Monday, April 16, 2007


The name echinoderm literally means "spiny skin." Echinoderms are characterized by their pentaradial symmetry, internal skeletons, a water vascular system, and tube feet. The phylum Echinodermata is split into 5 main classes. Asteroidea which includes sea stars, Crinoidea with the sea lillies and feather stars, Echinoidea including the sea urchins and sand dollars, Holothuroidea with the sea cucumbers and Ophiuroidea including brittle stars and basket stars. Echinodermata is the only phylum the exists exlusively in marine habitats.

Read more about the classes: click here.


Echinoderms have radial symmetry, which means they are divided into 5 equal sections, radiating from a central point. In each segment, there is an identical set of internal organs. The closest thing to eyes in the phylum is the light sensitive spots on their arms. Along with having no eyes, Echinoderms also have no heart or brain. Each tentacle-like structure is called a tube foot and aids in respiration and movement. Echinoderms are coelomates, which means they have a large body cavity completely lined with tissue.


Nearly all echinoderms release their solid wastes through the anus. It will then be released in the form of feces, except with the organism brittle stars, because they do no require an anus and thus leaving them to excrete through the mouth. Wastes are excreted through the tube feet and skin gills on the body, and also contain nitrogen in the form of ammonia.


All echinoderms are capable of sexual and asexual reproduction. Most are ethier male or female, but in some cases could be hermaphodites. Fertilization takes place externally, by both males and females realeasing their gametes into the environment during the reproductive season. The animals will release their sperm and eggs into the sea and detect gametes of their own species and then later, they will release theirs. Soon after this process, larvae will begin to swim around, along with plankton. Afterwards, the larvae, which have bilateral symmetry, swim to the floor of the ocean and start to mature and matamorphosose into adults which contain radial symmetry. Sea stars are unbelievable animals. They have the ability to heal themselves when been attacted by predators. Interesting fact, sea stars are able to regenerate into a brand new organism if pulled apart, but only as long as it has a portion of the middle area on the body. Sea stars are incredible and clever creatures that cause a lot of trouble for fisherman, especially the ones who earn their living catching mollusks.

To view a diagram of the reproduction and life cycle of a Sea Star, click here and scroll down the website:

Sunday, April 15, 2007


Echinoderms have a very uncomplicated nervous system. Most species have a nerve ring that surrounds the mouth with radial nerves that connect the central ring to other parts of the body. Some echinoderms also have sensory cells that recognize chemicals released by potential prey. Sea stars also have up to 200 light sensitive eye spots on the end of each arm able to detect whether they are in light or dark. Star fish and other echinoderms also have statocysts which tell the animal if it is upside down. Most echinoderms are motile except for sea lillies and feather stars who attach to rocks. Echinoderms use tube and thin layers of muscle latched onto plates of the endoskelton to move around. With the pumping of water in and out of the water vascular system the tube feet act as suction cups and make it extremely hard to move a sea star from where it is. To protect themselves from predators echinoderms have numerous defenses. Sea urchins have tons of long spikes (sometimes containing poison) which help to keep enemies away. Others, like the sand dollar, burrow away in the sand and mud where as the brittle stars shed a leg whenever they feel a predator close.


The task of internal transport relys on many different systems in the echinoderms. Gas exchange (respiration) and the elimination of wastes is taken care of by skin gills and tube feet. Tube feet are an essential part of the water vascular system, this is used for many things, one of which is the transportation of waste. It is made of channels connectig to several tube feet. Water first enters the madroporite, located on the dorsal side of the echinoderm, and then continues through the stone canal and is greeted by the ring canal. The water then diverges into five different radial canals, that further transform into a bubble like structure called, ampullae. The final exit point is the tube feet, which are connected to the ampullae. Echinoderms have an open circulatory system, which means blood does not travel within blood vessels.


Echinoderms, not unlike many other animals, have to exchange carbon dioxide for oxygen. In most echinoderms the outer tissue of the tube feet creates the main surface for respiration. In other echinoderms the exchange of carbon dioxide and oxygen occurs through skin gills or in respiratory trees.