Echinoderms are one of the animal groups most closely related to the vertebrates because echinoderms and vertebrates share a common ancestor and similar patterns of development. (Vertebrates [Chordates] includes man, and all other animals that have a backbone.)
The phylum Echinodermata consists of mostly marine, bottom-dwelling animals. The phylum is so named because of the presence of spiny plates (calcareous ossicles), which form a dermal skeleton. Echinoderms are typically radially symmetrical (although the larva is bilaterally symmetrical), and they have true coeloms [see-luh m] arising as outpocketings from embryonic mesoderm of the gut. It is on the basis of this last characteristic, and because the bipinnara larva more closely resembles the chordate larva, that the echinoderms are said to be more closely related to the chordates.
A unique feature of the echinoderms is a derivative of the coelom known as the water vascular system, a system of tubes that are filled with a watery fluid. Although a circulatory system is present, it is greatly reduced. Thus, the coelomic fluid acts as the principal medium for the transport of food and respiratory gases. Echinoderms are Deuterostomes as opposed to being Protostomes. The phylum Echinodermata is the only major group of invertebrates that undergoes deuterostomic (second mouth) development in that the embryonic gastropore becomes the anus and the "second mouth" forms as a new opening.