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Neural pathways exist both in the zooids and in the stem of siphonophores. Communication takes place between them, affording a primitive centralization of coordination. In fact, the neural tissue in the stem of some physonects and calycophores has coalesced to form giant axons that run along the midline of the stem (Mackie et al. 1987). Figure 3.37 is a “wiring diagram” that nicely explains the neural organization of a physonect, including a visualization of the epithelial pathways.

Conductive pathways are more easily defined in the Siphonophora than in the hydromedusae and scyphomedusae. This is partially because there has been more neurophysiological research on that group but is also because the neural network in siphonophores is less diffuse, unlike the medusae with their multiple nerve nets.

Figure 3.37 Simplified wiring diagram of a physonectid siphonophore. Only ectodermal nerve pathways are included. ex. ect., exumbrellar ectoderm; mu. circ., circular muscle; mu. long., longitudinal muscle; mu. rad. vel., radial muscle of velum (“fibres of Claus”), sub. end., subumbrellar endoderm.

Source: Adapted from Mackie et al. (1987), figure 35 (p. 187).

Figure 3.38 Porpitidae. (a) Pelagic polyp colony and medusa of Porpita porpita. (b) pelagic polyp colony (“by‐the‐wind‐sailor”) and medusa of Velella velella.

Sources: (a) Bouillon (1978); (b) Bouillon (1984).