering. Aboral surface is convex and bears rows of definitely arranged spines. Oral surface is concave, having a central mouth, communicating with five ambulacral groves, each bearing two double rows of tube feet. Pedicellariae are small and valvate type. Pentaceros is very harmful to pearl industry as it feeds on pearly oysters.
- Astropecten: -
Astropecten is a common starfish, inhabiting the sandy bottom of seas. Body consists of a large central disc with 5 tapering arms and each having 2 rows of conspicuous marginal plates bearing along spines. Oral surface has a centrally positioned mouth which comes out only occasionally and communicates with 5 ambulacral grooves. Water-vascular system includes two to four polian vesicles in each interradius and the stone canal is structurally complicated. There occurs no brachiolaria larva.
- Antedon: -
Antedon is commonly known as feather star. It occurs in sea waters of the Atlantic, Western Africa, the Mediterranean and West of Tropical America. It feeds on plankton and debris and has great power of regeneration and autotomy. Body consists of a central disc and a series of ten radiating arms. The upward directed oral surface bears mouth and a projecting papilla which bears the anus while downward directed aboral surface bears several long appendages called cirri for anchoring or crawling on the substratum. Tube feet or podia without suckers present along the edges of ambulacral grooves. Sexes are separate. Reproduction is sexual. Development includes a free swimming doliolaria larva.
Important point: -
- Phylum Echinodermata (Gr., echinos=spiny+derma=skin, ata=characterized by) includes exclusively marine invertebrates displaying pentamerous radial symmetry and an endoskeleton of calcareous plates and spines. Jacob Klein gave the name Echinodermata.
- Echinoderms are exclusively marine, free living and mostly bottom dwellers. Organ system grade of body organization.
- Asterias are free-living marine animals that can be found at all water depths as well as crawling over rocks and shells.
- Asterias feed mainly upon molluscs, especially bivalves and snails. In addition, they act as scavengers on any dead animals.
- Most Asterias possess a pentamerous radial symmetry, secondarily derived from a larval bilateral symmetry. The body is flattened and flexible, with a pigmented and ciliated epidermis. Average size of Asterias ranges from 10 to 25 cm in diameter.
- The aboral and abactinal surfaces are beset with numerous short but stout spines arranged in irregular rows parallel to the long axis of the arms.
- The mouth is situated in the center of the oral surface in the form of a five-rayed aperture.
- Pedicellariae of sea stars are minute, whitish jaw-like structures, found on both the body surfaces, in association with spines. The pedicellariae help in the capture and removal of debris and minute organisms, such as larvae, which may settle on the body surface and interfere with respiration by covering the dermal branchiae and tube feet. In some starfishes, the pedicellariae may help in capturing of small prey.
- Sea star possesses a complete digestive tract situated in the central disc. Between the mouth and anus, it can be differentiated into esophagus, stomach and intestine.
- Asterias is carnivorous, feeding on any slow-moving animal but mainly upon molluscs such as oysters, clams, mussels, snails, etc. The fish crabs and barnacles also make its food.
- Locomotion is performed with the help of water-vascular system which sets up a hydraulic pressure.
- Asterias or starfish is unisexual, but there is no sexual dimorphism. The reproductive organs of primitive type and lack copulatory organs, accessory glands and receptacles or reservoirs for storing sperms and ova.
- There are five pairs of testes or ovaries with one pair at the base of each arm lying freely between pyloric caeca and ampullae of the tube feet.
- Fertilization is external. The eggs and sperms are shed in seawater. The female lays about 200 million eggs in a season.
- The development of starfish includes the following larval stages- Dipleurula larva or early bipinnaria, Bipinnaria larva and Brachiolaria larva.
💦Self-Assessment question (Terminal Question): -
💥Long Answer type questions: -
2. Give an account of larval forms found in Sea star.
3. Discuss the digestive system of starfish.
4. Explain the reproduction of star fish.
💥Short Answer type Question :-
💦Multiple choice questions: -
1. Asterias belong to:
(a) Echinoidea (b) Asteroidea
(c) Ophiurodea (d) Holothuroidea
2. Common name of Asterias:
3. Locomotory organs of starfish:
4. Characteristic feature of echinoderm:
5. Larval stage in life history of starfish:
6. The eggs of Asterias are:
7. Excretory products:
8. Which of the following phyla is found only in sea water?
9. Most fragile arms are of:
10. Tube feet are locomotory in function and also perform additional:
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