ZOOLOGY LECTURE NOTES

Module 2. SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA). DIVISION PRIMITIVE MULTICELLULAR ANIMALS (PROMETAZOA). LOWER MULTICELLULAR ANIMALS (EUMETAZOA)

Content module 2.1. Subkingdom Multicellular Animals (Metazoa). General characteristics. Division Primitive Multicellular Animals (Prometazoa).

Topic 2.1.1. Phylum Placozoa. Phylum Sponges (Spongia)

General characteristics of the phyla. Origin, evolution, paleontological data, modern diversity, geographical distribution, and Significance in nature and human life. A list of key taxonomic groups important for human life and the functioning of natural ecosystems. Class Calcified Sponges (Calcispongia). CLASS Glass SPONGES (Hyalospongiae). Class Demosponges (Demospongiae). Class Archaeocyatha.

Phylum Placozoa. Placozoans inhabit the bottoms of coastal marine waters. These are simply structured Multicellular Organisms, with sizes measured in millimeters. They lack differentiated Tissues and Organs; the body consists of an external flagellated epithelium and an internal parenchyma containing several Cell types.

The phylum Placozoa includes Trichoplax adhaerens and Treptoplax reptans. Trichoplax adhaerens was first discovered in a marine aquarium at the University of Graz, Austria, as early as 1873 by the German zoologist F. Schulze. Ten years later, the Italian researcher Z. Monticelli described a closely related form, T. reptans, from a marine aquarium at the Naples Zoological Station. These findings aroused great interest among zoologists; however, some time later, the prominent German zoologist T. Krumbach (1907) suggested that these forms were degraded larvae of hydroid jellyfish. Consequently, researchers lost interest in these animals for the next 70 years. It was not until 1971 that the German scientist K. Grell rediscovered these animals and described their Structure using ultrathin sections and Electron Microscopy. He found egg Cells in various phases of Cleavage within the animals' bodies, which irrefutably proved Trichoplax to be an independent species and capable of reproduction.

Phylum Sponges, or Porifera (Spongia or Porifera). Sponges are lower Multicellular animals that are sessile, attached to a substrate, and feed by filtration. They inhabit aquatic environments, predominantly marine waters, and form colonies. Over 5,000 extant and more than 2,000 extinct species are known. In the freshwater bodies of Ukraine, no more than ten species have been found; in the Black Sea, due to low salinity, there are 29 species, and even fewer in the Sea of Azov. They lack true tissues and organs; the body consists of cells performing various Functions and having different structures, along with an intercellular substance—a metabolic product. Sponges are organisms with cellular differentiation, or pre-tissue organisms. In 1825, they were officially classified as animals. Their body shapes are diverse. Solitary forms are mostly cup-shaped, while colonial forms create encrustations on substrates in the form of bushes, crusts, or lumps. Sponge sizes range from 1–2 mm to 2 meters, and many

species are colored by special pigments in yellow, brown, orange, red, green, or violet. The body consists of two cell layers: the pinacoderm (covering the body externally) and the choanoderm (lining the paragastric cavity or flagellated chambers). Between these two layers lies the mesohyl (parenchyma, mesenchyme), composed of various cells and their secretion products, which contains skeletal elements. The bulk of the mesohyl is penetrated by canals that open onto the surface via pores (ostia). The structural types of sponges are ascon, sycon, and leucon, which derive their names from genera that exhibit these respective body plans.

The ascon-type sponge, such as Ascetta, resembles a goblet or small sac attached by its base (pedicel) to the substrate, with an opening (osculum) at the upper pole. This opening leads into the internal (paragastric or atrial) cavity. Further structural complexities are associated with the proliferation of the mesohyl and the outpocketing of the atrial cavity regions into it, forming radial canals whose walls are lined by the choanoderm. This is the syconoid type, characteristic of many solitary sponges (e.g., Sycon raphanus). The leuconoid structural type is the most common: the body wall thickens, and choanocytes concentrate in flagellated chambers within the parenchyma (mesohyl) connected to the external environment and the atrial cavity by a system of canaliculi. The canaliculi connecting the chamber to the external environment are called incurrent canals, while those connecting to the paragastric cavity are excurrent canals. The chambers can form multiple layers, representing a complex irrigation system that regulates Water flow. In sycon and leucon sponges, the paragastric cavity and canals are lined by the pinacoderm. The pinacoderm comprises two cell types.

Class Calcified Sponges (Calcispongia or Calcarea). These are exclusively marine (up to 7 cm) solitary or colonial sponges with a calcareous Skeleton, inhabiting shallow depths. The body of solitary sponges is barrel-shaped, tubular, or sac-like, with a spiky surface and a fringe of long spicules surrounding the osculum. Adults exhibit all structural types: ascon, sycon, and leucon.

Class Glass Sponges, or Hexactinellids (Hyalospongiae or Hexactinellida). These are exclusively marine, predominantly deep-water, large (up to 1 m) sponges with a siliceous skeleton, solitary or less commonly colonial. The body is goblet-shaped, tubular, or sac-like. It is soft and easily torn, or hard and brittle if the skeleton is heavily developed. Sponges can be gray, white, yellowish, or brown. The mesohyl is nearly reduced; living cells are represented by a syncytium that forms the dermal membrane, the walls of the flagellated chambers, and a mobile network of strands and partitions interconnecting all PARTS OF THE body. Choanocytes are absent. Instead, There is a continuous choanosyncytium from which individual collar complexes extend (a flagellum surrounded by a basket of microvilli). Numerous lacunae (cavities) circulate water between the strands and partitions. The irrigation system is of the syconoid type. Archaeocytes lack amoeboid activity and contain no digestive vacuoles (phagosomes). Myocytes or any contractile elements are absent.

Class Demosponges (Demospongiae). This is the most numerous class of sponges, known since Precambrian times (the end of the Proterozoic era). They are marine and freshwater, predominantly colonial (rarely solitary) forms of medium to large sizes. The irrigation system is of the leuconoid type, and the skeleton is siliceous, often featuring spongin fibers. Colonies vary in size, color, and shape—encrusting, spherical, branched, or goblet-shaped. The skeleton consists of monaxonal or tetraxonal spicules; spongin fibers form the organic component. Sometimes the skeleton is composed entirely of spongin (such as the bath sponge). The class includes one extinct and two extant orders of demosponges.