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The rarely seen slate pencil urchin (Heterocentrotus mamillatus) is a distinctive species, with thick, pink spines. No longer used as chalk, the unfortunate animals' attractive spines are in some areas now being made into wind chimes.

The bodies of most sea urchins seem roughly spherical, but in fact they are made up of five radial segments, in typical echinoderm fashion. Sea urchins develop a calcareous skeleton or test, which contains the feeding apparatus, the intestines, and the gonads. Prior to reproduction the gonads expand to fill the whole shell, and it is this rich substance that make sea urchins so attractive to hungry fish despite their spiny defenses.

The ripe gonads of the sea urchin Hemicentrotus pulcherrimus are prized in Japan for sushi; the taste of this uni is strong, but delicious.

Sea cucumbers. Though at first they look just like loose sacks, or large worms, sea cucumbers (class Holothuria) are constructed with the same five-sided symmetry typical of the echinoderms. Because they are so elongate, they have a "head" and a "tail," unlike the starfish or urchins. The head of a sea cucumber is not, however, particularly well developed, consisting of little more than a ring of tentacles around the mouth. Sea cucumbers are an important trade item in Indonesia. (See "Trepang Fishing," page 27.)