Recent work on functional morphology has revealed not only how a wide range of animals work, but shows the significance of their shapes in great detail. Also, sound evidence of evolution is provided. A new approach towards the understanding of Pycnogonida comes from an appreciation of the significance of their general habits and shapes and from the structure and mode of action of their legs. Recent fossil evidence shows that the arachnids had at least two terrestrial landings, occurring millions of years apart in time. At least two, but probably more, separate arachnid lines lived in the sea. It is concluded that pycnogonids evolved from one such aquatic group which never became terrestrial.