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Notes: Abstract  The tentacular apparatus of Coeloplana bannworthi consists of a pair of tentacles which bear, on their ventral side, numerous tentilla. Each tentacle extends from and retracts into a tentacular sheath. Tentacles and tentilla are made up of an axial core covered by an epidermis. The epidermis includes six cell types: covering cells, two types of gland cells (mucous cells and granular gland cells), two types of sensory cells (ciliated cells and hoplocytes), and collocytes, this last cell type being exclusively found in the tentilla. The core is made up of a fibrillar matrix, the mesoglea, which is crossed by nerve processes and two kinds of smooth muscle cells. Regular muscle cells are present in both the tentacles and tentilla while giant muscle cells occur exclusively in the tentilla. The retraction of the tentacular apparatus is an active phenomenon due to the contraction of both types of muscle cells. The extension is a passive phenomenon that occurs when the muscle cells relax. Tentacles and tentilla first extend slightly due to the rebound elasticity of the mesogleal fibers and then drag forces exerted by the water column enable the tentacular apparatus to lengthen totally. Once the tentacles and tentilla are extended, gland cells, sensory cells, and collocytes are exposed to the water column. Any swimming planktonic organism may stimulate the sensory cilia which initiates tentillum movements. Pegs of hoplocytes can then more easily contact the prey which results in a slight elevation of the nearby collocytes, the last being responsible for gluing the prey to the tentilla.

Notes: Summary The fine structure of the integument of Myzostoma cirriferum is described with special attention to the integument sensory areas. Hypotheses about the function and a functional model of these are proposed. The integument consists of an external pseudostratified epithelium with cuticle (the epidermis) covering a parenchymo-muscular layer (the dermis). The dermis includes two types of cells: muscular fibers of the double obliquely striated type and parenchymal cells. Differences occur in the epidermis, which consists either of a large non-innervated myoepithelial area (viz. the regular epidermis). or of several rather localized sensory-secretory areas associated with discrete nerve proceses (viz. the sensory epidermis). The regular epidermis is made up of three types of cell: covering cells, ciliated cells and myoepithelial cells. The sensory epidermis shows small or marked structural variations from the regular epidermis. Small variations occur in the cirri, the buccal papilla, the body margin, the parapodia and the parapodial folds where nerve processes insinuate between epidermal cells. They are thought to be mechanoreceptor sites that could give information on the structural variations of the host's integument and participate in the recognition of individuals of the same species. The sensory epidermis differs markedly from the regular eidermis in the four pairs of lateral organs. Each lateral organ consists of a villous and ciliated dome-like central part, surrounded by a peripheral fold. The epidermis of the fold's inner part (viz. the part facing the central dome) is made up of secretory cells, while that of the fold's outer part is similar to the regular epidermis. The epidermis of the dome includes vacuolar cells, sensory cells and a different type of secretory cell. Lateral organs are presumed to be both chemoreceptors and mechanoreceptors. They could allow the myzostomids to recognize the host's integument and prevent them from shifting on the surrounding inhospitable substrate.

Notes: Summary The spermatophore ofMyzostoma cirriferum is a white V-shaped structure up to ca. 500 μm long. It is formed by a translucent matrix which includes numerous cysts of two types that are very close together and tend to form interlacing twists. According to their contents, three spermatophoral regions can be distinguished: the body with the horns, the foot and the basal disc. The body-horns region forms the upper part of the spermatophore and extends over ca. 400 μm. This region includes mature spermiocysts which are formed by one cyst cell each including one to three groups of rolled up spermatozoons. Features of these cyst cells are their great length (up to 25 μm), their euchromatic nuclei each provided with a large nucleolus, their numerous mitochondria and osmiophilic vesicles included in the cytoplasm as well as cytoplasmic remnants of the residual bodies of the spermatids. Spermatozoons appear to be well adapted to the intradermic penetration occurring in this species in that all of them possess nuclei provided with dense nuclear grains, a hairpin-bent flagellum and a microtubular palissade. The spermatophore foot is located just below the body and extends over ca. 90 μm. It contains exclusively spermiocysts which include one to three abortive germinal cells. They differ also from the previous cysts by their smaller length (ca. 6–10 μm) and their more heterochromatic nuclei. The basal disc is the lower part of the spermatophore. It extends over ca. 10 μm and contains electron-dense vesicles in its upper part and vesicles with fibrillar material in its lower part. When mature myzostomids contact each other, a spermatophore is expulsed from one seminal vesicle of the donor myzostomid to the integument of the receiver myzostomid. The vesicles with fibrillar content are the first in contact with the cuticle of the receiver myzostomid. The material they include is supposed to have a histolytic action and to be responsible for the lysis of the cuticle and epidermal cells thus providing a passage for the spermatophore contents. Afterwards, cysts move as a result of the spermatozoons' beating and pass through the receiver's integument. At the time of penetration, cytoplasmic membranes of the cyst cells merge together forming an enormous syncytium extending into the whole receiver's body. This syncytium surrounds the spermatozoons and the abortive germinal cells. The whole process of intradermic penetration (i.e. from the fixation of the spermatophore to its reduction to an empty matrix) lasts from 1–5 h.