A competing theory proposes that pycnogonida belong to their own lineage, distinct from chelicerates, crustaceans, myriapods, or insects. This theory contends that the sea spider's chelifores, which are unique among extant arthropods, are not in any way homologous to the chelicerae in real chelicerates, as was previously supposed. Instead of developing from the deutocerebrum, they can be traced to the protocerebrum, the anterior part of the arthropod brain and found in the first head segment that in all other arthropods give rise to the eyes only. This is not found anywhere else among arthropods, except in some fossil forms like "Anomalocaris", indicating that the Pycnogonida may be a sister group to all other living arthropods, the latter having evolved from some ancestor that had lost the protocerebral appendages. If this is confirmed, it would mean the sea spiders are the last surviving (and highly modified) members of an ancient stem group of arthropods that lived in Cambrian oceans. However, a subsequent study using Hox gene expression patterns consistent with a developmental homology between chelicerates and chelifores, with chelifores innervated from a deuterocerebrum that has been rotated forwards; thus, the protocerebral Great Appendage clade does not include the Pycnogonida.