INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM PRIMITIVE MULTICELLULAR ANIMALS (PROMETAZOA)

PHYLUM PLACOZOA

Placozoans inhabit the bottom of shallow marine areas and are also found in marine aquaria. Only two species of the genus Trichoplax are known: T. adhaerens and T. reptans, which are grouped into a Class bearing the same name as the phylum.

These are simply constructed Multicellular Organisms measuring a few millimeters. The body consists of an outer flagellated epithelium and an inner mesenchyme (mesohyl),

which contains several Cell types and their secretory products.

Structure. The body of Trichoplax appears as a thickened plate (hence the phylum name) capable of changing its shape (Fig. 69). It is translucent, whitish-gray in color, up to 4 mm in diameter, and consists of several thousand Cells that vary in Structure and function. The outer layer of flagellated cells surrounds the internal environment—the mesohyl, which is composed of loosely arranged fiber cells. The flagellated Cells of the upper body surface are flat, and their outer surface is covered with numerous microvilli. Among them are specialized cells whose Cytoplasm contains spherical inclusions—vacuoles filled with a fat-like substance. These are believed to play a protective role: in case of danger, they are expelled outward to deter predators. This is precisely why placozoans are not eaten by predators. The flagellated cells of the lower body surface, upon which the animal crawls, are elongated and cylindrical, facilitating the animal's slow gliding movement. Interspersed among them are glandular cells that secrete digestive Enzymes.

Fig. 69. Trichoplax:

a — change in body shape every 10 min; b — diagram of structure based on Electron Microscopy data (after Malakhov):

1 — dorsal epithelial cell; 2 — fiber cell; 3 — mitochondrial complex; 4 — brown body; 5 — ventral epithelial cell; 6 — digestive vacuole; 7 — mucous cell; 8 — glandular cell; 9 — cell with a shiny sphere

The mesohyl consists of irregularly shaped fiber cells with long processes that connect them to each other as well as to the flagellated cells of the body surface. The fiber cells feature a unique chain-like structure—a mitochondrial complex consisting of large Mitochondria alternating with vesicles. These fiber cells enable changes in body shape through the contraction of microtubules and microfilaments located in the cytoplasm. Digestive vacuoles are also present in the cytoplasm of mesohyl cells.

Nutrition. Trichoplax feeds on Bacteria, Algae, and Protozoa. It is characterized by two modes of feeding: a peculiar form of external Digestion and phagocytosis. Creeping over clusters of food particles, it tightly presses against them with its ventral body surface and secretes digestive enzymes from glandular cells. The food is digested, and the nutrients are absorbed by epithelial cells via pinocytosis. Another mode of feeding involves the capture of food particles by the flagellated cells of the dorsal and ventral surfaces, which can form pseudopodia, as well as by fiber cells capable of squeezing their pseudopodia between the flagellated cells. Digestion takes place within the digestive vacuoles of both fiber and flagellated cells. In the process, the fiber cells become amoeboid, returning to their original shape once digestion is complete.

Reproduction is typically asexual, occurring by fission or budding. During fission of the parent individual, the daughter organisms remain interconnected for a considerable time. In the case of budding, small flagellated "swarmers" (20–40 µm in diameter) bud off from the dorsal surface of the parent Organism to serve a dispersal function (Fig. 70). Sexual reproduction is also known, but it remains poorly studied. Large oocytes at various Cleavage stages have been found within the mesohyl, while spermatozoa have been observed in the Water. Larvae have not yet been discovered.

Fig. 70. Reproduction in Trichoplax (after Malakhov): a — swarmer; b — successive stages of fission

For a long time, Placozoa were considered planula-like larvae of hydrozoans, and it was only in the late 1960s that these animals were established to be independent organisms capable of reaching sexual maturity; in 1971, they were classified into a separate phylum.