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

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Abstract
Cercariae of C. 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 1st series of epidermal cell bodies, formed immediately below and having the characters of protein synthesizing cells, become connected with it. The 1st cell bodies are replaced by a series of 5 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 4 of the 5 types are discharged into the outer cytoplasmic layer before the cercaria leaves the molluskan host [L. littorea], and remain there in the free swimming cercaria. The secretions of the 5th 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 2nd intermediate fish host.