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The animals on the cover of Asterisk: The Definitive Guide are starfish (Asteroidea), a group of echinoderms (spiny-skinned invertebrates found only in the sea). Most starfish have fivefold radial symmetry (arms or rays branching from a central body disc in multiples of five), though some species have four or nine arms. There are over 1,500 species of starfish.

Starfish live on the sea floor and in tidal pools, clinging to rocks and moving (slowly) using a water-based vascular system to manipulate hundreds of tiny, tube-like legs, called podia. A small bulb or ampulla at the top of the tube contracts, expelling water and expanding the starfish’s leg. The ampulla relaxes, and the leg retracts. At the tip of each leg is a suction cup that allows the starfish to pry open clam, oyster, or mussel shells. Starfish are carnivores; they eat coral, fish, bivalves, and snails.

Starfish can flex and manipulate their arms to fit into small places. At the end of each arm is an eyespot, a primitive sensor that detects light and helps the starfish determine direction. Starfish also have the ability to regenerate a missing limb. Some species can even regrow a complete, new starfish from a severed arm.

Many of the animals on O’Reilly covers are endangered; all of them are important to the world. The cover illustration is by Karen Montgomery, based on a black and white engraving from the Dover Pictorial Archive. The cover fonts are Gilroy Semibold and Guardian Sans. The text font is Adobe Minion Pro; the heading font is Adobe Myriad Condensed; and the code font is Dalton Maag’s Ubuntu Mono.