INVERTEBRATE ZOOLOGY IN THREE VOLUMES - BOOK 1 - H.I. Shcherbak - 1995

SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA)

GENERAL CHARACTERISTICS

Multicellular animals are characterized by the fact that their body consists of many Cells and their derivatives. However, unlike protozoan colonies, which also comprise A large number of cells, the cells in the body of Metazoa are differentiated and perform various Functions. They lose their individuality, become parts of a complex Organism, and, by uniting to perform specific functions in most multicellular animals, they form Tissues: epithelial, connective, muscular, and nervous. Tissues, in turn, form complex Organs and Organ Systems that operate in a coordinated manner within the organism, allowing it to function as a single whole. Thus, a characteristic feature of Metazoa is The Emergence of specialized tissues, organs, and organ systems.

A distinctive feature of multicellular animals, unlike protozoan colonies, is the multilayered arrangement of their cells, in which the outer cells form a continuous layer that separates the animal's body from the external environment. In this way, multicellular animals develop an internal environment of the organism that houses all its body cells and maintains the constancy of Physical and Chemical parameters.

Multicellular animals are characterized by both Selection/8.html">Asexual and sexual reproduction, though sexual reproduction is the predominant form, and in some groups, the only one.

Asexual (vegetative) reproduction in multicellular animals is a form of reproduction without the participation of sex cells. It can occur in two ways: by division (transverse, longitudinal, or irregular) and by budding (internal or external).

All multicellular animals reproduce sexually (sometimes parthenogenetically). Sexual reproduction in multicellular animals is carried out with the help of special generative (sex) cells. The remaining cells are somatic. Their somatic cells are diploid, whereas the Gametes formed from generative cells are haploid—gametic reduction (see Fig. 22). A characteristic feature of the Sexual process in multicellular animals is oogamy: eggs (macrogametes) and spermatozoa (microgametes) differ not only in size but also in Structure. Typically, a spermatozoon has The structure of a somewhat modified flagellate. A non-motile spermatozoon is called a sperm (nematodes, decapod crustaceans, etc.). Eggs (ova) are non-motile, devoid of flagella, and predominantly spherical in shape. Some of them move in an amoeboid manner (for example, in Hydra). The Cytoplasm of most animal eggs contains yolk granules—nutrient reserves (Proteins, Polysaccharides, fats)—as well as Nucleic Acids and Enzymes. Only in lower groups do the eggs lack yolk. Two poles are distinguished in the egg: the animal pole, which contains The Nucleus and the zone of intensive METABOLISM, and the opposite pole, the vegetative pole.

The egg is surrounded by one or more membranes. The Composition and Structure of these formations vary among different animals.

A special form of reproduction that evolved on The basis of sexual reproduction is parthenogenesis, in which a new organism develops from an unfertilized egg.

The life cycle in all Metazoa is characterized by complex individual development—ontogenesis—during which an adult organism develops from a fertilized egg.

The ontogenesis of multicellular animals consists of a series of stages. First, gametes are formed (gametogenesis). During this process, undifferentiated diploid cells undergo meiotic division to produce haploid eggs and spermatozoa. Following Fertilization, egg Cleavage begins, which has the character of palindomic division. Cleavage culminates in The formation of a single-layered embryo, the blastula. Upon completion of cleavage, The Cell divides monotomically, and the processes of Cell Differentiation and migration (Gastrulation) begin, initially leading to the formation of two or three germ layers (ectoderm, endoderm, and mesoderm), and later to organ primordia. The ontogenesis of primitive multicellular animals (Prometazoa), which lack germ layers, differs from this scheme. A significant part of ontogenesis (embryonic development) takes place under the protection of egg membranes (or within the maternal organism). After hatching (or birth), postembryonic development begins, which can be direct (when the young organism resembles the adult) or accompanied by metamorphosis. Ontogenesis ends with the formation of an adult animal ready for sexual reproduction.

Multicellular animals exhibit several types of egg cleavage. The type of cleavage largely depends on the amount and localization of yolk in the egg. If there is little yolk and it is distributed evenly, cleavage goes to completion, resulting in identical blastomeres (total, equal cleavage). If the yolk is concentrated at the vegetative pole, cleavage may be unequal: large yolk-rich macromeres are formed at the vegetative pole, and small micromeres at the animal pole. With an increase in The amount of yolk in the egg, the central mass of cytoplasm with yolk does not divide, and superficial incomplete cleavage occurs. In this case, the nucleus divides repeatedly, daughter nuclei migrate to the peripheral part of the cytoplasm, which divides into cells, while the yolk remains in the center. In eggs with a large amount of yolk concentrated at the vegetative pole, blastomeres are formed only at the animal pole, while the vegetative part of the egg does not divide.

