by Jason A. Dunlop*1 Introduction: Pycnogonida, or sea spiders, are not true spiders at all. They are in fact a group of — probably rather primitive — marine arthropods, characterized by a small, slender body and in many cases by correspondingly long legs (Fig. 1). So unusual is their morphology that many of their internal-organ systems have been displaced into the legs. Because of their strange appearance, older studies occasionally referred to them as ‘nobody crabs’ (literally crabs without a body) — although it is important to stress that they are not crustaceans, any more than they are spiders. Pycnogonids are thought either to have evolved right at the very base of the arthropod tree — and thus not to be closely related to any particular group of arthropods — or to be related to ara
by Alistair J. McGowan*1 Introduction: Biological diversity, or biodiversity, shot to prominence among non-specialists in 1992, after the Rio Earth Summit (Fig. 1). Media coverage of the summit did a tremendous amount to raise awareness of the need to gather baseline data on species, and of the spectre of extinction hanging over some of them. The international Convention on Biodiversity declared 2010 the International Year of Biodiversity, and 2011–20 the Decade of Biodiversity. The use of the term biodiversity in the media has increased greatly, and the word is now in general use. Many countries now have biodiversity action plans that start locally and move through various levels and habitat types to the national level (for example, see the United Kingdom’s Biodiversity Action Plan).
by Jason A. Dunlop*1 Introduction: Chelicerata is one of the main divisions of the arthropods, and essentially consists of arachnids and their closest relatives. The name was coined in 1901 by the Berlin-based zoologist Richard Heymons (Fig. 1). It means the ‘claw-bearers’, in reference to the claw- or fang-shaped mouthparts that characterize the group. In addition to the arachnids, Chelicerata also includes the horseshoe crabs (Xiphosura), the extinct sea scorpions (Eurypterida) and little-known chasmatapaids (Chasmataspidida), and the sea spiders (Pycnogonida). The inclusion of sea spiders within this group is controversial, as we shall see below, and arachnids, horseshoe crabs, eurypterids and chasmataspids are sometimes grouped together as the Euchelicerata. The name Merostoma