SEA STARS, BRITTLE STARS, FEATHER STARS, SEA URCHINS AND SEA CUCUMBERS—Phylum ECHINODERMATA

Including sea stars (starfish), brittle stars, basket stars, feather stars, sea urchins, sand dollars and sea cucumbers.
Each echinoderm has some manner of hard chalky skeleton within its skin, although the amount, size and compaction of these elements varies considerably. From fused, interlocking plates that are obvious in a sand dollar or a sea urchin, through the variably packed and often obvious ones of sea stars, to the tiny and widely separated ossicles (plate-like or spine-like skeletal structures) of most sea cucumbers, this internal skeletal system is a constant.
Radiating about a central mouth, the five rays (arms) of a typical sea star beautifully illustrate the definitive penta-radial (five-armed) symmetry of an echinoderm. This layout is also present in sea urchins, sea cucumbers and other echinoderms, although it is not as obvious.
Another definitive feature of each echinoderm is a water vascular system, which essentially comprises a single-opening, internal plumbing system with many interconnected fluid-filled canals. Rows of very many tube feet, extending over the body of each echinoderm, are the most obvious external evidence of this unique system, whose prime functions are breathing and supplying rigidity and locomotion.
The accompanying photograph illustrates a rainbow star (EC4), at left, regenerating two lost rays, a common occurrence. The other specimen, a cookie star (EC11), is “budding” a new arm near the tip of another—a very unusual event.
Nearly 300 species of echinoderms live in the marine waters of the Pacific Northwest.