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It will be useful first to point out the chief advances in organisation by which the simple Gastraea gradually became the more developed Chordaea. Its bilateral and tri-axial type indicates that the Gastraeads the common ancestors of all the Metazoa divided at an early stage into two divergent groups. Thus arose the typical bilateral form, which has three axes.
The same bilateral type is found in all our artificial means of locomotion carts, ships, etc.; it is by far the best for the movement of the body in a certain direction and steady position. Hence natural selection early developed this bilateral type in a section of the Gastraeads, and thus produced the stem-forms of all the bilateral animals.
In the struggle for life they have found shelter in the body-cavity of other animals. Modern gastraeads. In order to avoid confusion with these, I afterwards gave them the name of Prophysema. The whole mature body of the Prophysema is a simple cylindrical or oval tube, with a two-layered wall. The two strata of cells that form the wall of the tube are the primary germinal layers.
We can say, for instance, that we have inherited the oldest organs of the body, the external skin and the internal coat of the alimentary system, from the Gastraeads; the nervous and muscular systems from the Platodes; the vascular system, the body-cavity, and the blood from the Vermalia; the chorda and the branchial gut from the Prochordonia; the articulation of the body from the Acrania; the primitive skull and the higher sense-organs from the Cyclostomes; the limbs and jaws from the Selachii; the five-toed foot from the Amphibia; the palate from the Reptiles; the hairy coat, the mammary glands, and the external sexual organs from the Pro-mammals.
The fact that there are still in existence various kinds of gastraeads, or lower Metazoa with an organisation little higher than that of the hypothetical gastraea, is a strong point in favour of our theory.
We must grant, however, that in the whole stem-history of the Vertebrates the long stretch from the Gastraeads and Platodes up to the oldest Chordonia remains by far the most obscure section. We might frame another hypothesis to raise the difficulty namely, that there was a long series of very different and totally extinct forms between the Gastraea and the Chordaea.
When these are closed they resemble the Physemaria. Possibly the gastraeads that we call Physemaria are only olynthi with the pores closed. Prophysema primordiale, a living gastraead. The same in longitudinal section. Ascula of gastrophysema, attached to the floor of the sea. Olynthus, a very rudimentary sponge. The lowest form of the Cnidaria is also not far removed from the gastraeads.
On these principles I assume that the oldest and simplest Turbellaria arose from Platodaria, and these directly from bilateral Gastraeads. The chief advances were the formation of gonads and nephridia, and of the rudimentary brain.
In all these rudimentary living coelenteria the sexual cells of both kinds ova and sperm cells are formed by the same individual; it is possible that the oldest gastraeads were hermaphroditic. The sexual cells originally proceeded from the edge of the primitive mouth of the gastraead.
Both classes have a complete ciliary coat on the epidermis, a heritage from the Turbellaria and the Gastraeads; also, both have two openings of the gut, the mouth and anus, like the Gastrotricha. But we find also an important organ that is wanting in the preceding forms the vascular system.
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