Lecture Notes on Invertebrate Zoology

Content Module 2. Origin of Metazoa and Their Phyla: Flatworms, Roundworms, Annelids

Lecture Topic 4. Multicellular Invertebrates (Metazoa). Characteristics of the Phylum Porifera (Sponges) as Primitive Multicellular Animals

The main theories on THE ORIGIN OF Multicellular animals and their comparison: E. Haeckel's gastraea theory, I. Mechnikov's phagocytella theory, O. Zakhvatkin's and I. Hadži's theories. Sponges are the most primitive multicellular animals. Absence of cellular differentiation between the outer and inner layers, and the mesoglea. Main cellular elements. Body forms: ascon, sycon, and leucon. Reproduction and development of sponges, inversion of germ layers. Systematics of the phylum. Classes of calcareous, hexactinellid (Glass), and demosponges. Ecological significance and Structure/179.html">Practical Applications. Sponges are lower multicellular animals, sessile and attached to a substrate, feeding by filter feeding. Sponges lack well-defined Tissues and Organs; their body consists of numerous Cells with various Functions and structures, along with an intercellular substance—the secretory product of these cells. The sponge body is constructed of two Cell layers: the pinacoderm, which covers the exterior, and the choanoderm, which lines the paragastric cavity or flagellated chambers. Between these two layers lies the mesohyl (parenchyma, mesenchyme), which consists of various cells and their secretory products, and also contains skeletal elements. The thickness of the mesohyl is permeated by canals opening onto the surface through small pores. The entire diversity of sponge architecture can be divided into three types: ascon, sycon, and leucon. An asconoid sponge, such as Ascetta, has the shape of a goblet or small sac attached by its base (sole) to the substrate, with an opening (osculum) at the opposite upper pole. This opening leads into the internal (paragastric or atrial) cavity. Further structural complexities in sponges are associated with the proliferation of the mesohyl and the outpouching of Regions of the atrial cavity into it, forming radial canals whose walls are lined by the choanoderm. This is the syconoid type, found in many solitary sponges. The leuconoid structural type is characteristic of most sponges: the body wall thickens, and choanocytes concentrate in flagellated chambers located within the parenchyma (mesohyl) and connected to the external environment and the atrial cavity by a system of branching canaliculi. The canaliculi connecting the chambers to the external environment are called incurrent canals, and those connecting to the paragastric cavity are called excurrent canals. The chambers can form multiple layers, significantly increasing the volume of the sponge body. Thus, a complex irrigation system is formed that directs and regulates the Water flow. In syconoid and leuconoid sponges, the paragastric cavity and canals are lined with pinacoderm, just like the outer surface. The pinacoderm comprises two cell types: pinacocytes and porocytes. Pinacocytes, tightly adjoining one another, form the outer cover of sponges and line their internal cavities (except for the flagellated chambers). Porocytes are a variety of pinacocytes—cells pierced by a pore canal. A porocyte is capable of contraction, allowing it to open or close the pores and regulate the canal diameter. The choanoderm consists of collar cells, or choanocytes. These are cylindrical cells with a single flagellum. The mesohyl, or parenchyma, consists of intercellular substance densely populated by various cell types and skeletal elements. Among the mesohyl cells are: archaeocytes, myocytes, collencytes, lophocytes, spongocytes, and sclerocytes. The vast majority of sponge species possess a Skeleton that provides structural support for the body and canal walls, though it does not perform a locomotor function. It may be calcareous, siliceous (glassy), or horny (made of spongin). The composition and STRUCTURE OF THE Skeleton form The basis of sponge Classification. The mineral skeleton (calcareous or siliceous) consists of A large number of needles, or spicules, which vary in shape and arrangement within the sponge body. Spicules are formed within specialized cells called sclerocytes. Spicules may be mono-, tri-, or polyaxonic, or possess more complex structures such as stars, pins, loops, anchors, etc. Occasionally, spicules fuse into a continuous skeletal framework, which is most characteristic of glass sponges. The organic (horny) skeleton consists of Collagen fibers and spongin—a substance chemically similar to silk. Spongin fibers bind individual spicules into a unified skeleton. Sponges reproduce both sexually and asexually. Asexual (vegetative) reproduction occurs through external or internal budding, longitudinal fission, and fragmentation. During external budding, a bud forms on the parent Organism, into which all layers of the parent organism grow and the paragastric cavity extends. Internal budding is characteristic of the freshwater sponge Spongilla (badyaga). It takes place within the parenchyma. A group of amebocytes rich in nutrients is separated from other cells by a dense, complex outer envelope produced by collencytes. It consists of two layers of organic substance (spongin) with a foamy layer containing air situated between them. In some species of Spongilla, specialized skeletal elements—amphidiscs, which look like short rods with star-shaped discs at the ends—are localized in the foamy layer. They prevent the spongin layers from sticking together, while the foamy layer provides reliable thermal insulation. Such a wintering bud is called a gemmule.