This interesting blastula is very important. The conversion of the morula into a hollow ball proceeds on the same lines originally in the most diverse stems--as, for instance, in many of the zoophytes and worms, the ascidia, many of the echinoderms and molluscs, and in the amphioxus. Moreover, in the animals in which we do not find a real palingenetic blastula the defect is clearly due to cenogenetic causes, such as the formation of food-yelk and other embryonic adaptations. We may, therefore, conclude that the ontogenetic blastula is the reproduction of a very early phylogenetic ancestral form, and that all the Metazoa are descended from a common stem-form, which was in the main constructed like the blastula. In many of the lower animals the blastula is not developed within the foetal membranes, but in the open water. In those cases each blastodermic cell begins at an early stage to thrust out one or more mobile hair-like processes; the body swims about by the vibratory movement of these lashes or whips (Figure 1.29 F). We still find, both in the sea and in fresh water, various kinds of primitive multicellular organisms that substantially resemble the blastula in structure, and may be regarded in a sense as permanent blastula-forms--hollow vesicles or gelatinous balls, with a wall composed of a single layer of ciliated homogeneous cells. There are "blastaeads" of this kind even among the Protophyta--the familiar Volvocina, formerly classed with the infusoria. The common Volvox globator is found in the ponds in the spring--a small, green, gelatinous globule, swimming about by means of the stroke of its lashes, which rise in pairs from the cells on its surface. In the similar Halosphaera viridis also, which we find in the marine plancton (floating matter), a number of green cells form a simple layer at the surface of the gelatinous ball; but in this case there are no cilia.