[ULTRASTRUCTURAL ORGANIZATION OF CELLULAR ELEMENTS AND INTERCELLULAR CONNECTIONS IN STATOCYSTS OF TERRESTRIAL PULMONARY SNAIL H. LUCORUM].

The organ of mollusc equilibrium - statocyst appears to be the analogue of acoustic-vestibular system in vertebrate animals. In terrestrial pulmonary snail the epithelial lining of statocyst cavity is created by two types of the cells - a small amount of large cells, provided with kinocilia of sensitive cells and considerably a large number of small supporting or inserted cells, covered with the microvilli. By means of transmission and scanning electron microscopy the ultrastructure and intercellular connections of these cells were studied. The sensitive cells have in a certain way structured cytoplasm, which consists of three layers: ectoplasm, granular layer and hyaloplasm. Myelin-like bodies having the granular, vesicular and drop-like formations in the centre appear to be the special structure of the cytoplasm. In the cytoplasm there are areas, saturated with electron dense glycogen granules. On the electronograms sometimes it is observed how the pinocytic vesicles in the supporting cells are created from the diverticulum of plasmatic membrane of sensitive cells. The boundary areas of plasmatic membrane of adjacent cells (sensitive cells with supporting or supporting cells with the support) are also characterized by the presence of specialized contacts, which are analogous to desmosomes in the epithelial tissues, as well as by the existence of cellular desmosomes, interdigitations. Numerous lacunas have been revealed in the intercellular space, which are connected by the thin tubules and ducts resulting in the formation of a complicated configuration of extensive system of communicating with each other lacunae, which have the exit in statocyst cavity.

[LIGHT AND ELECTRON MICROSCOPY OF CELLULAR ELEMENTS IN STATOCYSTS OF TERRESTRIAL PULMONARY SNAIL HELIX LUCORUM].

Statocysts H. Lucorum appear to be the paired formations having a spherical form and locating on dorsolateral surface of pedal ganglia of subesophageal ganglionic complex. Using the method of reconstruction, as well as the method of scanning electron microscopy on paraffin and semithin slices it has been revealed that the epithelial lining of H. Lucorum appears to be a spatially ordered complex, consisting of 13 cell assemblies. in each of them one sensory cell is surrounded by the satellite cells. The sensory cell has a star shape on the front pole of statocyst because of many protoplasmic processes branching off from its body. The other 12 cells have polygonal shape and form three belts around the inside perimeter of statocyst: interior, middle or equatorial and posterior. There are 4 cells in each belt. The sensory cells forming anterior and posterior belts turned out to be displaced in relation to the cells of equatorial belt and for this reason the whole cell structure resembles a brickwork. The distribution of sensory cells on the inner surfce of statocyst creates the structural polarization of this formtion. First of all, this is evidenced by the presence of the sensory cell on the anterior pole of statocyst and by the absence of those on opposite, posterior pole. The topography of each sensory cell on the inner surface of statocyst, the sizes and shapes of the nuclei, the quantity of nucleoli, the peculiarities of cytoplasm vacuolation and interrelation of sensory and supporting cells have been established.

Morphological peculiarities statoconia in statocysts of terrestrial pulmonary snail Helix lucorum.

Vast majority of statoconia in statocysts of Helix lucorum are of oval shape and have smooth surface. Each statoconium in its central part has a nucleus, a spherical mass of 1.5 µ in diameter, surrounded by concentric structures. Minority of statoconia are of subcircular, elongated, rectangular, triangular, irregular, and sometimes fanciful shape and are structured around several nuclei or around small statoconia consolidated by shared growth layers. Apart from statoconia, spherical formations lacking mineral composition of 0.3-2.5 µ in diameter were also found in the statocyst cavity. Similar formations were found in vacuoles of sensory cells of statocyst epithelial lining. It is hypothesized that statoconium nuclei and growth layers around them are of different origin: the nuclei are formed by statocyst sensory cells, while the mineral component is a result of activity of supporting cells.