Echinodermata

Phylum Echinodermata​

By: Aanchal Narang

​Introduction: **Echinoderms **  have their nominal origin from the Greek words //echin // -spiny, and //derma // -skin. They are known as a clade of the subtaxa **[|Deuterostomia,] **  which falls under the **[|Bilateria] **  region of the subkingdom **Eumetazons. **  Echinoderms are deuterostomes which means that a small "pucker" develops inside the embryo during development. Although that doesn't seem like such a big deal, being a deuterostome is what relates echinoderms to chordates, the taxa humans are in. (EK) This subkingdom falls under the **Animal Kingdom. **  The Bilateria division includes all animals that have **<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">bilateral symmetry ** <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">, which is a symmetrical arrangement along the central axis of an organism that divides the body into two equivalent halves. All sea stars and most echinoderms can undergo some form of <span style="font-family: 'Times New Roman',Times,serif; font-size: 132%;">[|regeneration,] <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;"> whether it be an arm or an entire body.

Diagnostic Characteristics: <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;"> There are an estimated 7,000 discovered Echinoderms, and from these, six classes of the phylum have been established. These are Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea (sea urchins and sand dollars), Crinoidea (sea lilies and feather stars), Holothuroidea (sea cucumbers), and Concentricycloidea (sea daisies). Echinoderms are important biologically and geologically; biologically because few other groupings are so abundant in the biotic desert of the deep sea, as well as the shallower oceans, and geologically as their ossified skeletons are major contributors to many limestone formations, and can provide valuable clues as to the geological environment.(12DO). There are no parasitic species of echinodermata known, though a large number of these species live commensally with other marine life such as fish, mollusks, protozoa, and other echinoderms. (J. Sun 24)
 * Table of Contents: ||
 * <span style="font-family: 'Times New Roman',Times,serif;">Diagnostic Characteristics ||
 * <span style="font-family: 'Times New Roman',Times,serif;">Acquiring and Digesting Food ||
 * <span style="font-family: 'Times New Roman',Times,serif;">Sensing the environment ||
 * <span style="font-family: 'Times New Roman',Times,serif;">Locomotion ||
 * <span style="font-family: 'Times New Roman',Times,serif;">Respiration ||
 * <span style="font-family: 'Times New Roman',Times,serif;">Metabolic Waste Removal ||
 * <span style="font-family: 'Times New Roman',Times,serif;">Circulation ||
 * <span style="font-family: 'Times New Roman',Times,serif;">Self Protection ||
 * <span style="font-family: 'Times New Roman',Times,serif;">Osmotic Balance ||
 * <span style="font-family: 'Times New Roman',Times,serif;">Temperature Balance ||

<span style="font-family: 'Times New Roman',Times,serif; font-size: 132%;">Though there are six discrete classes of echinoderms, all echinoderms have specific characteristics. These characteristics include:

<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">· Bilateral Symmetry: symmetrical arrangement along the central axis of an organism that divides the body into two equivalent halves. The symmetry of an echinoderm can also be referred to as pentaradial since the echinoderm has five planes of symmetry. (J. Sun 24)<span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;"> · **<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">Water Vascular System: ** <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;"> a network of hydraulic canals that branch into extensions called **<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">tube feet ** <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;"> (another characteristic specific to echinoderms) that function in locomotion, feeding, and gas exchange. · **<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">Radial Cleavage: ** <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;"> Though Echinoderms are known to be bilateral, they do have some radial elements in their development. During the cleavage stage of the pre-embryo, the mass of cells that creates the organism shows **<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">radial symmetry. ** <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;"> Echinoderms are often referred to as having secondary radial anatomy because their internal and external body parts radiate from the center, in many cases as five spokes. (11RM) · **<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">Development of the coelom from the archenteron: ** <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;"> The **<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">coelom ** <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;"> is the **<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">body cavity ** <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;"> of the echinoderm, which is formed by the splitting of the embryonic mesoderm into two layers. This body cavity originates from the **<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">archenteron, ** <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;"> which is the central cavity, or the **<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">digestive cavity, ** <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;"> of the echinoderm. · **<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">Slow moving, ** <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;"> often attached to rocks or other bases · The internal and external parts of the animal radiate from the center · **<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">Thin skin ** <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;"> covered by endoskeleton of hard plates made of Calcium Carbonate · **<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">Prickly ** <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;"> from skeletal bumps and spines · **<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">Sexual Reproduction: ** <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;"> separate male and female individuals that release gametes into seawater (external reproduction). In laboratories, Echinoderms can be stimulated to give off gametes by a mild electric shock (JE). · **<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">Larvae ** <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;"> begin as **<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">bilateral larvae, ** <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;"> and through metamorphosis end as **<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">radial adults. ** <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;"> This is a secondary adaptation due to their sessile life-style. Because this is a secondary adaptation and not inherent in echinoderms, they are not truly radial in their anatomy. For example, the opening (madreporite) of a sea star’s water vascular system is not central, but to one side of the star.

