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

SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA)

SECTION PRIMITIVE MULTICELLULAR ANIMALS (PROMETAZOA)

PHYLA PLACOZOA

Placozoans inhabit the bottom sediments of coastal marine environments. These are extremely simply organized Multicellular Organisms measuring only a few millimeters across. Lacking differentiated Tissues and Organs, their body consists of an outer flagellated epithelium and an inner parenchyma containing several distinct Cell types.

The phylum Placozoa includes two known species: Trichoplax adhaerens and Treptoplax reptans.

T. adhaerens was first discovered in 1873 by German zoologist F. Schulze in a marine aquarium at the University of Graz, Austria. A decade later, Italian researcher Monticelli described a closely related form, T. reptans, from a marine aquarium at the Naples Zoological Station. These findings sparked great interest among zoologists; however, some time later, the prominent German zoologist T. Krumbach (1907) suggested that these forms were merely degraded larvae of hydrozoan medusae. Consequently, researchers largely ignored these animals for the next 70 years. It was not until 1971 that German scientist K. Grell rediscovered these creatures and detailed their Morphology using ultrathin sections and Electron Microscopy. He identified oocytes in various Cleavage stages within the animals' bodies, which irrefutably demonstrated that Trichoplax is a distinct, independent species capable of reproduction.

Trichoplax is a whitish-gray, translucent creature resembling a delicate plate up to 4 mm in diameter with an irregular and constantly changing shape (Fig. 67, a). The animal glides slowly across the substrate surface. Lacking distinct anterior and posterior ends, it constantly changes its direction of movement. Externally, the body is covered by a layer of flagellated Cells of varying Structure. The "dorsal" body surface is covered by a flat epithelium, whereas the "ventral" surface—upon which the animal crawls—is covered by a tall cylindrical epithelium (since these animals lack bilateral Symmetry, the terms "ventral" and "dorsal" are used conditionally). All these cells share an exceptionally primitive feature: they lack a basal epithelial membrane, a condition among Multicellular animals found only in the most primitive forms, such as Sponges or acoel turbellarians. Transmission electron microscopy revealed numerous microvilli On the surface of the dorsal epithelium (Fig. 67, b). The flagella of the epithelial cells of Trichoplax are embedded in deep pits, the walls of which are reinforced with specialized supportive rods.

Glandular cells containing vacuoles are scattered throughout the ventral epithelium. The dorsal epithelium contains specialized cells with spherical inclusions known as "shiny balls." These are large vacuoles filled with a lipid-like substance that presumably serve a defensive function, as they can be expelled outward from The Cell to deter predators with their contents. Interestingly, Trichoplax is rejected as food by both crustaceans and gastropods.

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Fig. 67. Trichoplax—changes in body shape every ten minutes (a); structural diagram based on electron microscopy data (b): 1 — dorsal epithelial cells; 2 — fiber cells; 3 — mitochondrial complex; 4 — brown body; 5 — ventral epithelial cells; 6 — digestive vacuoles; 7 — mucous cell; 8 — glandular cell; 9 — cell with a shiny ball

The parenchyma of Trichoplax contains fiber cells. These are irregularly shaped cells with long extensions that connect them to one another as well as to the ventral and dorsal epithelial cells. Parenchymal cells are tetraploid, whereas epithelial cells are diploid. The fiber cells contain a unique organelle not found in any other Organism: a mitochondrial complex consisting of large Mitochondria alternating with vesicles, forming a distinctive chain-like structure. Very small digestive vacuoles are localized in the Cytoplasm of both parenchymal cell types.

Through cytoplasmic contractions, the fiber cells drive Changes in the animal's body outline. Their cytoplasm contains a sophisticated system of microtubules and microfilaments responsible for non-muscular contraction. Trichoplax feeds on Bacteria, Algae, and flagellates. Crawling over clusters of unicellular algae, it applies its "ventral" surface tightly to food particles (aggregations of unicellular or filamentous algae), pours digestive secretions from the glandular epithelial cells onto them, and subsequently absorbs the liquid products of external Digestion via pinocytosis through the epithelial cells. Extracorporeal digestion of flagellates of the genus Cryptomonas by Trichoplax has been confirmed by live observations and electron microscopy.

A second feeding mechanism is phagocytosis, carried out by the Cells of the dorsal and ventral epithelia. Food particles are driven toward the cells by flagella and captured by pseudopodia. The parenchymal fiber cells also perform a phagocytic function. Some of these contain "brown bodies"—large inclusions (12–15 µm in diameter) that appear greenish-brown or pinkish-brown under a Light Microscope, depending on whether the animal has fed on green or red algae. These brown bodies are too large to be standard digestive vacuoles. They are believed to serve as sites for the accumulation of metabolic wastes and the breakdown products of algal pigments digested by Trichoplax.

Trichoplax reproduces both asexually and sexually. In asexual reproduction, the animal divides in half by constriction, with the daughter individuals remaining connected for a prolonged period by a narrow bridge that eventually ruptures (Fig. 68). Another mode of asexual reproduction is budding. Small spheres form on the dorsal surface of the animal, pinch off, and transform into free-swimming swarmers (a dispersal stage).

The swamer has a diameter of about 20–40 µm. It features an internal cavity whose walls are formed by several cell layers: an outer layer derived from the dorsal side, an inner layer from the ventral side, and parenchymal cells sandwiched between them. The swarmers swim freely in the Water for some time; lacking distinct anterior and posterior poles, their movement is random. Eventually, the internal cavity opens to the exterior via a pore through which the swarmer attaches to the substrate. Through this opening, the inner cell layer turns inside out, forming a flat plate with the typical structure of an adult Trichoplax (see Fig. 68).

Fig. 68. Reproduction of Trichoplax:

a — stage of swarmer development with an internal cavity; b — swarmer with a pseudopodium; c — young Trichoplax; d — division

Sexual reproduction in Trichoplax is not yet fully understood. Large (90–120 µm) oocytes undergoing total and equal cleavage are found within the parenchyma. Larvae have not been discovered.

Analyzing the data gathered on this organism, one may hypothesize that Trichoplax is a descendant of a hypothetical phagocytella that transitioned to a crawling benthic lifestyle.