They are formed of a peripheral denser layer of protoplasm and a central spongy mass. They usually undergo their entire development in the water. In some instances they have been successfully reared by artificial impregnation. As an example of the Calycophoridæ I shall take Epibulia aurantiaca, a form allied to Diphyes, the development of which has been studied by Metschnikoff[78]. [78] In my description of the development of the Siphonophora I employ Huxley's terminology. [FIG. 74. THREE LARVAL STAGES OF EPIBULIA AURANTIACA. (After Metschnikoff.) A. Planula stage. B. Six-days' larva with nectocalyx (_nc_) and tentacle (_t_). C. Somewhat older larva with gastric cavity. _ep._ epiblast; _hy._ hypoblast; _so._ somatocyst; _nc._ nectocalyx; _t._ tentacle; _c._ large yolk cells; _po._ polypite.] There is a regular segmentation, unaccompanied by the formation of a segmentation cavity. At its close the ovum becomes a spherical ciliated embryo. This embryo soon becomes elongated, and its cells differentiate themselves into a central and a peripheral layer--the epiblast and the hypoblast (fig. 74 A). At this stage the larva has the typical planula form. The epiblast is especially thickened at a pole, which may be called the oral pole, and towards the side of this, which will be spoken of as the ventral side. Adjoining this thickened layer of epiblast a special thin layer of hypoblast becomes differentiated, which in opposition to the main mass of large nutritive cells forms the true hypoblastic epithelium (fig. 74 B, _hy_). On this thickening two prominences make their appearance (fig. 74 B). The oral of these is the rudiment of a tentacle (_t_), and the aboral of a nectocalyx (_nc_). [FIG. 75. AN ADVANCED LARVA OF EPIBULIA AURANTIACA WITH ONE LARGE NECTOCALYX. (After Metschnikoff.) _so._ somatocyst; _nc._ second imperfectly developed nectocalyx; _hph._ hydrophyllium; _po._ polypite; _t._ tentacle.]