These twelve stems of the animal kingdom are, however, by no means co-ordinate and independent types, but have definite relations, partly of subordination, to each other, and a very different phylogenetic meaning. Hence they must not be arranged simply in a row one after the other, as was generally done until thirty years ago, and is still done in some manuals. We must distribute them in three subordinate principal groups of very different value, and arrange the various stems phylogenetically on the principles which I laid down in my _Monograph on the Sponges,_ and developed in the _Study of the Gastræa Theory._ We have first to distinguish the unicellular animals (_protozoa_) from the multicellular tissue-forming (_metazoa_). Only the latter exhibit the important processes of segmentation and gastrulation; and they alone have a primitive gut, and form germinal layers and tissues.
The metazoa, the tissue-animals or gut-animals, then sub-divide into two main sections, according as a body-cavity is or is not developed between the primary germinal layers. We may call these the _cœlenteria_ and _cœlomaria,_ the former are often also called _zoophytes_ or _cœlenterata,_ and the latter _bilaterals._ This division is the more important as the cœlenteria (without cœlom) have no blood and blood-vessels, nor an anus. The cœlomaria (with body-cavity) have generally an anus, and blood and blood-vessels. There are four stems belonging to the cœlenteria: the gastræads (“primitive-gut animals”), sponges, cnidaria, and platodes. Of the cœlomaria we can distinguish six stems: the vermalia at the bottom represent the common stem-group (derived from the platodes) of these, the other five typical stems of the cœlomaria—the molluscs, echinoderms, articulates, tunicates, and vertebrates—being evolved from them.
Man is, in his whole structure, a true vertebrate, and develops from an impregnated ovum in just the same characteristic way as the other vertebrates. There can no longer be the slightest doubt about this fundamental fact, nor of the fact that all the vertebrates form a natural phylogenetic unity, a single stem. The whole of the members of this stem, from the amphioxus and the cyclostoma to the apes and man, have the same characteristic disposition, connection, and development of the central organs, and arise in the same way from the common embryonic form of the chordula. Without going into the difficult question of the origin of this stem, we must emphasise the fact that the vertebrate stem has no direct affinity whatever to five of the other ten stems; these five isolated phyla are the sponges, cnidaria, molluscs, articulates, and echinoderms. On the other hand, there are important and, to an extent, close phylogenetic relations to the other five stems—the protozoa (through the amœbæ), the gastræads (through the blastula and gastrula), the platodes and vermalia (through the cœlomula), and the tunicates (through the chordula).