++ Most, if not all of these characteristics are necessary for the classification of Echinodermata. Each of the five classes also have their own individual constraints:

The subphyllum <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">**<span style="font-family: 'Times New Roman',Times,serif;">Stelleroidea ** <span style="font-family: 'Times New Roman',Times,serif;"> contains the two classes of sea stars (9VK) : <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">**<span style="font-family: 'Times New Roman',Times,serif;">Class Asteroidea (sea stars and sun stars (9 VK)): ** <span style="font-family: 'Times New Roman',Times,serif;"> Sea stars have five arms (sometimes more) which radiate from a central disk. The undersurfaces of the arms bear tube feet, each of which act like a suction disk. Asteroids have arms which seem to connect together in such a way as to make it difficult to discern where the arms end and the <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">**<span style="font-family: 'Times New Roman',Times,serif;">central disk ** <span style="font-family: 'Times New Roman',Times,serif;"> begins. (9 VK) <span style="font-family: 'Times New Roman',Times,serif; font-size: 120%;">**<span style="font-family: 'Times New Roman',Times,serif;">Class Ophiuroidea (brittle stars and basket stars (9 VK)): ** <span style="font-family: 'Times New Roman',Times,serif;"> Brittle Stars have distinct central disks, and the arms are long and flexible. Their tube feet lack suckers. Ophiuroidea have a distinct central body part called a <span style="font-family: 'Times New Roman',serif; font-size: 12pt;">**<span style="font-family: 'Times New Roman',Times,serif;">central disk ** <span style="font-family: 'Times New Roman',Times,serif;"> with arms radiating out from the body and the adjacent arms do not connect with each other (9 VK).

