On the ultrastructure of the larva ofKronborgia amphipodicolaChristensen and Kanneworff, 1964 (Turbellaria, Neorhabdocoela)

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Abstract
The newly hatched free-swimming larva of the endoparasitic turbellarian Kronborgia amphipodicola Christensen & Kanneworff, 1964 (fam. Fecampidae) was studied by means of the scanning and transmission electron microscopes. The larva is covered by epidermal cells provided with cilia and microvilli. Processes, each ending in a clup-shaped modified cilium, penetrate the epidermis on the anterior end of the larva. These structures are supposed to be of chemoreceptive nature. Different types of gland cells open throughout the larval surface. Below the epidermis two or more thin muscle layers are found, and below these there occur neoblast-like cells and cells containing organelles similar to the organelles of the epidermal cells. These latter cells are suggested to be epidermal-replacement cells. The larva forms a cyst on the cuticula of the crustacean host with secretory products from two gland types opening throughout the larval surface. The penetration through the host cuticula is probably carried out both mechanically and chemically as indicated by transmission electron microscopical observations. The newly entered larva in the host haemocoel does not differ essentially from the free-swimming larva. No host tissue reactions were observed. The structures of the larva are compared with those of other platyhelminthes and especially with the larvae of digenetic trematodes.