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The animal on the cover of WebSocket is a sea anemone (order Actiniaria).

More than 1,000 species of sea anemones are found throughout the world’s oceans. They are particularly abundant in coastal tropical waters. These organisms tend to remain rooted in one place, anchoring to hard surfaces such as coral reefs or rocks on the sea floor.

Closely related to coral and jellyfish, sea anemones are invertebrates with cylindrical bodies surrounded by tentacles. They vary in size, ranging from half an inch to six feet in diameter, and may possess anywhere from a dozen to several hundred tentacles. They also appear in an array of vivid colors, resembling flowers such as their namesake, the terrestrial anemone.

Despite their elegant beauty, these animals are quite predatory. Their tentacles are dotted with stinging cells that are used to immobilize and consume small fish and crustaceans. Accordingly, sea anemones have very few predators of their own, and many species live for more than 50 years.

Sea anemones are frequently cited for their symbiotic relationships with clownfish, which have a protective coating that renders them immune to the anemone’s lethal sting. The clownfish is safe from its enemies among the anemone’s tentacles, while the anemone enjoys food scraps from the clownfish’s meals.

Many of the animals on O’Reilly covers are endangered; all of them are important to the world. To learn more about how you can help, go to animals.oreilly.com.

The cover image is from loose plates (original source unknown). The cover fonts are URW Typewriter 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.