Hox genes and the body plans of chelicerates and pycnogonids

Hox genes are found in all metazoan phyla and are involved in specifying identity along the anterior-posterior body axis. In arthropods, ten different classes of Hox genes can be distinguished, which are expressed in a typical staggered array along the anterior-posterior axis of the embryo in characteristically stable domains. These features have been used to align segments between different arthropod groups and in this way have contributed to solving longstanding zoological questions. In this chapter I summarize Hox gene data from chelicerates, including the enigmatic pycnogonids (sea spiders) and how these data have helped us to understand the body plans of different arthropod taxa.