animals,” or metazoa, in contradistinction to the gutless protozoa. I have pointed out in my Study of the _Gastræa Theory_ [not translated] (1873) the important consequences of this conception in the morphology and classification of the animal world. I also divided the realm of metazoa into two great groups, the lower and higher metazoa. In the first are comprised the _cœlenterata_ (also called zoophytes, or plant-animals). In the lower forms of this group the body consists throughout life merely of the primary germinal layers, with the cells sometimes more and sometimes less differentiated. But with the higher forms of the cœlentarata (the corals, higher medusæ, ctenophoræ, and platodes) a middle layer, or _mesoderm,_ often of considerable size, is developed between the other two layers; but blood and an internal cavity are still lacking. To the second great group of the metazoa I gave the name of the _cœlomaria,_ or _bilaterata_ (or the bilateral higher forms). They all have a cavity within the body (cœloma), and most of them have blood and blood-vessels. In this are comprised the six higher stems of the animal kingdom, the annulata and their descendants, the mollusca, echinoderma, articulata, tunicata, and vertebrata. In all these bilateral organisms the two-sided body is formed out of four secondary germinal layers, of which the inner two construct the wall of the alimentary canal, and the outer two the wall of the body. Between the two pairs of layers lies the cavity (cœloma). Although I laid special stress on the great morphological importance of this cavity in my _Study of the Gastræa Theory,_ and endeavoured to prove the significance of the four secondary germinal layers in the organisation of the cœlomaria, I was unable to deal satisfactorily with the difficult question of the mode of their origin. This was done eight years afterwards by the brothers Oscar and Richard Hertwig in their careful and extensive comparative studies. In their masterly _Cœlum Theory: An Attempt to Explain the Middle Germinal Layer_ [not translated] (1881) they showed that in most of the metazoa, especially in all the vertebrates, the body-cavity arises in the same way, by the outgrowth of two sacs from the inner layer. These two cœlom-pouches proceed from the rudimentary mouth of the gastrula, between the two primary layers. The inner plate of the two-layered cœlom-pouch (the visceral layer) joins itself to the entoderm; the outer plate (parietal layer) unites with the ectoderm. Thus are formed the double-layered gut-wall within and the double-layered body-wall without; and between the two is formed the cavity of the cœlom, by the blending of the right and left cœlom-sacs. We shall see this more fully in Chapter X.