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

SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA)

SECTION PRIMITIVE MULTICELLULAR ANIMALS (PROMETAZOA)

PHYLUM DICYEMIDA

This phylum comprises parasites inhabiting the renal appendages of benthic cephalopod Mollusks, reaching up to 1 cm in length. Similarly to the preceding phyla, dicyemids are characterized by the absence of true Tissues and Organs, Mouth, gut, nerves, and Muscles; their life cycle is exceptionally complex and not yet fully understood. Unlike the previous phylum, both generations of dicyemids (sexual and asexual) are parasitic (Fig. 87).

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Fig. 87. Life Cycle of dicyemids — development within the host Organism (a); free-living larva (b):

1 — two-Cell embryo; 2 — founder nematogen; 3 — primary nematogen;

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

The body of the mollusk hosts a succession of generations: the founder nematogen, several generations of nematogens, and the rhombogen. The nematogen is elongated and vermiform in shape. Internally, it contains an elongated axial cell surrounded by ciliated epithelium. Eight to nine anterior epithelial Cells form a slight expansion known as the propore (or calotte), while 14–22 ciliated cells cover the trunk region. In addition to its own Nucleus, the Cytoplasm of the axial cell contains several smaller nuclei that divide mitotically to form Germ Cells, or axoblasts. Through these, new nematogens develop within the axial cell of the maternal nematogen, a process that repeats multiple times.

Using Electron Microscopy, specific comb-like outgrowths have been discovered On the surface of the ciliated cells, along with cytoplasmic bridges (desmosomes) between the outer and axial cells. The Mitochondria in dicyemid cells are tubular rather than lamellar, as is typical for most Multicellular animals.

These animals feed on dissolved nutrients through pinocytosis performed by the tegumentary cells. The pinocytotic vesicles are then transferred to the axial cell.

During The formation of a nematogen, the axoblast first increases in volume and then undergoes unequal mitotic

division, resulting in one large central cell (macromere) surrounded by several small cells (micromeres) that develop into the ciliated epithelium. Subsequently, the macromere again divides unequally into a large axial cell and a small cell that penetrates the interior of the large cell, divides, and gives rise to the axoblasts. Following these processes, the embryo grows via cellular enlargement, leaves the maternal individual, and begins an independent parasitic life. After several generations of nematogens, rhombogens appear in a similar manner. Their epithelium features specialized wart-like cells containing dark excretory granules. The majority of axoblasts within the rhombogens degenerate, while the remaining ones develop into a new generation: the infusorigenes.

The infusorigen consists of an axial somatic cell, inside which several aflagellate spermatozoa (sperms) arise, surrounded by oocytes. Meiosis is observed during Spermatogenesis and oocyte maturation. Following the maturation of the Gametes, the infusorigen disintegrates and degenerates, with this stage occurring entirely within the rhombogen. After Fertilization within the axial cell of the rhombogen, the zygote develops into a dispersal larva known as the infusoriform (Fig. 88). It is covered with cilia and internally possesses two light-refractive bodies containing guanine, as well as axoblasts. Released with the mollusk's urine, the infusoriform swims near the bottom for some time before infecting a young cephalopod mollusk. Inside the mollusk, the axoblasts develop into lamarckian larvae that penetrate the Kidneys, where they transform into founder nematogens.

Fig. 88. Diagram of The Structure of the infusoriform:

1 — light-refractive bodies; 2 — ciliated cell; 3 — axoblast

Some researchers consider axoblasts to be agametes, in which case the infusoriform, nematogen, and rhombogen represent asexual generations; others suggest that axoblasts are parthenogenetic eggs, implying that all the aforementioned generations are parthenogenetic. This issue remains a subject of debate. The only undisputed fact is that the infusorigen represents the sexual generation.

A representative of this phylum is *Dicyema*. The systematic position and origin of dicyemids, much like those of ortonectids, are subjects of scientific debate; they are sometimes considered simplified trematodes or even relatives of Ciliates. The more widely accepted view is that dicyemids, akin to ortonectids, are descendants of Mechnikov's phagocytella.