Eugen Kolisko, M.D.
“The primary division of the animal kingdom is into phyla. Each phylum is sharply characterised by the possession of a plan of structure in the adult which is peculiar to it, differing from that proper to every other phylum in such ways that it is, in general, incapable of derivation from any other.” (From an article on Zoology in the 14th Edition of the Encyclopaedia Britannica.) If we compare all the different systems of the animal kingdom we find that the distinction between the Vertebrates and the Invertebrates (animals with and without backbones) always occurs. In all the systems too, the Vertebrates are divided into five classes, the whole group being treated as one phylum.
In the Invertebrates there are divisions into phyla, and both Haeckel and Huxley described this group as consisting of seven phyla. These two zoologists differed in their descriptions of any one phylum, but were agreed on the sevenfold division
It appears that we need something in the nature of a key in order to be able to approach the door of understanding of the animal kingdom. The key turns out to be the arrangement of the animals in twelve main divisions or phyla. Seven of them embrace the totality of the Invertebrate Sub-Kingdom. These animals have a soft body and no real skeleton. They are the left-overs of a stage of World- evolution in which the endoskeleton had not been reached. The five phyla of the Vertebrates show the five ways in which the true skeleton can be manifested.
Thus the best way is to describe the characteristic qualities of the twelve phyla or groups in order that the reader may be aware of the archetypal forms of the animal kingdom.
1. The PROTOZOA (animals consisting of a single cell) are the primitive form of organic life, but nevertheless they embrace, in a microscopic appearance, nearly every macroscopic being in all the Kingdoms of Nature. Plants and animals consist in their bodies of cells, and even in the Mineral Kingdom the crystals can be compared with a mineralised cellular structure. Because plants and animals are composed of cells, in the Protozoan group there is no sharp dividing line between the plant and animal kingdom. The great difference between Protozoa (animals consisting of one cell) and Metazoa (all the organisms consisting of many cells) arises from the opposite tendencies of division and cohesion.
In no living organism is it possible to think of the body or soma as being merely the sum total of all the cells—rather has the life of the cell an antagonistic tendency to the metazoic life. This is shown in all infectious diseases caused by Bacteria and Protozoa, because they have in themselves the power of disintegration of the life of the higher organisms. In cancer and other tumours, and especially the malignant tumours, this tendency of the single cell to grow and increase takes place at the expense of the higher organ. In cancer it is not separate and individual cells which cause the disease, but the too-great multiplication of the cells of a tissue. A normal cellular activity becomes malignant in the human being. For this reason no bacterium can be found in cancer.
There is no possibility for further evolution for the Protozoa. They are left-overs in the march of evolution. This class of animals can be thought of as having created all the forms of organic life, by remaining in cellular form in the tissues. But if they appear without the power of further development, they bear in themselves the powers of destruction of all higher life.
2. The COELENTERATA—hydra polyps and medusae (animals such as sea anemones and those which build coral) and all the sponges (which were included in this group in all the older classifications) are characterised by the polarity between the inside and the outside of the organism. These animals are composed of two layers of tissue, the endoderm and the ectoderm (literally, the inner skin and the outer skin). The basic plan of these animals is a cup, the outer layer of which is formed by the ectoderm and the inner by the endoderm. The different species show variations of this plan, as for instance, the beautiful fringe of tentacles around the rim of the cup of the sea anemone. In this group we find for the first time the formation of inner organs. The differentiation begins between the digestive and the nervous system, the latter forming the outer layer.
Haeckel applied his knowledge of embryology (the study of development of living organisms from their earliest rudiments) to his study of the phylum, and by this means he realised that that stage in embryonic evolution in which the embryo loses its spherical shape in order to form something like a cup, and is called a gastrula, can be compared with an adult hydra. These animals are free-living beings corresponding to the gastrula stage of embryonic life.
At this point in evolution, the difference between the animal and vegetable kingdoms becomes evident. Those animals which are fixed to the substratum can, however, be thought of as being similar to plants. In this group there may be seen a characteristic dualism which leads to the double generative forms of polyps and medusae. In some species these two forms alternate. In one class of the Coelenterata, the sponges, sea anemones and corals, the polypus-form only is developed, and it leads to a fixed mode of life and then to a plant-like, vegetative growth and solidification. A great part of the solid earth is formed by the deposits of such organisms as rocks (coral rocks, limestone, etc.).
The other path of evolution leads to mobility, to soft and pliable tissues, to sexual differentiation and a more animal-like condition. The summit of this is reached in the floating colonies of medusae (soft beings rather like jellyfish in form, and stacked up on one another rather like a stack of plates) colonies where the polypus generation is under the direction of a highly developed medusa. Such a medusa is transformed into an air-bladder and causes the whole colony to float. It is interesting to compare these organisms with the plant. The corals, which become permeated with mineral substance, correspond to the root organisation. The floating colony is more like a blossom, and the intermediate forms of hydra are more like leaves. So we find the dualism appearing in the anatomical structure, in the endoderm and ectoderm, in the digestive and nervous systems (in the functions of digestion and perception) and in the polarity of polyps and medusae, and also in the division of the species into a coral type, in fixed and floating colonies.