Volume II Part 6 (2/2)
Out of the Synambae, in the early Laurentian period, there afterwards developed a fourth primary form of the animal kingdom, which we shall call the ciliated germ (Planaea). This arose out of the Synamba by the outer cells on the surface of the cellular community beginning to extend vibrating fringes called cilia, and becoming ”ciliated cells,” and thus differentiating from the inner and unchanged cells. The Synambae consisted of completely equi-formed and naked cells, and crept about slowly, at the bottom of the Laurentian primaeval ocean, by means of movements like those of an Amba. The Planaea, on the other hand, consisted of two kinds of different cells-inner ones like the Ambae, and external ”ciliated cells.” By the vibrating movements of the cilia the entire multicellular body acquired a more rapid and stronger motion, and pa.s.sed over from the creeping to the swimming mode of locomotion. In exactly the same manner the _Morula_, in the ontogenesis of lower animals, still changes into a ciliated form of larva, which has been known, since the year 1847, under the name of _Planula_. This Planula is sometimes a globular, sometimes an oval body, which swims about in the water by means of a vibrating movement; the fringed (ciliated) and smaller cells of the surface differ from the larger inner cells, which are unfringed. (Fig. 4 of the Frontispiece.)
Out of this Planula, or fringed larva, there then develops, in animals of all tribes, an exceedingly important and interesting animal form, which, in my Monograph of the Calcareous Sponges, I have named _Gastrula_ (that is, larva with a stomach or intestine). (Frontispiece, Fig. 5, 6). This Gastrula externally resembles the Planula, but differs essentially from it in the fact that it encloses a cavity which opens to the outside by a mouth. The cavity is the ”_primary intestine_,” or ”primary stomach,” the _progaster_, the first beginning of the alimentary ca.n.a.l; its opening is the ”_primary mouth_” (prostoma). The wall of the progaster consists of two layers of cells: an outer layer of smaller ciliated cells (outer skin, or ectoderm), and of an inner layer of larger non-ciliated cells (inner skin, or entoderm). This exceedingly important larval form, the ”Gastrula,” makes its appearance in the ontogenesis of all tribes of animals-in Sponges, Medusae, Corals, Worms, Sea-squirts, Radiated animals, Molluscs, and even in the lowest Vertebrata (Amphioxus: compare p. 200, Plate XII., Fig. _B_ 4; see also in the same place the Ascidian, Fig. _A_ 4).
Definition of the _forms_ | +Ontogenesis.+ | +Phylogenesis.+ of the five first stages | The five first stages | The five first stages of the development of | of the individual | of the phyletic or the animal body. | development. | historical development.
-------------------------------+------------------------------+------------------------ | | _First Stage of Development._ | 1. | 1.
| +Monerula.+ | +Moneron.+ A simple cytod (a | | plastid without a kernel.) | Animal egg without a | Most ancient animal | | kernel (when the egg-kernel | Monera, originating by | | has disappeared, | spontaneous generation.
| | after being fructified). | | | | | | | | | | | | _Second Stage of Development._ | 2. | 2.
| +Ovulum.+ | +Ambae.+ A simple cell (a | | plastid containing a | Animal egg with kernel | Animal Ambae.
kernel.) | (a simple egg-cell). | | | | | | | | | | | | | | | | | _Third Stage of Development._ | 3. | 3.
| +Morula.+ | +Synamba.+ A community (an | (_Mulberry form._) | aggregation of identical | | An aggregation of simple cells). | Globular heap of h.o.m.ogeneous | Ambae.
| | ”cleavage spheres.” | | | | | | | | | | | | _Fourth Stage of Development._ | 4. | 4.
| +Planula.+ | +Planaea.+ A solid or bladder-shaped, | (_Ciliated larva_.) | globular, or oval | | Many-celled primaeval body, _composed of two | Many-celled larva | animal without kinds of different cells_: | without mouth, composed | mouth, composed of externally ciliated, | of different cells. | two kinds of different internally non-ciliated | | | cells.
cells. | | | | | | | | | | | | | | _Fifth Stage of Development._ | 5. | 5.
