A phylogenetic tree consisting of 69 OT, OT-like, VT and the human vasopressin receptors and their respective ligands is shown and has been prepared by sequence alignment using ClustalW. Receptor clusters for OT, isotocin and mesotocin are indicated in dark grey and VT/vasopressin receptors (presumably the ancestral group) are highlighted in light grey. Receptors of species within the same class or order are highlighted by broken lines. The UniProtKB entry numbers of the receptors are shown at the end of each branch. During production of the present review, two research groups independently reported the discovery and functional characterization of an OT/VT-like signalling system in Caenorhabditis elegans [36,37]. The two nematocin receptors and their peptide ligands are involved in the nematode's gustatory associative learning and reproductive behaviour. In contrast with humans and other animals, these nematodes produce an undecapeptide that is only functional at full length. It contains the typical six-residue N-terminal ring, but has an additional two residues at its C-terminus. The evolutionary perspective of this modification, its influence on ligand binding and the phylogenetic relationship of the receptors have yet to be established.