Sea Urchin Larva

Word Roots and Origins ossicle from the Latin ossiculum, meaning "little bone"

Also, in deuterostomes, the anus forms from the blastopore, and the mesoderm arises from outpockets of the endoderm. Because they develop as deuterostomes, echinoderms are more closely related to chordates than they are to other invertebrates. Chordates are also deuterostomes and are discussed in the second part of this chapter.

Echinoderms have four major characteristics that are not shared by any other phylum. (1) Most echinoderms have a type of radial symmetry called pentaradial symmetry, in which the body parts extend from the center along five spokes. (2) They have an endoskeleton composed of calcium carbonate plates known as ossicles. The ossicles may be attached to spines or spicules that protrude through the skin. The name echinoderm means "spiny skin." (3) They have a water-vascular system, which is a network of water-filled canals inside their body. (4) They have many small, movable extensions of the water-vascular system called tube feet, which aid in movement, feeding, respiration, and excretion. The water-vascular system and tube feet will be discussed in more detail.

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