| +Gastrula.+ | +Gastraea.+ A globular or oval | (_Larva with mouth._) | _body with simple intestinal | Many-celled with intestines | Many-celled primaeval cavity and mouth-opening. | and mouth; intestinal | animal with intestine Body wall composed | wall with two | and mouth; intestinal of two layers_; an | layers. | wall with two externally ciliated ectoderm | | layers. (Primary form (dermal layer), an | | of zoophytes and internally non-ciliated | | worms.) entoderm (gastral layer). | |
From the ontogenetic occurrence of the Gastrula in the most different animal cla.s.ses, 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 animal tribes, which in all essential points was formed like the Gastrula, and which we shall call the Gastraea. This Gastraea 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 Gastraea 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 Gastraea 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 Gastraea; 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 Gastraea; out of it develops the inner membrane (epithelium) of the intestinal ca.n.a.l 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 Amba; (3) the Synamba; (4) the Planaea; and (5) the Gastraea. 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 cla.s.s 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 Primaeval animals (Protozoa), which also comprises the still living Infusoria and Gregarinae.
The phyletic development of the six higher animal tribes, which are all derived from the Gastraea, deviated at this point in two directions. In other words, the _Gastraeads_ (as we may call the group of forms characterized by the Gastraea-type of structure), divided into two divergent lines or branches; the one branch of Gastraeads 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 Gastraeads 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 cla.s.ses, 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 relations.h.i.+p 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 Primaeval animals (Protozoa), including the Infusoria and Gastraeads. 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 cla.s.ses distinguished in them, have pa.s.sed through (p. 132). There is a much larger number of cla.s.ses 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 cla.s.ses and nineteen cla.s.ses, we have to distinguish, at least, sixteen main cla.s.ses and thirty-eight cla.s.ses 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.
The group of _Primaeval animals_ (Protozoa) within the compa.s.s which we here a.s.sign to this tribe, comprises the most ancient and the simplest primary forms of the animal kingdom; for example, the five oldest phyletic stages of development previously mentioned, and besides these the Infusoria and Gregarinae, as well as all those imperfect animal forms, for which, on account of their simple and indifferent organization, no place can be found in any of the other six animal tribes. Most zoologists, in addition to these, include among the Protozoa a larger or smaller portion of those lowest organisms, which we mentioned in our neutral kingdom of Protista (in Chapter XVI.). But these Protista, especially the large division of the Rhizopoda, which are so rich in forms, cannot be considered as real animals for reasons previously given. Hence, if we here leave them out of the question, we may accept two main cla.s.ses or provinces of real Protozoa, namely, _Egg animals_ (Ovularia) and _Germ animals_ (Blastularia). To the former belong the three cla.s.ses of Archezoa, Gregarinae, and Infusoria, to the latter the two cla.s.ses of Planaeads and Gastraeads.
SYSTEMATIC SURVEY
_Of the 16 Main Cla.s.ses and 38 Cla.s.ses of the Animal Kingdom._
------------------+-----------------------+----------------------+----------------- _Tribes or Phyla_ | _Main Cla.s.ses_, | _Cla.s.ses_ |_Systematic Name_ _of the_ |_Branches or Clades_ | _of the_ | _of the_ _Animal Kingdom._ | _of the_ | _Animal Kingdom._ | _Cla.s.ses._ | _Animal Kingdom._ | | ------------------+-----------------------+----------------------+------------------
A. { =Primaeval= { I. Egg-animals { 1. Archaic animals 1. Archezoa =Animals= { _Ovularia_ { 2. Gregarines 2. Gregarinae { { 3. Infusoria 3. Infusoria +Protozoa+ { {II. Mulberry animals { 4. Planaeads 4. Planaeadas { _Blastularia_ { 5. Gastraeads 5. Gastraeadas
B. { =Animal= { III. Sponges { 6. Sponges 6. Porifera =Plants= { _Spongiae_ { { +Zoophyta+ { IV. Sea-nettles { 7. Corals 7. Coralla { _Acalephae_ { 8. Hood-jellies 8. Hydromedusae { { 9. Comb-jellies 9. Ctenophora
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