<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">**<span style="font-family: 'Times New Roman',Times,serif;">Class Echinoidea (sea urchins, sand dollars): ** <span style="font-family: 'Times New Roman',Times,serif;"> These have no arms but have five rows of tube feet. Sea urchins are spherical in shape, whereas sand dollars are flattened and disk-shaped. <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">**<span style="font-family: 'Times New Roman',Times,serif;">Class Crinoidea (sea liles): ** <span style="font-family: 'Times New Roman',Times,serif;"> These are always attached to some surface in the water (rocks, other organisms, etc.) Currently, there are only a couple hundred known species of Crinoidea, but from fossil evidence, there used to be many more. (14 HL) <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">**<span style="font-family: 'Times New Roman',Times,serif;">Class Holothuroidea (sea cucumbers): ** <span style="font-family: 'Times New Roman',Times,serif;"> Sea cucumbers do not generally look like echinodermata at first look. They lack spines, and their hard skeleton is much more relaxed. They are elongated in the oral-aboral axis. When closer examined, one finds that sea cucumbers have five rows of tube feet. <span style="font-family: Arial,Helvetica,sans-serif; font-size: 10pt;">Several important characteristics of sea cucumbers include: 1. Oral tentacles 2. A calcareous ring that encircles the pharynx to which the tentacles are attached 3. The exoskeleton is reduced to microscopic ossicles (DPOD 21). <span style="color: #1f984d; font-family: Tahoma,Geneva,sans-serif; font-size: 260%;">Acquiring and Digesting Food: ** Class Asteroidea (sea star): ** The suction tubes that sea stars have on their feet are used to grab prey (clams and oysters). The arms of the sea star embrace the oyster or clam, and then the stomach of the sea-star is turned inside out. The stomach then comes out of the sea star, and enters the small opening between the closed shells of the oyster or clam. In this way the sea star has entered the bivalve and begins to secrete juices that digest the mollusk in its own shell. The digested mollusk is then absorbed into the stomach of the sea star. (11 AL) The size of sea stars is not based upon how old they are but upon how much food they consume. (11 AL) <span style="color: #22873b; font-family: Tahoma,Geneva,sans-serif; font-size: 270%;">Locomotion: **<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">Class Asteroidea: ** <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;"> The suction disks on the tube feet are used to coordinate the sea stars tube feet from rock to rock. Tube feet are small projections that are arranged in grooves along each arm and which operate through hydraulic pressure. (7 AS) <span style="font-family: 'Times New Roman',Times,serif; font-size: 132%;"> **<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">Class Ophiuroidea: ** <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;"> Their tube feet lack suckers, and they move by serpentine lashing of the arms. <span style="font-family: 'Times New Roman',Times,serif; font-size: 132%;"> **<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">Class Echionoidea: ** <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;"> Sea urchins and sand dollars have no arms, but they do have five rows of tube feet that function in slow movement. sea urchins also have muscles that pivot their long spines, which aids in moving. <span style="font-family: 'Times New Roman',Times,serif; font-size: 132%;"> **<span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">Class Crinoidea: ** <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;"> Sea lilies are always attached to a base, and do not tend to move. Feather stars are very sessile, but crawl around using their one long flexible arm. <span style="color: #1e7b3d; font-family: Tahoma,Geneva,sans-serif; font-size: 260%;">Respiration: <span style="color: #000000; font-family: 'Times New Roman',Times,serif; font-size: 110%;">The gills of a **star fish** are simple tubular projections of skin. The core of each gill is an extension of the body cavity (coelom). Gas exchange occurs by diffusion across the gill surfaces, and fluid in the coelom circulates in an out of the gills, aiding gas transport. The surface’s of a sea star’s tube feet also function in gas exchange. The tube feet are highly specialized for respiration in that they are flattened and thin-walled to create a larger surface area for the exchange of gases (CS 13). <span style="font-family: 'Times New Roman','serif';">The water vascular system, which is a network of hydraulic canals branching into extenstions called **tube feet**, also aids in gas exchange.
 * Class Ophiuroidea (brittle star): ** Some species of Ophiuroidea are suspension feeders, which are animals that feed on small particles suspending in water, and others are predators or scavengers.
 * Class Echinoidea (urchins):** The mouth of an urchin is ringed by complex jaw-like structures adapted for eating seaweeds and other food.
 * Class Chrinoidea (sea lily):** All species of class Chrinoidea use their arms in suspension feeding.
 * Class Holothuroidea (sea cucumber): ** Some of the tube feet around mouth serve as feeding tentacles.



