Metazoa

“ There is the fundamental fact that the plants and animals with which we are familiar—Metaphyta and Metazoa—are formed by the aggregation of units homologous with Protozoa. These units, often conspicuously showing their homology in early embryonic stages, continue some of them to show it throughout the lives of the highest type of Metazoa, which contain billions of units carrying on a protozoon life. Of the protoplasts not thus active the great mass, comparatively little transformed in low organisms, become more and more transformed as the ascent to high organisms goes on ”

“ This antagonism, seen in these incipient Metazoa which show us none of that organization characterizing the Metazoa in general, is everywhere in more or less disguised forms exhibited by them—must necessarily be so if growth of the individual is a process of integration while formation of new individuals is a process of disintegration. And, primarily, it is an implication that whatever furthers the one impedes the other. ”

“ On any view of the origin of the metazoa, this diverse or developmental reproduction is a new protoplasmic faculty; on any view, it must have been transmitted, for otherwise the metazoa would have ceased to exist. This new faculty of developmental reproduction, then, with the inception of the metazoa, takes its place among other protoplasmic faculties, and, with the progress of differentiation and the division of [144] labour, will become the special business of certain cells. ”

“ The whole of the animal kingdom is divided into two great groups, which are called the Protozoa and the Metazoa. Similarly, the whole of the vegetable kingdom is divided into the Protophyta and the Metaphyta. The characteristic feature of all the Protozoa and Protophyta is that the organism consists of a single physiological cell, while the characteristic of all the Metazoa and Metaphyta is that the organism consists of a plurality of physiological cells, variously modified to subserve different functions in the economy of the animal or plant, as the case may be. ”

“ Von Siebold pointed out that in the Protozoa the individual was always a single vital unit or cell, in contrast with the higher division of the animal kingdom, the Metazoa, in which the body is generally, though not universally, regarded as composed of many such units. ”

“ But in the Metazoa and Metaphyta at large, the associated units are, with the exceptions just indicated, completely subordinated. The unit-life is so far lost in the aggregate life that neither locomotion nor the relative motion of parts remains; and neither in shape nor composition is there resemblance to protozoa. Though in many cases the internal protoplasm continues to carry on vital processes subserving the needs of the aggregate, in others vital processes of an independent kind appear to cease. ”

“ Insectivorous Plants that have the power of capturing and digesting insects have no real internal cavity. Animal-feeding Protista take in their food into the interior of their protoplasm and digest it therein, and the Metazoa have an internal cavity or stomach for the same purpose. Here again there are exceptions in the case of certain internal parasites, such as the Tapeworms and Acanthocephala (Vol. II. pp. 74, 174) , which have no stomachs, living as they do in the dissolved food-supplies of their hosts, but still possessing the general tissues and organs of Metazoa. ”

“ Although no one of the functions of the body is necessarily attended by the limited duration of the cells which perform it, as is proved by the persistence of unicellular forms, yet any or all of them might lead to such a limitation of existence without in any way injuring the species, as is proved by the Metazoa. But the reproductive cells cannot be limited in this way, and they alone are free from it. They could not lose their immortality, if indeed the Metazoa are derived from the immortal Protozoa, for from the very nature of that immortality it cannot be lost. ”

“ Sporozoa in the persistence of the encysted stage are very plant-like, though they are often intracellular and are parasitic in living Animals. On the other hand, the Infusoria for the most part answer to all the physiological characters of the Animal world, but are single cells, and by the very perfection of their structure, all due to plasmic not to cellular differentiation, show that they lie quite off the possible track of the origin of Metazoa from Protozoa. ”

“ Had it done no more than give to Protozoa and Protophyta that cell-form which characterizes them—had it done no more than entail the cellular composition which is so remarkable a trait of Metazoa and Metaphyta—had it done no more than cause the repetition in all visible animals and plants of that primary differentiation of outer from inner which it first wrought in animals and plants invisible to the naked eye; it would have done much towards giving to organisms of all kinds certain leading traits. ”

“ The original idea that in the Rhombozoa and Orthonectida we had animals intermediate between the Protozoa and Metazoa is no longer widely held. The modern view is that the simplicity of their structure is secondary and not primary, and is correlated with their parasitic habit of life. ”

“ Attention has already been drawn to the difference between those lowly organisms, each of which is composed of a single cell—the protozoa, as they are termed—and those higher organisms, called metazoa, in which there are many cells with varied functions. ”

“ As in the case of sexual propagation, so in that of karyokinesis, processes which are common to all the Metazoa are not wholly without their foreshadowings in the Protozoa. And seeing how greatly exalted is the office of egg-cells—and even of tissue-cells—as compared with that of their supposed ancestry in protozoal cells, it seems to me scarcely to be wondered at if their specializations of function should be associated with corresponding peculiarities of structure—a general fact which would in no way militate against the doctrine of evolution. ”

“ Life-cycles of the Protozoa.—It is probable that in all Protozoa, as in the Metazoa, the life-history takes its course in a series of recurrent cycles of greater or less extent, a fixed point, as it were, in the cycle being marked by the act of syngamy, or conjugation, which represents, apparently, a process for recuperation of the waning vital powers of the organism. ”

“ It was there pointed out that in animal organisms the various structures are so arranged that evolution of a small amount of energy in one, sets up evolution of a larger amount of energy in another; and often this multiplied energy undergoes a second multiplication of like kind. If this view is tenable, we may now suspect that this method displayed in the structures of the Metazoa was initiated in the structures of the Protozoa, and consequently characterizes those homologues of them which compose the Metazoa. ”

