Metazoa, metazoa
All multicellular animals in the animal kingdom except protozoa. It is characterized in that the body is composed of a large number of cells with morphological differentiation and functional division; Different from protozoan cells with both nutritional and reproductive functions, their reproductive cells and vegetative cells have obvious differentiation. According to the symmetry of the system shape, metazoa can be divided into asymmetric animals (multi-legged animals) and radiation symmetric animals (coelenterates, ctenophora, echinoderms; The symmetry of the latter is secondary, ctenophore and some corals are left-right radiation symmetry) and bilateral symmetry animals (all other categories). Metazoa have germ layer differentiation during embryonic development, in which laver only has the initial differentiation of endoderm and ectoderm, and coelenterate has mesoderm between endoderm and ectoderm. All the phyla from Platycladus orientalis are trilobites. According to the existence and structure of body cavity, metazoa can be divided into dehydrated animals, including foraminifera, coelenterates and oblate animals. Prosthetic cavity animals, including linear phylum, Gastropoda, etc. Coelenterates, including all the animal classes after annelids.
Metazoa can be divided into four levels according to their morphological structure: ① blastocyst level. For example, some metazoans consist of only one layer of cells and an inner cavity, while others consist of several cells crowded together to form a solid tubular body or a plate-like structure composed of many cells and several layers of cells. Because the cell arrangement of these animals is similar to that of higher animals in blastocyst stage, some people call them solid sac larvae. ② At the cellular level, Porphyra, for example, consists of two layers of cells, namely the outer cortex and the inner stomach. Various functions of the body are accomplished by cells that live more or less independently, such as cervical cells. ③ At the tissue level, there are not only cells, but also non-cellular substances (matrix, fiber, etc. ) in the organization. For example, coelenterates began to differentiate into epithelial tissue (with nerve-like conduction function). ④ At the level of organ system, animals have different organ systems, which are composed of different cells and tissues from Platycladus orientalis and have different structures and functions.
Among metazoans, Neodymia is the first to have a complete digestive tube, with one end for the mouth and the other end for the anus. In addition, the digestive tract is separated from the circulatory system, and the nervous system is higher than that of flat animals.
Evidence from paleontology, morphology and embryology shows that multicellular metazoa come from unicellular protozoa. Before the rise of multicellular animals, single-celled animals have flourished. There is no absolute macro-structure between unicellular animals and multicellular animals, and hollow spherical Volvariella volvacea, which is composed of many small cells (sometimes up to 50 thousand cells), is an intermediate type between unicellular animals and multicellular animals.
In what way did single-celled animals evolve into multicellular animals? Different factions have different views. The main theories are:
(1) Protozoan theory, 1874, Meikel first proposed that the earliest ancestors of multicellular animals were spherical groups similar to Volvariella volvacea, and these single-celled animals invaginated to one side to form the ancestors of multicellular animals. Because they are very similar to gastrula, with two germ layers and a primitive mouth, Meikel called them gastrula and his theory gastrula. (2) According to the theory of devouring insects, Mechnikov observed the embryonic development of many lower-cell animals and found some lower species, whose gastrula was not mainly formed by invagination, but by inward migration. He also found that some multicellular animals mainly rely on phagocytosis for intracellular digestion and hate extracellular digestion. Therefore, he reasoned that the earliest multicellular animal was intracellular digestion, and then extracellular digestion was developed. In a single-celled colony like Volvariella volvacea, some cells devour food and enter the colony to form endoderm. The result is a diederm animal, which is solid at first, and then gradually forms a digestive cavity. Therefore, Mechnikov called the ancestors of this hypothetical multicellular animal phagocytes. (3) According to syncytial theory, Hatch and Hansen believe that the ancestor of metazoa should be a unicellular animal similar to polynuclear ciliates, whose cell membranes have been segmented and evolved into multicellular animals. According to this theory, anorectal animals are the most primitive metazoans. Protozoan ciliates have evolved into intestines with symmetrical cilia on both sides but no large and small nuclei, and another branch has evolved into higher ciliates with large and small nuclei.
