Echinodermata is the largest phylum that does not have any of its organisms living on land or in fresh water. The most common Echinoderms are starfish, sea cucumbers, sea urchins, and sand dollars. The name literally means, in Greek, "spiny skinned," because that's what echinoderms look like. 
An Echinoderm has five-fold radial symmetry when it's an adult with five arms and spines or bumps all over its body. They have a calcite endoskeleton composed of many tiny plates and spines. This forms, within the tissues, a kind of firm support for the organism. These are microscopic in sea cucumbers. That is why they look softer. In sea urchins and sand dollars, the tiny bones are fused together to form a solid, rigid skeleton.
They don't have a heart, brain or eyes. Their mouth is situated on the underside and their anus on top of their body, with a few exception: feather stars, sea cucumbers and some urchins.
On their extremities they have tube-feet with suction cups, which are controlled by a water vascular system. Tube feet have muscles to retract them, but no muscles to extend them. These tube feet stick out through holes in the skeleton. The suction causes movement, which is usually very slow. The water vascular system is a network of passages that are filled with fluid, that have the purpose of gas exchange and movement.
They have a complete digestive tubular gut, and an open circulatory system. Their nervous system is simple, but no echinoderms have a brain. Some have ganglia which is a group of nerve cells forming a nerve center.
There are both male and female Echinoderms. They have sexual reproduction and they can also regenerate limbs if they are chopped off or bitten off. This process of regeneration allows echinoderms to reproduce asexually. If a limb is detached with a piece of the central body mass attached the rest of the starfish body will regenerate. Our God gave starfish the ability to regenerate, in prime situations, into many new starfish within a couple of months if cut into radial pieces.
In sexual reproduction, the male will release its sperm into the water. The female releases her eggs into the water, sometimes up to a hundred million at once. Fertilization will occur externally. Most echinoderms are not good parents. They don't take care of their young at all, but some sea urchins will incubate their eggs in special little pouches.
All echinoderms live only in salt water. Most echinoderms live on the floor of the body of salt water, but there is always a few exceptions, in that some can swim or float. Some echinoderms like starfish and sea urchins like to live in shallow water, while others like holothurians and ophiuroids live on the deep sea floor.
Many are carnivorous predators, like the starfish. Many are also detritus foragers, and feed on the debris on rocks, like some sea cucumbers. Echinoderms are protected through their spiny skins, but they are still preyed upon by some shells, fish, crabs, shrimps, and by other echinoderms like starfish.
The larvae of echinoderms are ciliated, free-swimming organisms. They tend to look like embryonic chordates because they organize themselves bilaterally. As the organism grows, one side of the body grows more then the other, and it is eventually absorbed by the larger side. After that, the body is formed into five parts around a central axis. Then the echinoderm has radial symmetry.
- Starfish Monroe County Women's Disability Network
- Echinoderm by Wikipedia
- Echinoderm Life History and Ecology University of California, Berkeley, Museum of Paleontology