4. The morula of most animals further changes into a _Blastula_, a hollow ball filled with fluid, the wall being formed by a single layer of cells, the blastoderm or germinal layer. This modification is brought about by the action of the cells--they conveying nourishing fluid into the interior of the whole cell colony and thereby being themselves forced towards the surface. The Blastula of most Invertebrata, and even that of Amphioxus, is possessed of fine ciliæ, or hair-like processes, the vibrating motion of which causes the whole organism to rotate and advance in the water. Living representatives of such Blastæads, namely, globular gelatinous colonies of cells enclosing a cavity, are Volvox and Magosphæra. 5. The Blastula of most animals assumes a new larval form called _Gastrula_, in which the essential characteristics are that a portion of the blastoderm by invagination converts the Blastula into a cup with double walls, enclosing a new cavity, the primitive gut. This invagination or bulging-in obliterates the original inner cavity of the Blastula. The outer layer of the Gastrula is the ectoderm, the inner the endoderm; both pass into each other at the blastoporus, or opening of the gut cavity. The Gastrula is a stage in the embryonic development of the various great groups of animals, and some such primitive form as ancestral to all Metazoa is thus indicated. This hypothetical _Gastræa_ is still very essentially represented by the lower Coelenterates--_e.g._, Olynthus, Hydra. 6. The sixth stage--that of the _Platodes_, or flat-worms--is very hypothetical. They are bilateral gastræads, with a flattened oblong body, furnished with ciliæ, with a primitive nervous system, simple sensory and reproductive organs, but still without appendages, body cavity, vent, and blood-vessels. The nearest living representatives of such creatures are the acoelous Turbellarians--_e.g._, Convoluta, a free-swimming, ciliated creature. 7. The next higher stage is represented by such low animals as the _Gastrotricha_--_e.g._, Chætonotus among the Rotatoria, which differ from the rhabdocoelous Turbellarians chiefly by the formation of a vent and the beginnings of a coelom, or cavity, between gut and body wall. The addition of a primitive vascular system and a pair of nephridia, or excretory organs, is first met with in the _Nemertines_. 8. These, together with the _Enteropneusta_ (Balanoglossus), are comprised under the name of Frontonia, or Rhynchelminthes, and form the highest group of the Vermalia.