metazoan
Except protozoa, it is the general name of all other animals (metazoa). Among metazoa, eumetazoa is the only metazoa except mesophytes and lateral animals (sponges), including groups from coelenterates (Echinopoda and Ctenomedusae) to vertebrates. Metazoa are multicellular animals, but they are not only a cell group, but also called Keimblattie- re, which is arranged into two or three layers of germ layers according to cells. In addition, because these cells differentiate into tissues of various shapes in morphology and function, they are also called tissue animals (except mesophytes and sponges). The differentiation of germ layer and tissue forms various organs, so metazoa generally maintain the system of higher animals. Related to this, the individual development of metazoa is generally complicated. A single-celled egg divides to form a germ layer and develops into an embryo. While the tissues differentiate and form a certain shape, some of them have to undergo extremely complicated metamorphosis to form adults. Although sometimes asexual reproduction is carried out, sexual reproduction is the main method. Metazoa are undoubtedly ancestors from protozoa, but there are many discussions about what kind of protozoa their ancestors are. It can be roughly divided into: (1) is the result of cell differentiation of polynuclear protozoa [Hatch's theory of ciliate origin holds that primitive oblate animals are produced by polynuclear ciliates, and coelenterates are secondary animals]; (2) According to the old theory, it is thought that it evolved from flagella groups such as Volvariella volvacea. Among these two theories, it is generally believed that the argument of the former is insufficient [Hadji's theory is more striking than that of G. de Beer], and more people are in favor of the latter. The latter's argument: this group structure is considered as the unchangeable germ of metazoa; The individuals in the group make structural changes according to the polar axis; Divided into nutritional components and reproductive components; In the process of forming a group, it is particularly similar to the individual development of sponge animals; The male germ cells of metazoa keep the shape of flagellates and so on. Haeckel inferred from these materials that the ancestors of metazoa and intestines appeared at the same time. E. Metchnikoff (1886) thought that the floating wave larvae developed from coelenterate larvae. According to the opinions of many scholars, the second theory is close to the real situation. Metazoa are related to the inner cavity of the body and can be divided into coelenterates and coelenterates. Body cavity motors can be divided into original body cavity motors and true body cavity motors [Ziegler, 1889]. In addition, some people fall into two categories: old animals and new animals [C. Graby, 1908]. Others can be divided into two categories: terminal cell stem and intestinal cavity stem related to mesoderm or body cavity formation. The lower animals in acrosome are equivalent to primordial reproductive adenomas, while other higher species and intestinal reproductive adenomas are true reproductive adenomas. The acrosome is consistent with the old animals, and the small intestinal adenoma is consistent with the new animals. If these two dry species are higher animals, they can see the somites. The above viewpoints are of great significance to the relationship between metazoa systems. In this way, it is easy to distinguish between vertebrates and invertebrates. Therefore, the systematic relationship between vertebrates and chordates including invertebrates is attracting people's attention.
A branch of protozoa that has nothing to do with other metazoa. Protozoan flagellates are generally considered to be the direct ancestors of lateral animals.
Epiphytes are just a phylum of sponges, and their development process is from single-layer cell blastocyst to two-layer cell adult, which is formed by the animal hemisphere of blastocyst falling into the plant hemisphere. In other words, the epidermis is equivalent to the endoderm of other animals, and the jaw cell layer is equivalent to the ectoderm of other metazoans, which is just the opposite of other metazoans. Mesenchymal layer originated from cortex and jaw cell layer, among which amoeba-like cells have digestive function, but they are closely related to endoderm of body cavity animals.
Because the evolution process of lateral animals is different from other metazoa, sponges and coelenterates are the most primitive parts of metazoa, but there is no genetic relationship between them. Therefore, sponges cannot be classified as twins, nor can they be called intestinal worms. They should belong to a branch of metazoa, and they are called lateral animals. Unlike other metazoa, other metazoa are called true metazoa. (Xie Ji, Huang Ruichang) Protozoa are the most primitive and lowest animals in the animal kingdom. Most of them are single-celled organisms. In cell structure, the single cell of protozoa is similar to that of multicellular animals, and it can also be divided into cytoplasm and nucleus, and the surface of cytoplasm is surrounded by cell membrane. Functionally, this protozoan cell is a complete organism, which can perform the life functions of multicellular animals, such as nutrition, respiration, excretion, reproduction and response to external stimuli. These functions are accomplished by cells or organelles differentiated from cells. Therefore, different organelles are functionally equivalent to organs and systems of multicellular animals. They perform the same physiological function at different structural levels. The diversity and complexity of the structural and functional differentiation of this cell that constitutes protozoa are incomparable to any cell in multicellular animals, so at the cellular level, the cell that constitutes protozoa is the most complex cell. A few protozoa are composed of several or several cells. There may be no morphological and functional differentiation between cells, or there may be preliminary morphological and functional differentiation, but each cell still maintains a certain degree of independence. We call these protozoa colonies, such as Gonium and Pleodorina.