Kostenko & Mamkaev 1990 (citation) Description. External morphology. The body is elongated, worm-like; lateral edges are never bent under the ventral side; anterior end is narrowed, the posterior one is bluntly rounded (the body shape is reminiscent of small monocelids). The length of mature specimens is ca. 1.5 mm. The mouth opening is located in the end of the first quarter of the body length. The statocyst, two pigmented eyespots placed by the sides of the statocyst at the level of its front margin, a flexible elastic bursal nozzle, as well as female and male gonopores positioned one after another in the last quarter of the body are easily discernable on live squeezed specimens. Four longitudinal nerve trunks approximately equidistant from each other and from the body edges are readily visible on unsqueezed juvenile specimens. The offshoots of irregular shapes are sometimes discernable. The body coloration is bright green, typical of “green convolutas.” In addition to that, orange-red rhabdite glands could also be observed. Sagittocysts are absent (Fig. 1). Epidermis. The whole body surface of P. shikoki is covered with cilia. The length of the cilia in cross-sections is ca. 5 µm. The basal apparatus of the cilia is clearly evident, forming a layer of 2.5-3 µm in width under the surface of the body. The staining with Heidenhain’s azan imparts more intense color to the epidermis than to the adjacent peripheral parenchyma. Thanks to that, it has become possible to distinguish insunk epithelial cells and cytoplasmic bridges connecting them to the epidermal epymium. The nuclei of the epithelium are oval, with sizes of 4-5 x 2-2.5 µm. Karyoplasm is filled with numerous chromatin lumps. The nucleolus was never observed. In single instances, the nuclei, as well as symbiotic algae can be found outside muscular layers. The body-wall musculature is represented by the outer circular and inner longitudinal layers. The circular layer is composed of loosely arranged fine myofibrils running in 2-3 rows. The longitudinal layer is represented by isolated, strong muscle fibers 1.25-1.5 µm in width, having larger sizes toward the dorsal aspect of the animal. In front of the statocyst and posterior to the copulatory organ, the longitudinal fibers become considerably thinner, many of them pass down in the parenchyma; owing to that, the posterior end is provided merely with extremely thin longitudinal fibers, while the anterior end is nearly devoid of longitudinal musculature. Epidermal glands are relatively small and not very numerous. They are more densely distributed at the anterior and posterior ends, as well as along the dorsal side, though relatively scarce on the ventral surface. Epidermal glands commonly occur within the boundaries of the peripheral parenchyma, but sometimes could also be found in the central region of the body. Upon the staining with Heidenhain’s azan their secretion becomes light-blue, which indicates its mucous character. The frontal organ is located at a considerable distance in front of the statocyst; it has the length of 40-45 µm, and opens with a terminal pore. The secretion of the frontal glands stains much more intense, than the secretion of the epidermal glands. Nervous apparatus and sense organs. The brain is orthogonal. The main tangle of nerve fibers is located in front of the statocyst. The dorsal and ventral transverse fibers are clearly visible; their dorsoventral branches are connected with two stems that extend backwards and form a pair of ganglionic swellings behind the statocyst. Above the statocyst or behind it they are linked together by a highly developed transverse fiber. In lateral direction on both sides, this fiber splits into two branches that pass in lateral and ventral trunks. In some sections, three pairs of nerve trunks could be traced behind the statocyst. Dorsal trunks extend backwards from two ganglionic formations situated by the sides of the statocyst; lateral from the dorsal ones at nearly the same depth run dorsolateral trunks; close to the ventral wall and slightly laterad from these pass ventral trunks. The statocyst has a typical structure. Its capsule (as seen in sections) has an oval shape, with the size of 16 X 21 Ìm. Two parietal cells border the capsule; they have elongated flat nuclei 6.5-7 X 2 Ìm in size. The lithocyte 7.5-8 Ìm in diameter is situated in the statocyst lumen. The bean-shaped nucleus occupies its upper part (7.5 X 2.5 Ìm), while the cup-shaped statolith is located in its lower region (5 X 3 Ìm). The eyes have an appearance of oval spots with granular pigment. In live specimens inside the eyespots, we observed an area devoid of pigment with neighboring clump of refractive granules having a greenish hue that corresponds to the reflective vacuole studied under the electron microscope (Popova & Mamkaev 1985). The shape of the eyespots is varied but, on average, their length never exceeds 40 Ìm. Elongated sensillae, resembling those of S. psammophila, were not found. Parenchyma could be divided into peripheral and central. In the peripheral parenchyma, numerous nuclei are visible, belonging both to the parenchymal and epithelial as well as glandular and reproductive