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It is an interesting fact that in the plant kingdom also the simple hollow sphere is found to be an elementary form of the multicellular organism. The botanist Schmitz gave them the name of Halosphaera viridis in 1879. The next stage to the Blastaea, and the sixth in our genealogical tree, is the Gastraea that is developed from it.
The first duty and first need of every organism is self-preservation. This is met by the functions of the nutrition and the covering of the body. When, therefore, in the primitive globular Blastaea the homogeneous cells began to effect a division of labour, they had first to meet this twofold need. One half were converted into alimentary cells and enclosed a digestive cavity, the gut.
The Gastraea itself has originated from the simple multicellular vesicle, the Blastaea, and this in turn must have been evolved from the lowest circle of unicellular animals, to which we give the name of Protozoa. We have already considered the most important primitive type of these, the unicellular Amoeba, which is extremely instructive when compared with the human ovum.
We may call it the Blastaea. Its body consisted, when fully formed, of a simple hollow ball, filled with fluid or structureless jelly, with a wall composed of a single stratum of ciliated cells. There were probably many genera and species of these blastaeads in the Laurentian period, forming a special class of marine protists.
As we have already seen, this ancestral form is particularly important. The actual ontogenetic development of the gastrula from the blastula furnishes sound evidence as to the phylogenetic origin of the Gastraea from the Blastaea.
In this fact, and the fact we have already established that the Gastraea has been evolved from the hollow vesicle of the one-layered Blastaea, and this again from the original unicellular stem-form, we have obtained a solid basis for our study of evolution. The second section, that leads from the Gastraea to the Prochordonia, is much more difficult and obscure.
The depression would grow deeper and deeper. In time the vegetal function of taking in and digesting food would be confined to the cells that lined this hole; the other cells would see to the animal functions of locomotion, sensation, and protection. This was the first division of labour among the originally homogeneous cells of the blastaea.
In explaining the phylogenetic origin of the gastraea in the light of this ontogenetic process, we may assume that the one-layered cell-community of the blastaea began to take in food more largely at one particular part of its surface. Natural selection would gradually lead to the formation of a depression or pit at this alimentary spot on the surface of the ball.
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