Introduction

The phylum Echinodermata contains over 6000 species. This phylum includes the sea stars, brittle stars, sea urchins and sea cucumbers.Its name comes from the Greek meaning "spiny skin" since their endoskeleton is made of hard calcium carbonate plates.There are five classes of echinoderms: Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea (sea urchins), Crinoidea (feather stars) and Holothuroidea (sea cucumbers). In general, all echinoderms have a five-part radial symmetry, a water vascular system, structures called tube feet and they live only in salt water.

Internal Transport

In echinoderms, food, oxygen and wastes are transported by different systems. Gas exchange and the removal of metabolic wastes occur through the skin gills and tube feet found all over the body; while, nutrients are distributed by the digestive glands and the fluid within the body cavity.

The Water Vascular System
The water vascular system is an internal network of fluid-filled canals that are connected to the tube feet. In echinoderms, this system is involved in feeding, respiration, internal transport, excretion, as well as in movement.
So, how does the water vascular system work? This system opens to the outside through the madreporite, which is a sievelike structure. The madreporite connects to the ring canal, which is connected to five radial canals that extend into each body segment, and is attached to hundreds of tube feet. The water vascular system is able to propel water in and out of the tube feet, creating a partial vacuum that act like living suction cups.

Feeding & Excretion

Echinoderms have diverse methods in feeding: Some, such as the sea star, are carnivores, where their tube feet are used to pry open other animals that have shells. Once the shell is open, the starfish everts its stomach out of its mouth, secretes digestive enzymes on their prey and digests the prey in the prey's own shell. Once the starfish is finished feeding, it moves its stomach back into its shell. While, herbivores, such as sea urchins, use their five-part jaw to scrape algae off rocks. Filter feeders, such as basket stars, use their tube feet to capture floating materials in the water. Detritus feeders collect large amounts of sand along the sea floor and filters out the waste materials in the form of feces.

Excretion in echinoderms is very simple. Almost all echinoderms release solid wastes (feces) through the anus. One exception is the brittle star, which releases solid wastes through the mouth because they lack an anus. Also, nitrogenous cellular wastes (ammonia) are release through the tube feet and the skin gills.

Respiration

In echinoderms, their respiration varies depending on the species. Mostly, they all respire by exchanging oxygen and carbon dioxide through the thin-layered tissues on their tube feet. Also they use some of the bumps and spines on the surface of their body to take in oxygen. Some echinoderms have a gill structure called skin gills that also allows for respiration. In sea cumcumbers, they take in oxygen with their respiratory trees which gets oxygen through the anus. The sea cucumbers contract and expand the muscle walls of their anus to draw in and release water in a slow rhythm. By creating this water movement, it allows the respiratory tree to take in the oxygen for breathing.

Movement & Response

Echinoderms have many tube feet on each of their "arms" and with the combination of the water vascular system, it makes up the movement of echinoderms. By expanding and contracting certain chambers in the water system, it moves the water into certain feet and that allows the muscle to extend which makes the feet retract. When the echinoderms expand and contract the correct tube feet in proper order, it allows walking to occur. In sea stars which belong to the class Asteroidea, there is a muscular pump that squeezes water out of the tube feet and that causes it to extend and straighten. Also in the class Ophiuroidea, which includes brittle stars, their tube feet are pointed without the suction movement, so they manouver around by rowing their "arms".
Echinoderms do not have anything known as a brain or any type of cephalization. They only have a nerve ring surrounding their mouth which then extends a series of nerve networks towards each of the body's region. In some echinoderms, ganglia is also present along the nerve networks. With the nerve ring, it allows the echinoderm to track down food around them. In addition to the nerve ring, echinoderms have ocelli which are like "eyes" located at the tips of the arms and they are light sensitive cells that alert the echinoderm if they are in light or dark areas. Also the sense organ, statocyst, helps with the echinoderm to determine the position the organism is in in relation to being upright or upside-down. These sensory organs are what makes up the echinoderms' nervous system.

Reproduction

Most echinoderms have separate sexes, but some are hermaphrodites. In sea stars, the sperm are produced in the testes, while the eggs are produced in the ovaries. When they detect gametes of their own species in the water, the sperm and eggs are released from the arms. Fertilization takes place externally, forming larvae, which are bilaterally symmetrical. Eventually, the larvae swim to the bottom of the ocean, mature and metamorphose into the different echinoderms we see today, which are radially symmetrical.
Also, echinoderms have the incredible ability to regenerate: if one part of their body is damaged, every piece of the torn starfish can develop into a whole new organism.

Diversity

sea star (class Asteroidea)



sea urchin & sand dollar
(class Echinoidea)












feather star (class Crinoidea)








sea cucumber (class Holothuroidea)










brittle star (class Ophiuroidea)

Facts

1. Some members of the classes Asteroidea, Echinoidea and Holothuroidea are poisonous and can cause serious injuries to humans.

2. Some echinoderm-like fossils have been suspected of being chordates.

3. In the past, scientists once thought there was a sixth class of echinoderms called the Concentricycloidea.

4. Each skeletal element of echinoderms are actually a finely branched cystal of calcium carbonate.

5. The biggest cucumbers can grow to 2 m high.

Work cited

Bird, Jonathan. "Echinoderms-The Spiny Animals". Oceanic Research Group. 1997.
<http://www.oceanicresearch.org/echinoderm.html>.

Denning, David, Russell, Eric R. "Branches of the Tree of Life: Echinoderms". BioMedia
Associates. 2000-2007.
<http://ebiomedia.com/prod/BOechinoderms.html>.

"Phylum Echinodermata". Invertebrates. <http://www.cyhaus.com/marine/Echinoderm.htm>.

"Phylum Echinoderm". Starfish. <http://www.virted.org/Animals/Starfish.html>.

"What are the Echinodermata". The Diversity of Living Organisms: Themes of Adaptation
and Evolution.
<http://science.kennesaw.edu/biophys/biodiversity/animalia/echinoderm.html>.

Zubi, Teresa. "Echinoderms". Invertebrates. 2007.
<http://www.starfish.ch/reef/echinoderms.html>.