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. [Illustration: 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], male gonophore; M[female], 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.