PARALLELISM OF ONTOGENY AND PHYLOGENY
| Definition of the forms of the five first stages of the development of the animal body. | Ontogenesis. | Phylogenesis. |
| The five first stages The five first stages of the individual development. | The five first stages of the phyletic or historical development. | |
| First Stage of Development. | 1. Monerula. | 1. Moneron. |
| A simple cytod (a plastid without a kernel.) | Animal egg without a kernel (when the egg-kernel has disappeared, after being fructified). | Most ancient animal Monera, originating by spontaneous generation. |
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| Second Stage of Development. | 2. Ovulum. | 2. Amœbæ. |
| A simple cell (a plastid containing a kernel.) | Animal egg with kernel (a simple egg-cell). | Animal Amœbæ. |
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| Third Stage of Development. | 3. Morula. (Mulberry form.) | 3. Synamœba. |
| A community (an aggregation of identical simple cells). | Globular heap of homogeneous “cleavage spheres.” | An aggregation of Amœbæ. |
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| Fourth Stage of Development. | 4. Planula. (Ciliated larva.) | 4. Planæa. |
| A solid or bladder-shaped, globular, or oval body, composed of two kinds of different cells: externally ciliated, internally non-ciliated cells. | Many-celled larva without mouth, composed of different cells. | Many-celled primæval animal without mouth, composed of two kinds of different cells. |
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| Fifth Stage of Development. | 5. Gastrula. (Larva with mouth.) | 5. Gastræa. |
| A globular or oval body with simple intestinal cavity and mouth-opening.Body wall composed of two layers; an externally ciliated ectoderm (dermal layer), aninternally non-ciliated entoderm (gastral layer). | Many-celled with intestines and mouth; intestinal wall with two layers. | Many-celled primæval animal with intestine and mouth; intestinal wall with two layers. (Primary form of zoophytes and worms.) |
From the ontogenetic occurrence of the Gastrula in the most different animal classes, from Zoophytes up to Vertebrata, we may, according to the biogenetic principle, safely draw the conclusion that during the Laurentian period there existed a common primary form of the six higher anima tribes, which in all essential points was formed like the Gastrula, and which we shall call the Gastræa. This Gastræa possessed a perfectly simple globular or oval body, which enclosed a simple cavity of like form, namely, the progaster; at one of the poles of the longitudinal axis the primary intestine opened by a mouth which served for the reception of nutrition. The body wall (which was also the intestinal wall) consisted of two layers of cells, the unfringed entoderm, or intestinal layer, and the fringed ectoderm, or skin-layer; by the motion of the cilia or fringes of the latter the Gastræa swam about freely in the Laurentian ocean. Even in those higher animals, in the ontogenesis of which the original Gastrula form has disappeared, according to the laws of abbreviated inheritance (vol. i. p. [212]), the composition of the Gastræa body has been transmitted to the phase of development which directly arises out of the Morula. This phase is an oval or round disc consisting of two cell-layers or membranes: the outer cell-layer, the animal or dermal layer (ectoblast), corresponds to the ectoderm of the Gastræa; out of it develops the external, loose skin (epidermis), with its glands and appendages, as well as the central nervous system. The inner cell-layer, the vegetative or intestinal layer (hypoblast), is originally the entoderm of the Gastræa; out of it develops the inner membrane (epithelium) of the intestinal canal and its glands. (Compare my Monograph of the Calcareous Sponges, vol. i. p. 466, etc.)
By ontogeny we have already gained five primordial stages of development of the animal kingdom: (1) the Moneron; (2) the Amœba; (3) the Synamœba; (4) the Planæa; and (5) the Gastræa. The former existence of these five oldest primary forms, which succeeded one another, and which must have lived in the Laurentian period, follows as a consequence of the biogenetic principle; that is to say, from the parallelism and the mechanico-causal connection of ontogenesis and phylogenesis. (Compare vol. i. p. [309].) In our genealogical system of the animal kingdom we may class all these animal forms, long since extinct, and, which on account of the soft nature of their bodies could leave no fossil remains, among the tribe of Primæval animals (Protozoa), which also comprises the still living Infusoria and Gregarinæ.
The phyletic development of the six higher animal tribes, which are all derived from the Gastræa, deviated at this point in two directions. In other words, the Gastræads (as we may call the group of forms characterized by the Gastræa-type of structure), divided into two divergent lines or branches; the one branch of Gastræads gave up free locomotion, adhered to the bottom of the sea, and thus, by adopting an adhesive mode of life, gave rise to the Protascus, the common primary form of the Animal-plants (Zoophyta). The other branch of the Gastræads retained free locomotion, did not become adherent and later on developed into the Prothelmis, the common primary form of Worms (Vermes). (Compare p. [133].)
This latter tribe (as limited by modern zoology) is of the greatest interest in the study of genealogy. For among Worms, as we shall see later, there are, besides very numerous peculiar families, and besides many independent classes, also very remarkable forms, which may be considered as forms of direct transition to the four higher animal tribes. Both comparative anatomy and the ontogeny of these worms enable us to recognize in them the nearest blood relations of those extinct animal forms which were the original primary forms of the four higher animal tribes. Hence these latter, the Molluscs, Star-fishes, Articulated animals, and Vertebrate animals, do not stand in any close blood relationship to one another, but have originated independently in four different places out of the tribe of Worms.
In this way comparative anatomy and phylogeny lead us to the monophyletic pedigree of the animal kingdom, the outlines of which are given on p. [133]. According to it the seven phyla, or tribes, of the animal kingdom are of different value in regard to genealogy. The original primary group of the whole animal kingdom is formed by the Primæval animals (Protozoa), including the Infusoria and Gastræads. Out of these latter arose the two tribes of Animal-plants (Zoophyta) and Worms as diverging branches. Out of four different groups of the Worm tribe, the four higher tribes of the animal kingdom were developed—the Star-fishes (Echinoderma) and Insects (Arthropoda) on the one hand, and the Molluscs (Mollusca) and Vertebrated animals (Vertebrata) on the other.
Having thus sketched out the monophyletic pedigree of the animal kingdom in its most important features, we must now turn to a closer examination of the historical course of development which the seven tribes of the animal kingdom, and the classes distinguished in them, have passed through (p. [132]). There is a much larger number of classes in the animal than in the vegetable kingdom, owing to the simple reason that the animal body, in consequence of its more varied and perfect vital activity, could differentiate and develop in very many more different directions than could the vegetable body. Thus, while we were able to divide the whole vegetable kingdom into six main classes and nineteen classes, we have to distinguish, at least, sixteen main classes and thirty-eight classes in the animal kingdom These are distributed among the seven different tribes of the animal kingdom in the way shown in the Systematic Survey on pages 132 and 133.