“ Among the lower Metazoa, such as the polypes, the capacity for reproduction still exists to such a degree in the somatic cells, that a small number of them are able to give rise to a new organism,—in fact new individuals are normally produced by means of so-called buds. ”

“ It is to the metazoa that we must go for the higher stages of evolution. Of the varied phases of this type of life, we can refer only to those of general character. No matter upon what planet life may have originated, we can not well avoid the conclusion that it must have had the organic cell as its unit, and that everywhere in its upward progress the many-celled self-moving form, feeding upon organic nutriment. must have been reached, as a stage superior to the minute single-celled animal, or the immobile plant, fed with inorganic nutriment. ”

“ The blastula germ form corresponds to a Volvox or Magosphæra, a similar ancestral form; the Gastrula is the inherited repetition of the Gastræa, the common stem form of the entire series of Metazoa. All these typical ancestral forms man shares with all the other Metazoa, that is to say with all other animals except the unicellular Protozoa. Every man, without exception, begins his individual existence in the form of a spherical egg cell, barely visible to the naked eye, as a very small dot, and the special characters of this egg-cell are exactly the same in man as in all other mammals. ”

“ But Darwin ably argues in favour of each of them by pointing to well-known analogies, drawn from the vital processes of living cells both in the protozoa and metazoa. For example, it is already a well-recognized doctrine of physiology that each cell of a metazoon, or multicellular organism, though to [4] a large extent dependent on others, is likewise to a certain extent independent or autonomous, and has the power of multiplying by self-division. ”

“ This we find in the lower cnidaria and worms, as well as in the more highly-developed molluscs, echinoderms, articulates, and vertebrates.
In all these metazoa, or multicellular animals, the chief embryonic processes are substantially alike, although they often seem to a superficial observer to differ considerably. The stem-cell that proceeds from the impregnated ovum always passes by repeated cleavage into a number of simple cells. ”

“ By the development of this process, multicellular Metazoa arose from colonies of similar Protozoa, and at length culminated in the higher animals and man. All the cell-series are immortal, but they all must die because the structure which is built up by them collectively is mortal. The reproductive cells are the only kind adapted for existence outside the body, and from time to time some of the human reproductive cells succeed in conjugating, and from them a new individual arises. ”

“ The lowest metazoa we know—namely, the lower zoophyta (sponges, simple polyps, etc.) —remain throughout life at a stage of development which differs little from the gastrula; their whole body consists of two layers of cells. This is a fact of extreme importance. We see that man, and also other vertebrates, pass quickly through a stage of development in which they consist of two layers, just as these lower zoophyta do throughout life. If we apply our biogenetic law to the matter, we at once reach this important conclusion. ”

“ It must first be pointed out that those lower Metazoa, like the Hydroid polyps in particular, which are endowed with such a high and general power of regeneration, do actually require this for their safety; they are not only soft, easily injured and torn, but they are most severely decimated by many enemies. ”

“ Whereas the protozoan is represented by a single cell that comprehends all the vital functions, the functions of reproduction as well as those of nutrition and relation, the metazoan, on the other hand, is composed of an aggregation, of a colony of distinct cells, among which a division of labor has been effected varying in complexity with the height that the animal has attained in the classificatory scale. ”

“ We find the highest development of the animal cell-soul in the class of ciliata, or ciliated infusoria. When we compare their activity with the corresponding psychic life of the higher, multicellular animals, we find scarcely any psychological difference; the sensitive and motor organella of these protozoa seem to accomplish the same as the sense-organs, nerves, and muscles of the metazoa. ”

“ In the Protozoa we have the globular form, diversified by temporary, improvised limbs, or by permanent organs. In the Metazoa variation from the globe takes place in axial directions—the fixed animals having usually but two axes of departure from the sphere; the moving animals having ordinarily three axes—a longitudinal, a vertical, and a transverse. ”

“ As they find their plasm directly in the decayed matter in the woods, they have lost their chlorophyll and green leaves. Among the metazoa many of the vermalia, and some of the higher animals, such as the rain-worm and many tube-dwelling annelids (the mud-eaters, limicolæ) , etc., live on putrid matter. The organs which their nearest relatives use for obtaining, breaking up, and digesting food (eyes, jaws, teeth, digestive glands) have been entirely or mostly lost by these saprosites. Many of them form a transitional type to the parasites. ”

“ We are now in a position to appreciate the fact that the old and more popularly know division of animals into vertebrata and invertebrata scarcely represents the facts of the case, that the primary division should be into protozoa and metazoa, and that the vertebrata are one of several groups of metazoa with a fundamental bilateral symmetry and imperfect metameric segmentation. ”

“ For germ-plasm as it occurs in the germ-cells of multicellular organisms must have a constitution greatly more complex even than that which it has in unicellular organisms—seeing that in the former case, and by hypothesis, it bears a living record of the whole phylogeny of the Metaphyta and Metazoa in all their innumerable branchings. ”

“ This is true of cell-communities and the second-grade individuals or metazoa or multicellular animals and plants to which they give rise, [22] and also of the communities of metazoa and the third-grade individuals to which they give rise, whether the members of such communities of higher grade are physically bound together, as in a Hydroid or a Portuguese Man-o’-War, or united only by mental bonds, as are the communities of ants and bees and termites. As we have seen, the individuals are differentiated structurally for the different functions which they have to perform. ”