INVERTEBRATE ZOOLOGY - H. Y. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE MULTICELLULAR ANIMALS (EUMETAZOA)

SECTION TRIPLOBLASTICA (TRIPLOBLASTIC) OR BILATERIA (BILATERALLY SYMMETRICAL ANIMALS)

SUBSECTION SPIRALIA

PHYLUM ORTHONECTIDA

Minute organisms (up to 1 mm in length) that parasitize the body cavity and Gonads of marine invertebrates, including turbellarians, nemerteans, polychaetes, Mollusks, and ophiuroids. They are characterized by an alternation of free-living and parasitic generations. The free-living generation consists of males and females, although hermaphroditic species are also known.

The phylum Orthonectida includes a single Class of the same name. A representative species is the ophiuroid parasite Rhopalura ophiocomae.

The body Structure of orthonectids is relatively simple. The body is covered by an epithelium, with some Cells bearing cilia. Beneath the body wall, females possess elongated contractile cells. In males, these cells are arranged around the Testis and contain microfilaments extending longitudinally along the body; additionally, supporting cells with thick inner fibers run along the central axis of the body. At the anterior end of the body beneath the epithelium, both sexes feature a group of undifferentiated, parenchyma-like cells. Muscles, digestive, nervous, and excretory systems are entirely absent.

The Reproductive System is also unformed. The interior of the female body contains numerous ova (up to 500–1000), whereas males possess a testis filled with spermatozoa. While there are no specialized efferent reproductive ducts, a genital pore is present within the male epithelium.

Reproduction and life cycle (Fig. 147). Sexually mature individuals living in seawater congregate during the reproductive period. Males release sperm, which penetrates the females and fertilizes their eggs. Ciliated larvae hatch from the eggs and leave the maternal Organism. The larva is covered by a layer of ciliated cells and contains numerous Germ Cells along with two light-refracting bodies inside. After swimming in the Water Column for some time, it penetrates a host. Within the host's body, the ciliated cells degenerate, while the germ cells fuse to form a plasmodium. The parasitic generation appears as a multinucleated plasmodium that feeds via pinocytosis and phagocytosis. The nuclei of the plasmodium divide mitotically as it grows, differentiating into vegetative and generative nuclei.

Cytoplasmic regions segregate around the generative nuclei, forming generative cells known as axoblasts. Within the plasmodium, these axoblasts develop into sexual individuals—some plasmodia producing females, others producing males. These sexual individuals then emerge into the surrounding water as the free-living generation.

Fig. 147. Life Cycle of orthonectids (after Malakhov): a - male; b - female; c - larva; d - plasmodium in the host organism;

e - Development of the ciliated larva of Rhopalura ophiocomae from a fertilized egg:

1 - unspecialized cells; 2 - spermatozoa; 3 - ova; 4 - vegetative nuclei; 5 - development of the sexual individual;

6 - generative cells (axoblasts); 7 - female; 8 — male

The phylogenetic position of orthonectids among Multicellular animals remains a subject of ongoing debate. For a long time, they were assigned significant phylogenetic importance as transitional forms between unicellular and multicellular animals. However, the prevailing view today is that they are descendants of more highly organized Multicellular Organisms—specifically Flatworms or closely related groups—that have undergone secondary simplification due to parasitism.

These are parasites inhabiting the Kidneys of benthic cephalopod mollusks, with body lengths reaching up to 1 cm.

The phylum Dicyemida includes a single class of the same name. A representative genus is Dicyema.

Structure and Life cycle. Several successive generations parasitize the kidneys of mollusks: the founder nematogen, several generations of nematogens, the rhombogen, and the infusorigen (Fig. 148). The nematogen has a worm-like shape, containing an elongated axial Cell surrounded by ciliated epithelium. In addition to its own Nucleus, the Cytoplasm of the axial cell contains several nuclei that divide mitotically to produce generative cells, or axoblasts. The axoblasts, in turn, divide mitotically within the axial cell of the maternal nematogen to develop into new nematogens. These leave the maternal individual (which subsequently dies) and lead an independent parasitic existence. This process is repeated numerous times. After several generations of nematogens, rhombogens appear through a similar pathway; they are distinguished by specialized wart-like cells in their epithelium containing dark granules of secretion. Within the rhombogens, most axoblasts degenerate, while the remaining ones develop into a new generation known as infusorigenes, which reside inside the rhombogens. An infusorigen consists of a somatic cell within which several non-flagellated spermatozoa (sperms) and surrounding ova are formed. Following the maturation of the Gametes, the infusorigenes disintegrate and degenerate, while larvae adapted for dispersal—known as infusoriforms—develop from the fertilized eggs within the axial cell of the rhombogen. Such larvae are covered with cilia and contain axoblasts internally. Excreted with the host's urine, the infusoriforms emerge into the external environment, swim near the bottom for a short period, and subsequently infect young cephalopod mollusks. Inside the mollusk's body, the axoblasts develop into vermiform larvae (lamerie larvae) that penetrate the kidneys and transform into founder nematogens. While some researchers consider infusoriforms, nematogens, and rhombogens to be asexual generations, others regard them as parthenogenetic. The only undisputed fact is that the infusorigen represents the sexual generation.

Fig. 148. Life cycle of dicyemids (after Malakhov): a - development in the host organism; b - free-living larva:

1 - two-cell embryo; 2 - founder nematogen; 3 - primary nematogen;

4 - rhombogen; 5 - infusorigen within the axial cell of the rhombogen; 6 — infusoriform

The systematic position of dicyemids has not yet been definitively established. Like ortonectids, they were once classified among the most primitive multicellular organisms, but the prevailing view in recent times is that they are secondarily simplified animals closely related to flatworms. This is also supported by molecular genetic studies.