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The origin and development of the epidermis and associated structures in the cercaria of Cryptocotyle lingua (Creplin) (Digenea: Heterophyidae) from Littorina littorea (L.)

Cercariae of Cryptocotyle lingua develop from intraredial germinal cells which divide to form ‘naked’ cell aggregates and later germ balls covered, first, by a syncytial primitive epithelium and later a syncytial epidermis formed, in sequence, from superficial cells of the embryo. The primitive epithelium is soon lost. The original nuclei of the syncytial epidermis degenerate when the first series of epidermal cell bodies, formed immediately below and having the characters of protein synthesizing cells, become connected with it. The first cell bodies are replaced by a series of five types of epidermal (secretory) cell bodies developing in the parenchyma and giving off cytoplasmic processes which become connected, in sequence, with the outer cytoplasmic layer. Secretion bodies from four of the five types are discharged into the outer cytoplasmic layer, before the cercaria leaves the molluscan host, and remain there in the free swimming cercaria. The secretions of the fifth type are retained in their epidermal (secretory) cell bodies. The arrangement of the secretion bodies in the outer cytoplasmic layer and their histochemical reactions suggest possible functions concerned, later, with entry into and encystment within the second intermediate fish host.