Sensing the Environment: <span style="color: #000000; font-family: 'Times New Roman','serif'; font-size: 110%;">The echinodermata class Asteroidea (sea stars) has a nerve net that plays the major role of the nervous system. A **nerve net** is a simple nervous system. Because echinodermata do not have brains or any distinct sensory organs, this type of nervous system allows the class to respond to physical contact. The sea star has a nerve ring connected to radial nerves that link to a nerve net in each arm. This allows for more complex movements than the other organisms with nerve nets. <span style="color: #1e7b3d; font-family: Tahoma,Geneva,sans-serif; font-size: 270%;">Metabolic Waste Removal: <span style="color: #000000; font-family: 'Times New Roman',Times,serif; font-size: 110%;">Because the class Echinodermata consists of **ectotherms** (organisms that absorb external heat rather than generating their own), less nitrogenous waste is produced than an **endotherm** (organism that uses metabolic energy to maintain a constant body temperature). This is because ectotherms do not use much of their own energy to maintain a specific temperature, and therefore do not have much metabolic waste. The primary waste product of Echinoderms is Ammonia, which is very water-soluble. <span style="color: #1e7b3d; font-family: Tahoma,Geneva,sans-serif; font-size: 290%;">Circulation: <span style="color: #000000; font-family: 'Times New Roman',Times,serif; font-size: 110%;">Echinoderms have an open circulatory system which comprises of a central ring and five radial vessels. Their body cavities are lined with cilia which create an internal current. Also found in echinoderms' cavities are amoebocytes. They are large phagocytic cells that function in the transport of food and storage of insoluble wastes. (CH 4) Echinoderms do not have hearts and their blood does not have hemoglobin, and is therefore not a part of gas exchange and respiration. <span style="color: #000000; font-family: 'Times New Roman',Times,serif; font-size: 45%;"> <span style="color: #1e7b3d; font-family: Tahoma,Geneva,sans-serif; font-size: 280%;">Self Protection: <span style="font-family: 'Times New Roman',Times,serif; font-size: 110%;">Echinoderms have very thin skin, but the skin is covered by an endoskeleton of hard calcareous plates. The plates are prickly from skeletal bumps and spines. These help to protect the animal from predators. Some Echinoids have highly poisonous, pincerlike organs (pedicellariae), which may cause intense pain and fever among humans. (12 DC) Species within the class Asteroidea can also have brightly colored pustules and spots on their arms that act as warning coloration or camouflage against predators. (AS) Sea cucumbers discharge entangling web when threatened. (EK24) Osmotic Balance: <span style="color: #000000; font-family: 'Times New Roman',Times,serif; font-size: 110%;">The water vascular system moves water in, out, and through the body of the echinoderm. Such a process is known as simple diffusion. Wastes are collected by coelomocytes in vesicles and brought to discharge sites (6 JSun). Many echinoderms have an exterior opening called a madreporite, which takes in water from outside the individual's body. If on the inside, fluid is taken from inside the body; the fluid or water is then pumped from the center vessel along the radial canals to the tube feet. (AR 8) <span style="color: #1e7b3d; font-family: Tahoma,Geneva,sans-serif; font-size: 260%;">Temperature Balance: <span style="color: #000000; font-family: 'Times New Roman',Times,serif; font-size: 110%;">Because echinoderms are ectotherms, they tend to have body temperatures close to environmental temperature, and generate little of their own body heat. Regeneration: ====<span style="font-family: 'Times New Roman',Times,serif;">All sea stars and most echinoderms are able to undergo some form of regeneration, whether it be the regeneration of an arm or a whole body. Successful regeneration requires that certain body parts be present in lost pieces. Many sea stars, for example, can only regenerate a lost portion if some part of the central disk is present (2 MB). Some species such as the Linckia multifora use regeneration to undergo asexual reproduction, during which one of the arms of the starfish will pull itself away and then regenerate into an entirely new animal. Most sea stars also use regeneration as a defense mechanism by losing a body part to escape a predator rather than being consumed entirely. This process of self-amputation is called autotomy (3 KA). ====

__Review Questions:__ 1) How do echinoderms protect themselves from predators and organisms that wish to settle on them? (AW) 2) Explain and relate the acquiring and digesting of food to the metabolic waste removal of an Echinodermata? (11-SC) 3) Why do echinoderms make little nitrogenous waste? How do they get rid of the little waste they do make (16T2)? 4) Why, if they seem to display radial symmetry, are echinoderms categorized as "Bilateria" along with chordates, arthropods, etc. and not with cnidarians and ctenophorans? Describe the symmetry of echinoderms at various points in the organisms' development. (SW 18) 5) Compare and contrast the 6 phyla of enchinoderms. (EG)

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