INVERTEBRATE ZOOLOGY IN THREE VOLUMES - VOLUME 1 - G.Y. Shcherbak - 1995

SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA)

SECTION TRUE MULTICELLULAR ANIMALS (EUMETAZOA)

True Multicellular animals are characterized by the presence of differentiated Tissues and, in most of them, The Emergence of Organs and Organ Systems. Eumetazoans possess an intestine (which may be reduced in some parasitic forms), a Nervous System, a Reproductive System, and Sensory Organs; most groups also develop an excretory system, while higher animals feature circulatory and respiratory systems. During embryonic development, Eumetazoa form two or three germ layers, which give rise to the tissues and Organs of the adult animal.

True multicellular animals exhibit diverse body plans and varying Levels of Organization. Among them, Cnidarians (phylum Cnidaria) and Comb Jellies (phylum Ctenophora) occupy the lowest evolutionary rung. They lack true organs and organ systems, possessing instead polyfunctional layers—the epidermis and gastrodermis—which develop from two embryonic germ layers (the ectoderm and endoderm, respectively). In all other Eumetazoa, a third germ layer—the mesoderm—is laid down during embryonic development, giving rise to a significant portion of the Internal Organs.

The level of animal organization is determined by the presence and Nature of the body cavity. The body cavity is defined as the space between the body wall (consisting of the integument and adjacent musculature) and the intestine. In Flatworms (phylum Plathel — minthes), Ribbon Worms (phylum Nemertini), and Kamptozoans (phylum Kamptozoa), there is no body cavity; the space is filled with mesodermally derived Connective Tissue known as parenchyma. In Pseudocoelomates (phylum Nemethelminthes), Rotifers (phylum Rotifera), Spiny-headed Worms (phylum Acanthocephala), and Cephalorhynchs (phylum Cephalorhyncha), the body cavity (referred to as the primary body cavity, or schizocoel) is filled with fluid that bathes internal organs and Functions as a hydroskeleton against which the Muscles act. The body cavity also plays a vital physiological role in the Organism: the coelomic/body cavity fluid transports oxygen, nutrients, and Metabolic waste products.

All other animals, starting from Annelids (phylum Annelida), possess a secondary body cavity, or coelom. Morphologically, the coelom differs from the schizocoel by having its own distinct walls, which during ontogeny invariably form from the mesoderm. These walls consist of a single layer of epithelium known as the coelomic or peritoneal epithelium. The coelomic epithelium lines the inner surface of the body wall and envelops the intestine. Through specialized channels called coelomoducts, the coelom communicates with the external environment. Thus, the coelom is not merely a space between internal organs, but a fully formed organ whose Structure varies across different animal groups. It performs A number of crucial functions within the organism. First and foremost, the coelom plays an essential supportive role. When the body wall muscles contract, pressure is transmitted to the coelomic fluid, which functions as a hydroskeleton. The coelom is also the site where Germ Cells mature. Like the schizocoel, it performs a transport function. In some animals, various cavities are combined. For instance, Mollusks (phylum Mollusca) possess a coelom, a schizocoel, and parenchyma, though these formations are developed to varying degrees across different classes. In Arthropods (phylum Arthropoda), coelomic sacs form during embryonic development but subsequently break down, and their cavity merges with the primary body cavity to form a mixed cavity, or mixocoel.

The section Eumetazoa unites multicellular animals that comprise the majority of modern animal phyla—ranging from Cnidaria to Chordata.