Sea spiders get their name from their vague similarity to "true" spiders. In a different class, sea spiders live in the ocean from shallow areas to thousands of meters below the surface of the water. Like other arthropods, they have a central body with legs around it. But, their bodies are tiny, with almost no abdomen and lots of appendages. Starting at the front end, a large proboscis with three lips is used to suck up food. Commonly, a stalk (tubercle) supports four eyes. Behind the proboscis may be long, clawed legs specialized for reproduction (ovigers). Males use them to court females and carry eggs. Walking legs come next and most sea spiders have four pairs but some have five or six. The walking legs are sturdy with eight joints, and each pair is on a separate body segment. Finally, at the tail end of a sea spider is an abdomen so small that it looks like a head and gives the spider the appearance of walking backwards.

Arthropods are the most successful group of organisms on Earth in terms of numbers, including almost half of all known species. The hallmark of their success is a body plan that makes them durable and adaptable. All arthropods have external skeletons made of hard material (chitin embedded in a protein matrix). The exoskeleton protects them from predators, weather conditions, and other threats. As they grow, arthropods typically shed the exoskeleton to reveal a bigger, fresh one underneath. The two lengthwise halves of an arthropod body are a mirror image of each other (bilateral symmetry) and typically divided into segments. Each segment has appendages that are specialized for the many activities of the arthropod. An arthropod may use appendages to feed (mouthparts), to breathe (gills, tracheae, book lungs), to reproduce (genitalia), and to move around (walking, swimming, flying). Having a modular body plan with multifunctional appendages has allowed arthropods to thrive in an impressive variety of environments.