Up to this time the larva has retained a more or less spherical form, and the cavity on the under side of the umbrella has not yet become developed. The latter now becomes established by the whole disc assuming a vaulted form with the concavity directed downwards. The lining of the cavity so formed is derived from the epiblast of the disc already spoken of. The exact mode of formation of the gastrovascular canals has not been worked out. It has however been established by the researches of the Hertwigs (No. 146) and Claus (No. 153) that the radial and circular vessels of this system are connected together in adult Medusæ by an hypoblastic lamella; so that these canals would seem to be the remnants of an once-continuous gastric cavity. This mode of formation is established in the case of the medusiform buds; and it would therefore seem, as pointed out by the Hertwigs, a fair deduction that it occurs in the larva--a conclusion which is confirmed by the primitive extension of the gastric cavity to the edge of the disc at the time when its walls give rise to the solid axes of the tentacles. In the course of the subsequent retirement of the gastric cavity from the edge of the disc the gastrovascular canals probably take their origin, though Fol was unable to follow the changes which result in their formation. On the completion of the above changes the larva has become a fully formed Medusa, but it undergoes a not inconsiderable metamorphosis before the attainment of the adult state. [FIG. 73. A THREE-DAYS' LARVA OF ÆGINOPSIS WITH TWO TENTACLES. (After Metschnikoff.) _m._ mouth; _t._ tentacle.] Two species of Æginidæ have been studied by Metschnikoff (163), viz. _Polyxenia leucostyla_ (_Ægineta flavescens_), and _Æginopsis mediterranea_. In both of these forms the segmentation results in the formation of an elongated two-layered ciliated planula, without a central cavity. The two ends of this grow out into two long processes--the rudiments of a pair of at first aborally directed arms--which contain a solid hypoblastic axis (fig. 73). At this stage the larva closely resembles the larva of Tubularia. An alimentary cavity is hollowed out in the centre of the hypoblast which soon opens by a wide oral aperture (_m_). A second pair of arms becomes formed, which are at first much shorter than the original pair; with their formation a radial symmetry is acquired. Sense organs become at the same time developed, and the whole embryo assumes a medusiform character. Fresh tentacles arise, the velum and cavity of the umbrella become established, but these changes do not involve any points of very special interest. Siphonophora. The development of the Siphonophora has been the subject of careful investigation by Haeckel (158) and Metschnikoff (163). The ova are large and usually (except Hippopodius) without a membrane.