Phylum Echinodermata​

Patrick Star, from the class Asteroidea, Phylum Echinodermata
Patrick Star, from the class Asteroidea, 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 bilateral symmetry, 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 regeneration, whether it be an arm or an entire body.

Table of Contents:
Diagnostic Characteristics
Acquiring and Digesting Food
Sensing the environment
Locomotion
Respiration
Metabolic Waste Removal
Circulation
Self Protection
Osmotic Balance
Temperature Balance
Diagnostic Characteristics:
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)

From Left Top, to Right Bottom: A Sea Star-class Asteroidea, a Brittle Star-class Ophiuroidea, a Sea Urchin-class Echinoidea, a Sea Lily-class Crinoidea
From Left Top, to Right Bottom: A Sea Star-class Asteroidea, a Brittle Star-class Ophiuroidea, a Sea Urchin-class Echinoidea, a Sea Lily-class Crinoidea

Though there are six discrete classes of echinoderms, all echinoderms have specific characteristics. These characteristics include:

· 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)
·
Water Vascular System: a network of hydraulic canals that branch into extensions called tube feet (another characteristic specific to echinoderms) that function in locomotion, feeding, and gas exchange.
·
Radial Cleavage: 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 radial symmetry. 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)
·
Development of the coelom from the archenteron: The coelom is the body cavity of the echinoderm, which is formed by the splitting of the embryonic mesoderm into two layers. This body cavity originates from the archenteron, which is the central cavity, or the digestive cavity, of the echinoderm.
·
Slow moving, often attached to rocks or other bases
· The internal and external parts of the animal radiate from the center
·
Thin skin covered by endoskeleton of hard plates made of Calcium Carbonate
·
Prickly from skeletal bumps and spines
·
Sexual Reproduction: 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).
·
Larvae begin as bilateral larvae, and through metamorphosis end as radial adults. 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
Stelleroidea contains the two classes of sea stars (9VK) :
Class Asteroidea (sea stars and sun stars (9 VK)): 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 central disk begins. (9 VK)
Class Ophiuroidea (brittle stars and basket stars (9 VK)): 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 central disk with arms radiating out from the body and the adjacent arms do not connect with each other (9 VK).

Class Echinoidea (sea urchins, sand dollars): 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.
Class Crinoidea (sea liles): 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)
Class Holothuroidea (sea cucumbers): 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.
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).

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)
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.

Locomotion:
Class Asteroidea: 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)
Class Ophiuroidea: Their tube feet lack suckers, and they move by serpentine lashing of the arms.
Class Echionoidea: 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.
Class Crinoidea: 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.
Respiration:
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). The water vascular system, which is a network of hydraulic canals branching into extenstions called tube feet, also aids in gas exchange.




This sea star is a great example of the characteristics of coelomates, including a fluid-filled body cavity completely lined by mesoderm tissue (6 J. Stein).
This sea star is a great example of the characteristics of coelomates, including a fluid-filled body cavity completely lined by mesoderm tissue (6 J. Stein).


Sensing the Environment:
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.
Metabolic Waste Removal:
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.
Circulation:
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.
Self Protection:
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:
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)
Temperature Balance:
Because echinoderms are ectotherms, they tend to have body temperatures close to environmental temperature, and generate little of their own body heat.
Regeneration:

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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13.

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