2. In the Siphonanthae, i.e. in all other Siphonophores, the ancestral form was a Siphonula, a bilaterally symmetrical Anthomedusa with a single long tentacle (cf. Corymorpha), which became displaced from the margin to the sub-umbrella. The Siphonula produced buds on the manubrium, as many Anthomedusae are known to do, and these by reduction or dislocation of parts gave rise to the various appendages of the colony. Thus the umbrella of the Siphonula became the protocodon, and its manubrium, the axis or stolon, which, by a process of dislocation of organs, escaped, as it were, from the sub-umbrella through a cleft and became secondarily attached to the ex-umbrella. It must be pointed out that, however probable Haeckel’s theory may be in other respects, there is not the slightest evidence for any such cleft in the umbrella having been present at any time, and that the embryological evidence, as already pointed out, is all against any homology between the stem and a manubrium, since the primary siphon does not become the stem, which arises from the ex-umbral side of the protocodon and is strictly comparable to a stolon.
Classification.—The Siphonophora may be divided, following Delage and Hérouard, into four sub-orders:
I. Chondrophorida (Disconectae Haeckel, Tracheophysae Chun). With an apical chambered pneumatophore, from which tracheal tubes may take origin (fig. 70); no nectocalyces or bracts; appendages all on the lower side of the pneumatophore arising from a compact coenosarc, and consisting of a central principal siphon, surrounded by gonosiphons, and these again by tentacles.
Three families: (1) Discalidae, for Discalia and allied genera, deep-sea forms not well known; (2) Porpitidae for the familiar genus Porpita (fig. 69) and its allies; and (3) Velellidae, represented by the well-known genus Velella (figs. 70, 71), common in the Mediterranean and other seas.
II. Calycophorida (Calyconectae, Haeckel). Without pneumatophore, with one, two, rarely more nectocalyces.
Three families: (1) Monophyidae, with a single nectocalyx; examples Muggiaea, sometimes found in British seas, Sphaeronectes, &c.; (2) Diphyidae, with two nectocalyces; examples Diphyes (fig. 72), Praya, Abyla, &c.; and (3) Polyphyidae, with numerous nectocalyces; example Hippopodius, Stephanophyes and other genera.
| From G. H. Fowler, modified after G. Cuvier and E. Haeckel, Lankester’s Treatise on Zoology. |
| Fig. 75.—A. Physalia, general view, diagrammatic; B, cormidium of Physalia; D, palpon; T, palpacle; G, siphon; GP, gonopalpon; M ♂, male gonophore; M ♀, female gonophore, ultimately set free. |
III. Physophorida (Physonectae + Auronectae, Haeckel). With an apical pneumatophore, not divided into chambers, followed by a series of nectocalyces or bracts.
A great number of families and genera are referred to this group, amongst which may be mentioned specially—(1) Agalmidae, containing the genera Stephanomia, Agalma, Anthemodes, Halistemma, &c.; (2) Apolemidae, with the genus Apolemia and its allies; (3) Forskaliidae, with Forskalia and allied forms; (4) Physophoridae, for Physophora (fig. 73) and other genera, (5) Anthophysidae, for Anthophysa, Athorybia, &c.; and lastly the two families (6) Rhodalidae and (7) Stephalidae (fig. 74), constituting the group Auronectae of Haeckel. The Auronectae are peculiar deep-sea forms, little known except from Haeckel’s descriptions, in which the large pneumatophore has a peculiar duct, termed the aurophore, placed on its lower side in the midst of a circle of swimming-bells.
IV. Cystophorida (Cystonectae, Haeckel). With a very large pneumatophore not divided into chambers, but without nectocalyces or bracts. Two sections can be distinguished, the Rhizophysina, with long tubular coenosarc-bearing ordinate cormidia, and Physalina, with compact coenosarc-bearing scattered cormidia.