The types of cleavage are also determined by the relative arrangement of blastomeres (Fig. 63). Thus, in Coelenterates and some Flatworms, blastomeres are arranged irregularly (anarchic cleavage). Radial cleavage is known in Echinoderms: the first two cleavage furrows run from pole to pole meridionally, and the third runs along the equator; then meridional and equatorial furrows alternate, and blastomeres are arranged in regular horizontal and vertical rows. During spiral cleavage, mitotic spindles are positioned at a 45° angle to the longitudinal axis of the egg—either clockwise or counter-clockwise—with the directions of division alternating in each successive division, resulting in a spiral arrangement of blastomeres. As a rule, cleavage is unequal in this case—each blastomere divides into a macromere located closer to the vegetative pole and a micromere located near the animal pole. Such cleavage is known, for example, in Annelids.

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Fig. 63. Types of egg cleavage — radial (a), spiral (b), bilateral (c), superficial (d):

1 — cytoplasm; 2 — yolk granules; 3 — nucleus

Nematodes exhibit bilateral cleavage, in which blastomeres are distributed symmetrically on either side of the longitudinal plane of Symmetry, which is also retained in the adult animal.

Cleavage ends with the blastula stage, which can have various structures, but in most cases appears as a spherical or flattened formation consisting of a single layer of cells with a more or less developed fluid-filled cavity, the blastocoel, in the center.

Next, the germ layers are formed—ectoderm, endoderm, and mesoderm—from which specific Tissues of the adult animal develop. The Theory of germ layers, present in all multicellular animals (except Prometazoa), was developed in the 19th century by A number of scientists: H. Pander, K. Baer, A. O. Kovalevsky, I. I. Mechnikov, and E. Haeckel.

The germ layers are formed as follows. First, a two-layered embryo, the gastrula, is formed. It consists of the ectoderm and the endoderm, which lines the cavity of the blind-ending primary gut opening via the blastopore at the vegetative pole. The process of gastrulation—the Formation of the gastrula—occurs differently in various animals. Several types of gastrulation are distinguished.

Immigration is characterized by the migration of some blastoderm cells into the blastocoel, where they form the endoderm layer. Immigration can be multipolar, occurring across the entire blastula, or unipolar, when cells migrate only at the vegetative pole. In this case, the blastopore is often indistinct (Fig. 64).

Fig. 64. MAIN TYPES OF gastrulation — delamination (a), multipolar immigration (b), unipolar immigration (c), invagination (d):

1 — ectoderm; 2 — endoderm

Delamination involves the blastula cells dividing in half parallel to The surface of the blastula, simultaneously forming both the ectoderm and endoderm.

Invaginative gastrulation is characterized by the infolding of the blastoderm wall at the vegetal pole, transforming the gastrula into a two-layered sac featuring a blastopore and a primitive gut, the cavity of which is known as the archenteron or gastrocoel.

Epiboly involves the larger Cells of the vegetal pole being overgrown by the smaller cells of the animal pole; in this process, a gastrocoel does not form, and the blastopore appears as a depression at the vegetal pole not covered by the migrating micromeres of the ectoderm.

The modes of formation of the third germ layer—the mesoderm—are quite diverse and will be examined in the sections dedicated to the respective taxonomic types.

Postembryonic development occurs either directly or through metamorphosis. In direct development, the individual hatching from the egg resembles the adult except for its smaller size and underdeveloped reproductive organs (such as in oligochaetes, leeches, most turbellarians, etc.). Development with metamorphosis is characterized by various larval stages that differ structurally and ecologically from the adults. In many invertebrates possessing an exoskeleton or a thick cuticle (nematodes, Arthropods), larval growth is accompanied by periodic molting.

Multicellular animals vary in their level of Organization, the presence and number of germ layers, the degree of cellular and organ differentiation, and The Development of specific organ systems. We adopt the division of Metazoa into two major groups: Prometazoa and Eumetazoa.