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Morphology

Most sea spiders have a narrow elongate body, divided into a number of segments. At the anterior end, there is a cephalon with a proboscis, bearing a terminal mouth. On the dorsal side of the cephalon there is an ocular tubercle bearing the eyes, one pair directed forwards and the other pair posteriorly. In some species all eyes are the same size, but in a few the anterior eyes are larger than the posterior ones. The cephalon is fused with the first trunk segment and the last trunk segment has an abdomen or anal process, with a terminal anus. In most adults the trunk segmentation is visible externally.
The appendages consist of a pair of palps carrying sense-organs, chelifores which may have terminal, functional chelae, used for food gathering or perhaps gripping the substratum, ovigerous legs, and four pairs of ambulatory (walking legs) or natatory (swimming) legs. Some families are without palps or chelifores, or both. In some species the ovigerous legs or false legs are present only in the males, whilst in others, although present in females, they are reduced. The terminal segments of the ovigerous legs may bear a number of serrated spines, which help them to fulfil their secondary function of cleaning the body, their primary function being to carry the eggs. Each body segment has a prominent lateral process with which the legs articulate. These legs consist of three coxae, a femur, two tibiae, a tarsus, a propodus and a terminal claw, with some auxiliary claws associated with it, or with the propodus.
Some juvenile stages have fewer than four pairs of legs and cannot be identified with this Picture Key.

Source: Hayward and Ryland, 1990 & King, 1974.