heading of "gut animals," or metazoa, in contradistinction to the
gutless protozoa.

I have pointed out in my Study of the Gastraea 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 coelenterata (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 coelentarata (the corals, higher medusae, ctenophorae,
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
coelomaria, or bilaterata (or the bilateral higher forms). They all
have a cavity within the body (coeloma), 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 (coeloma).

Although I laid special stress on the great morphological importance
of this cavity in my Study of the Gastraea Theory, and endeavoured to
prove the significance of the four secondary germinal layers in the