cells. The peripheral parenchyma is penetrated by numerous dorsoventral muscle fibers. When approaching the mouth, the peripheral parenchyma becomes looser and passes into the central, or digestive, parenchyma. The mouth leads into a rather spacious cavity, in which the remnants of the digested foodstuffs could be found. In addition to this cavity, numerous digestive vacuoles occupy the central parenchyma. Their chief content is diatom algae, but they also may contain the semidigested symbiotic algae and pyknotic nuclei. The proper nuclei of the central parenchyma are scanty. The dorsoventral fibers are nearly absent here. The main bulk of the symbiotic green algae is located in the peripheral parenchyma immediately under the muscular layer; moreover, they can force the epithelial nuclei deep inside the animal. The symbiotic algae contain chloroplasts that form in aggregate a goblet-like figure, and the eosinophilic pyrenoid, and are almost identical to those of S. psammophila and S. bifoveolata. Reproductive system. The testes are paired, ventro-lateral in position. They usually start in front of the mouth and extend in form of two longitudinal bands to the level of the male gonopore. Each testis is composed of numerous, adjoining bunch-like clusters of spermatocytes and spermatozoa. Ripening spermatogonia are scattered along the whole length of the testis that suggests the present of a diffuse germinal zone. Spermatogonia are distinguished by large round nuclei with distinct nucleoli. The diameters of the nuclei are about 7.5-8 Ìm. The most mature regions of the testes are represented by tangles of the mature filiform spermatozoa. The ripe sperm do not form distinct strips. There is a loose cluster of parenchymal cells with nuclei around the antrum that is penetrated by large muscle fibers running in various directions, but mostly horizontally and dorsoventrally. It is around here that the mature spermatozoa reach into the animal and from here they permeate into the male antrum. The true seminal vesicle is absent. The length of the mature spermatozoa in vivo is about 140-150 Ìm. The antrum represents a deep invagination of the coverings inside the body. The antrum is ciliated, but its cilia are distributed less regularly than those of the body surface. In addition to that, they are 1.5 longer than the cilia of the epidermis, and single cilia often stick together into separate tufts. The wall of the antrum is wider than the epidermis and contains nuclei. Circular muscles are located immediately beneath the epithelium of the antrum; these muscles are organized in several layers and represent a continuation of the circular musculature of the body wall. Next to them lie the longitudinal muscles arranged in several rows and representing derivatives of both the longitudinal body-wall muscles and dorsoventral fibers (Fig. 2). The germinal zone of the paired ovaries is located at the level of the front margin of the testes or slightly behind them. The ovaries lie closer to the center and to the ventral side than the testes. The ripening oocytes are surrounded by the coating cells [?] with easily distinguished nuclei. These cells of oblong, rhomb-like shape produce a kind of envelope around a ripening egg. Presumably, we can observe here the incipient formation of the tunica propria of an ovary, inherent to the members of other turbellarian orders. The ovary terminates in front of the bursal nozzle. Female copulatory organs start with the female gonopore leading into the long ciliated vagina. The cilia of the vagina are similar to the cilia of the male antrum. Its epithelium is also wider than the epithelium of the body surface. The nuclei of the vaginal wall are positioned both above and beneath the subjacent muscle fibers. These muscle fibers are represented by thin longitudinal fibrils extending along the whole length of the vagina and continuing into the muscular envelope of the seminal bursa [bursa copulatrix?]. Their association with the circular fibers of the body wall could be easily followed. On the other hand, the longitudinal fibers of the body musculature form a strong sphincter of the female gonopore. In the mid-region of the vagina, two parallel groups of eosinophilic glands of unknown function open into the lumen. The seminal bursa represents a long formation with thick cytoplasmic wall and numerous irregular cavities. Round nuclei are strewn in the bursal lining. Around the periphery, large dorsoventral muscles penetrate the bursal wall. Closer to the center these are absent. In the bursal cavities, the irregularly scattered spermatozoa usually occur. Their largest cluster forms in the cavity adjoining the bursal nozzle. A thin, elongated, and elastic nozzle is directed slantwise forward and down to the ventral wall, on which it rests upon. Its length, after the measurements made on live specimens, is about 40-45 Ìm. The diameter of the proximal part is approximately 11 Ìm. The proximal glands in the base of the nozzle are absent. The nozzle is encircled by cells of the matrix with oval nuclei containing easily distinguished nucleoli.[From Kostenko AG, Mamkaev YuV (1990)]
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