INVERTEBRATE ZOOLOGY IN THREE VOLUMES - VOLUME 1 - H.Y. Shcherbak - 1995

SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA)

SECTION PRIMITIVE MULTICELLULAR ANIMALS (PROMETAZOA)

PHYLUM ORTHONECTIDA

Minute organisms (up to a millimeter in size) parasitizing the body cavity and Gonads of marine invertebrates, including turbellarians, nemerteans, polychaetes, Mollusks, and ophiuroids. Similar to Placozoa and Sponges, they lack true Tissues and Organs; nerve, Muscle, and digestive Cells are absent. They are characterized by a regular alternation of a free-living (males and females) and a parasitic generation (Fig. 86).

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Fig. 86. Life Cycle of orthonectids — male (a), female (b), larva (c), plasmodium within the host Organism (d), Development of the ciliated larva Rhopalura ophiocomae from a fertilized egg (e):

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

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

The free-living sexual generation usually consists of males and females, although hermaphroditic species are also known. The bodies of the sexual individuals are covered with ring-like arrangements of epithelial cells, some of which bear cilia. Beneath the epithelium, females contain numerous eggs (up to 500–1000), while males possess a Testis with spermatozoa. A specialized genital opening is present within the epithelium. Surrounding the testis in males are elongated contractile cells with microfilaments extending along the entire body, while supporting cells with internal thick fibers run along the central axis of the body. The elongated contractile cells of females are located beneath the integumentary cells.

At the anterior end of the body in both females and males, There is a group of undifferentiated cells resembling parenchyma.

The sexual individuals leave the host's body and congregate in seawater. The males release sperm, which penetrates the female through the genital opening and fertilizes her eggs. The eggs develop entirely within the female.

As a result of total and unequal Cleavage, a solid ball of cells (morula) is formed, consisting of a single large central Cell surrounded by numerous smaller peripheral cells. A ciliated larva develops from the morula and emerges from the maternal organism (see Fig. 86). The larva is covered with a layer of ciliated

cells and contains two light-refracting bodies inside, along with numerous Germ Cells. The larva swims for some time, finds a host, and penetrates it, likely with the aid of the light-refracting bodies. Inside the host, the ciliated cells perish, while the germ cells merge to form a plasmodium.

The parasitic asexual generation appears as a multinucleate plasmodium that feeds via pinocytosis and phagocytosis using numerous extensions that invade the host's tissues. The nuclei divide mitotically, and the plasmodium grows. The nuclei of the plasmodium differentiate into vegetative and generative types. Portions of Cytoplasm segregate around the generative cells, forming generative cells known as axoblasts. Some researchers consider the subsequent development of sexual individuals from them to be a form of asexual reproduction, while others view it as parthenogenetic (sexual, without Fertilization). In some plasmodia, the axoblasts give rise to males, and in others, to females. The sexual individuals then emerge into the Water (the non-parasitic generation).

A typical representative of the orthonectids is the ophiuroid parasite Rhopalura ophiocomae.

The phylogenetic position of orthonectids within the animal kingdom remains a subject of debate. Some scientists consider them to be simplified descendants of higher Multicellular animals—such as Flatworms, rotifers, dinophilids, or echiurans—owing to their parasitic lifestyle. Others view them as one of the groups of primitively simple organisms that have remained at the level of the phagocytella grade of Organization. The latter hypothesis is currently the most probable. Since orthonectids are diploblastic animals, the outer ciliated layer of their body can be homologized to the kinoblast, and the internal cells to the phagocytoblast. Their Transition to parasitism across various groups of marine invertebrates has led to an increasingly complex life cycle and the specialization of the parasitic generation.