INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

Multicellular Organisms have colonized all major habitats: aquatic, terrestrial-aerial, subterranean, and within the bodies of other living creatures. They comprise over 1.5 million species.

The body of a multicellular Organism consists of numerous Cells which, unlike the cells of protozoan colonies, differ in both Structure and function. In the vast majority of multicellular organisms, similar cells are aggregated into Tissues that form Organs and Organ Systems.

Multicellular animals are capable of both Selection/8.html">Asexual and sexual reproduction; however, sexual reproduction is the predominant form, and in some groups, the only one.

Asexual (vegetative) reproduction in multicellular organisms involves The formation of new individuals without the participation of sex cells. It can occur in various ways: by division (transverse, longitudinal, or irregular), budding (external or internal), laceration (fragmentation of the parent organism into several parts, with each part regenerating a whole organism), and so on.

Sexual reproduction takes place with the participation of specialized generative (sex) cells. Unlike somatic cells—which are diploid in multicellular organisms—Gametes, derived from generative cells, are haploid, formed through a reductional division (Meiosis). That is, their nuclear cycle is accompanied by gametic chromosome reduction (Fig. 1). The only exceptions are rotifers, which exhibit a life cycle featuring intermediate reduction. The Sexual process in multicellular organisms is oogamous: egg cells (macrogametes) and spermatozoa (microgametes) differ not only in size but also in structure. Egg cells are immotile, lack flagella, and are typically spherical in shape; they often contain yolk, which consists of stored nutrient granules necessary for embryonic development. Two poles are distinguished in an ovum: the animal pole, which contains The Nucleus and the zone of intense metabolic activity, and the opposite, vegetative pole. A typical spermatozoon has The structure of a modified flagellate. An immotile spermatozoon is termed a sperm. Following Fertilization, the egg becomes enclosed in one or more membranes and is referred to as an ovum (or egg). One form of sexual reproduction is parthenogenesis, in which a new organism develops from an unfertilized egg that does not undergo reductional division.

The life cycle of multicellular organisms is characterized by complex individual development, or ontogeny, during which a sexually mature organism arises from a fertilized (or, in the case of parthenogenesis, unfertilized) egg. Ontogeny consists of several stages.

1. Initially, gametogenesis takes place, resulting in the formation of haploid egg cells and spermatozoa from undifferentiated diploid cells via meiosis. Following egg fertilization, Cleavage begins. The types of cleavage vary and depend on the amount and distribution of yolk in the egg: holoblastic and equal, resulting in identical cells (blastomeres), if the yolk is scant and evenly distributed; unequal, if the yolk is concentrated at the vegetative pole, in which case large, yolk-rich macromeres form at this pole, while small micromeres form at the animal pole. If yolk is abundant, superficial cleavage occurs, where blastomeres are distributed superficially while the central mass of yolk remains undivided, or blastomeres form exclusively at the animal pole while the vegetative portion containing the yolk does not divide.

Cleavage types are also defined by the spatial arrangement of the resulting blastomeres (Fig. 54). Archaic cleavage occurs when blastomeres are arranged irregularly. During spiral cleavage, mitotic spindles are oriented at a 45° angle to the longitudinal axis of the egg, either clockwise or counter-clockwise, alternating direction with each subsequent division, which results in a spiral arrangement of blastomeres. In radial cleavage, the first two cleavage furrows run meridionally from pole to pole, and the third runs equatorially. Subsequently, meridional and equatorial furrows alternate, and blastomeres are arranged in orderly horizontal and vertical rows. Bilateral cleavage occurs rarely, with blastomeres distributed symmetrically relative to the longitudinal plane of Symmetry.

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Fig. 54. Types of egg cleavage

(from Ivanova-Kazas, modified): a - radial; b - spiral; c - bilateral; d - superficial:

1 - Cytoplasm; 2 - yolk granules; 3 — nucleus

Cleavage culminates in the Formation of the blastula stage. Most commonly, it appears as a sphere formed by a Superficial layer of cells enclosing a more or less developed fluid-filled cavity, the blastocoel. Occasionally, the entire embryo is filled with cells, in which case it is called a morula.

2. Next, Cell movement processes—Gastrulation—begin, leading to the formation of two or three germ layers: ectoderm, endoderm, and mesoderm (Fig. 55). Initially, a two-layered embryo, the gastrula, is formed. It consists of a superficial cell layer, the ectoderm, and an inner layer, the endoderm, which lines the cavity of the primitive gut, or gastrocoel. The latter possesses an opening at the vegetative pole known as the primary Mouth, or blastopore.

Fig. 55. MAIN TYPES OF gastrulation (from Ivanova-Kazas, modified): a - delamination; b - multipolar immigration; c - unipolar immigration; d - invagination: 1 - ectoderm; 2 - endoderm

Several types of gastrulation are distinguished:

immigration - a portion of the blastula cells migrates into the blastocoel, where they form the endoderm layer. Immigration can be multipolar, occurring across the entire surface of the blastula, or unipolar, if cells migrate exclusively from the vegetative pole. In this process, the blastopore is often indistinct;

delamination - blastula cells divide in half parallel to the surface, thereby forming the ectoderm and endoderm simultaneously;

invagination - the blastula wall bends inward into the blastocoel at the vegetative pole, forming a gastrula in the shape of a two-layered sac with a blastopore and a primitive gut; the cavity between the ectoderm and endoderm is termed the blastocoel;

epiboly - large Cells of the vegetative pole are overgrown by smaller cells of the animal pole; in this case, a gastrocoel is not formed, and the blastopore appears as a depression at the vegetative pole.

The modes of formation of the third germ layer, the mesoderm, are quite diverse and will be discussed in the respective sections.

3. The cells of the germ layers differentiate to form tissues and organ primordia. The ectoderm gives rise to the superficial layer of The Cytology/cytology/66.html">Skin and its Derivatives (Skin glands, cuticle, shell, etc.), The Nervous system and Sense Organs, the epithelium of the foregut and hindgut, excretory organs (protonephridia, metanephridia, Malpighian tubules of insects and myriapods), and invertebrate respiratory organs; the endoderm gives rise to the midgut, digestive glands, and Malpighian tubules in arachnids; the mesoderm gives rise to the parenchyma, Gonads, coelomic epithelium, and Muscles.

Embryonic development takes place beneath the protective membranes of the egg or within the body of the maternal organism.

4. After hatching (or birth), postembryonic development begins, which can be either direct or involve transformation, known as metamorphosis. In direct development, a young organism emerges from the egg resembling the adult, differing only in size and undeveloped reproductive organs. Development with metamorphosis is characterized by various larval stages that differ from adult individuals in structure and lifestyle. In many invertebrates with an external Skeleton or thick cuticle (such as nematodes and Arthropods), larval growth is accompanied by molting.

Ontogeny concludes with the formation of an adult animal ready for sexual reproduction. The ontogeny of early multicellular organisms (Prometazoa), which lack germ layers, deviates from this pattern.

Depending on the level of Organization, the kingdom Multicellular Animals (Metazoa) is divided into two subkingdoms: Early Multicellular Animals (Prometazoa) and True Multicellular Animals (Eumetazoa).