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.
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].
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).