The development of the tail and the excretory system in the cercaria ofCryptocotyle lingua(Creplin) (Digenea: Heterophyidae fromLittorina Littorea(L.))

Abstract

The tail of the cercaria ofCryptocotyle linguadevelops within the redia (intraredial stage) the process being completed in the molluscan haemocoel (intramolluscan stage). During growth epidermal cell bodies are carried into the proximal region of the tail while the single type of epidermal (secretory) cell body, containing two types of secretions develops later in the same region. The vesicular secretion bodies disperse throughout the outer cytoplasmic epidermis while the clear secretion bodies collect between the inner plasma membrane of the outer cytoplasmic epidermis and the basement lamina, near the tail root, forming the proximal caudal dilation. The epidermal and epidermal (secretory) cell bodies soon degenerate. The longitudinal caudal muscles, at first similar to and continuous with those of the body, gradually develop into the separate system of striated caudal muscles characteristic of the free swimming cercaria. The nuclei of most of the muscle cells degenerate before emer­gence of the cercaria. A secondary excretory pore is formed at the posterior end of the body, in the intraredial cercaria, the caudal excretory vessel and primary pores subsequently degenerating. The tail is shed when the cercaria contacts the fish second intermediate host. The mechanical break at the body–tail junction is caused by the movement of the tail against the resistance of the fixed body. The resulting ‘wound area’ on the body, at first enclosed in the caudal pocket, is soon covered by a layer of outer cytoplasmic epidermis.