. Text-fig. 6. A, Actinula of Tubularia. After Leloup (1929). B, Conaria of Porpita. Based on Delsman (1923, figs. 16, 17, 18). XY= homologous planes. Text-fig. 6 will make this point clear. There is no need to endeavour, as Garstang did, to 'close the gap' between Disconanths and Siphonanths. The old order Siphonophora without doubt had a double origin, as Haeckel realized. As to the position of Pelagohydra, one is struck by the origin of the tentacles between the axial parenchyma in the meshes between the endoderm canals, from which the gonophores (gonozooids now reduced) spring. In Chondrop
. Text-fig. 6. A, Actinula of Tubularia. After Leloup (1929). B, Conaria of Porpita. Based on Delsman (1923, figs. 16, 17, 18). XY= homologous planes. Text-fig. 6 will make this point clear. There is no need to endeavour, as Garstang did, to 'close the gap' between Disconanths and Siphonanths. The old order Siphonophora without doubt had a double origin, as Haeckel realized. As to the position of Pelagohydra, one is struck by the origin of the tentacles between the axial parenchyma in the meshes between the endoderm canals, from which the gonophores (gonozooids now reduced) spring. In Chondrophora Bigelow's sections show that the tentacles arise from the canals themselves. The difficulty of exactly homologizing the structure of the two forms was stressed by Garstang (1946, p. 126), and of course Pelagohydra has no pneumatophore. Garstang maintained that the ' float' of Pelagohydra represents the hydrocaulus of Corymorpha, on account of the position of the gastric diaphragm, and that the gonophores are cauline. In the Chondrophora the gonophores are on the oral cone. Possession by hydroids of cauline gonophores is probably a later evolutionary stage than that of oral ones and in the opinion of my colleague Dr W. J. Rees leads on to a hydrorhizal position. The Chondrophora appear to be aberrant, pelagic, tubularian Anthomedusae. They have lateral (radial) extensions of the walls of the aboral chamber, the edge of the extension being provided with
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