Figure 1.

(a) Phylogenetic relationships of cnidarians (after [4]). As the phylogenetic position of ctenophores and sponges is still not completely resolved, their lineages are marked with a dashed line [3,5–8]. The length of branches is for illustrative purposes only and does not represent time of divergence. (b) Life cycle of Nematostella vectensis, with both sexual and asexual reproduction. (c) Schematic of the body plan of N. vectensis throughout development. The lower panel represents a section through the polyp at the dashed line. (d) Schematic of the nervous system of N. vectensis throughout development. Neurons are depicted in brown. The schematic is based on several studies (see below), but it probably does not represent the whole neuronal population. All stages are showed as sections, except for the primary polyp. The section through the polyp (at the dashed line) shows the distribution of neurons in different layers.