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In order to elucidate the functional role of cercomer in larvae of the monocercus type their transplantation from the specific host Chironomus obtusidens to Gammarus lacustris was conducted. At early stages after the transplantation proceeds an increase in the functional activity of the tegument of follicles of the cercomer followed by their complete destruction in 3 or 4 days. On the surface of the exocyst membrane an adhesion of the host's haemocytes occurs, which becomes more distinct in 3-4 days when the process acquires a character of local encapsulation. Within the same period, in the places of haemocytes aggregation, a local resorption of the exocyst external membrane takes place. Later intensification of the host response to transplant is associated with the destruction of follicles of the cercomer. In one case the occurrence of follicles of the cercomer in the cavity of cysticercoid was observed that is caused by the microbe affection of the latter. In the zone of contact of the tegument of scolex and neck with follicles of the cercomer an increased secretion (the microapocrine type) of the tegument, disturbance of the microvillous tegument of the cercomer's follicles and their destruction are observed. Incompatibility of the tegument of definitive departments and cercomer, which arises during differentiation of larvae, is supposed to affect the formation of scolex invagination in the evolution of larvae of Hymenolepis.

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Histochemical reaction with glyoxylic acid has revealed a specific fluorescence caused by the presence of indolamine (apparently serotonin) in the nervous system of larva and mature cestode. Fluorescence manifests itself in neurons and nerve fibres of the central ganglion and its commissure, in nerve cells of the proboscis, in longitudinal trunks and transverse commissures, and in the nerve elements connected with genital system.

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At the early stages of asexual breeding, the daughter larvae of T. crassicaps display destructive changes of the tegument due to its hyperfunction. The reparation of tegumental cytons is followed by the appearance of specific "brushes" of fibrillar material on the karyolemma of their nuclei. They are supposed to occur as a result of protein synthesis necessary for the reconstruction of cellular structures, mainly, of the nucleus. The abundant synthesized fibrillar falls into decay being transformed into bodies containing crystals which are transported to the superficial synthytium to be eliminated there.

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There are glandular elements of syntitial structure in parenchyma of scolex base of T. crassiceps larvae. They are located behind the suckers at the invaginated state of scolex. Perikaryons of gland contain dense rodshaped, spheric granules and great accumulations of electron-dense substance in vacuoles. The same types of secretary product are found in the rostellar tegument. The glandular elements are mostly developed in the period of the formation of rostellar hooks, but they are well expressed too in mature larvae thus providing morphofunctional specialization of rostellar tegument.

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Electron microscopical studies of the cyst of 1-month-old H. diminuta larvae revealed considerable thickening of the superficial syncytium of tegument and formation of a large number of cavities and canals. The cytoplasm of the tegument is filled with microtubules produced by cytons of parenchymal layer which are analogs of tonofibrils. These peculiarities, together with numerous processes of the external fibrous layer, are regarded as adaptative to the changes of the hydrostatic pressure of the hemocoel of the intermediate host. The superficial syncytium is covered with microvilli and keeps polycellular cytoplasmatic bonds with cytons, which ensures energetic and plastic requirements for the stabilization of the hypertrophied syncytium and its physiological regeneration. Accumulations of fibres identical with those of the connective tissue were found in the cytons of the tegument. The cyst parenchyma consists mostly of cells with widened canals of granular endoplasmic reticulum associated with the microfibrils and production of fibrous filaments. The internal fibrous layer is produced by typical fibroblasts. The tegument of the cercomer is thinner and is considered to be less differentiated.

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Three layers are tentatively recognized in the wall of a blood capillary of nucleus praeopticus (NP) of the carp, Cyprinus carpio L: the inner layer--endothelium, the middle layer--basement membrane with pericytes, and the outer broken layer containing mostly leucocytes and fibroblast-like cells. 1). The plasmalemma of the neurosecretory cells (NSC) makes a direct contact with a capillary: (a) with the plasmalemma of a sole-like process of the endothelial cell (0.1%), (b) with the basement membrane of the capillary (4.9%), (c) with the plasmalemma of the outer layer cells (41.3%). 2) The plasmalemma of the NSC is separated from the capillary by a glial process (53.7%). The first kind of the contact is frequent in the ventral part (3.7%), and the second one--in the dorsal part (29.4%) of NP. The different relations of the NSC--the capillary of the ventral and dorsal parts are due to heterochronism of their development in ontogenesis.

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The preoptic neurosecretory cells (NSC) in sexually mature female Cyprinus carpio L. have been studied with electron microscope in winter. Separate weakly differentiated NSC and differentiated NSC of small and mean size are localized in the ventral part of Nucleus preopticus. The big highly differentiated NSC are mostly localized in the dorsal part of Nucleus preopticus. Among these types of the cells are established the light and dark forms. The "picnomorphic" cells are found only in the dorsal part of Nucleus preopticus. The light NSC are subdivided into two following kinds of cells: 1. NSC with a great amount of moderately dilated channels of the rough endoplasmic reticulum and scarce neurosecretory granules; 2. NSC with a small amount of narrow channels of the rough endoplasmic reticulum and many neurosecretory granules. The various types of differentiated NSC reflect the asynchronous formation of neurosecretory granules. The Nucleus preopticus of C. carpio has mostly the light highly differentiated NSC. It is supposed that most of the NSC in Nucleus preopticus produce and storage neurosecretory granules.

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With electron microscope two types of synaptic buttons (SB) are found on the nuerosecretory cells (NSC) in the nucleus preopticus of the mature carp. SB type I contain dense core vesicles and synaptic vesicles, whereas SB type II have only the latter. SB type II is considered to be functionally more active. The density of SB on the NSC body surface varied inversely proportionally to the NSC size, being in average 1.7+/-0.2 on the section of one small NSC in the ventral part of the nucleus preopticus, and 1.0+/-0.1 on the section of a large one in the dorsal part (P less than 0.05). In large NSC type II SB are found more frequent than in small NSC, the synaptic apparatus of NSC in the dorsal part of the nucleus preopticus is in a more active stage, than of NSP in the ventral part. The structural organization of the complex SB-NSC of large cells is more complicated compared to that of small cells. The large cells are supposed to be more differentiated and functionally more active. The synapses of NSC of the nucleus preopticus of the carp are supposed to belong usually to axons of monaminergic neurons.

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The ultrastructure of synaptic endings of the neurosecretory cells of the nucleus preopticus was examined in adult Cyprinus carpio L. Two of synpatic endings occur: type I--small agranular vesicles and large granular vesicles, type II--only agranular vesicles. The functioning of the nucleus preopticus neurosecretory cells in Cyprinus carpio L is presumably controlled by the synpatic endings of the adrenergic (synaptic endings of type I) as well as of the cholinergic (synaptic endings of type II) origin. By visual and morphometric methods different kinds of synpatic endings are distinguished among both the types of synapses according to their particular functional states. A quantitative analysis of the correlation of these kinds of synpatic endings allows a suggestion in respect to the state of the synaptic apparatus on the perikaria of neurosecretory cells.

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