Discussion of Metacercaria

Several differences between the anatomy of the metacercaria and cercaria are evident. These include for the metacercaria the possession of the sucker and gland cells in place of the cystogenous organ and four lips at the anterior end of the body; a more elongate excretory bladder; granulation of the excretory bladder and intestine; spination, and some further differentiation of the reproductive system. The relationship between the cystogenous organ and four lips at the anterior end of the body of the cercaria and the sucker and gland cells of the metacercaria was not determined. Accordingly, it is not known whether the sucker is a completely new development or develops by modifications of pre-existing structures at the anterior end of the body.

Although tentacles are not developed in the metacercariae examined, the present species can be confidently assigned to the genus Bucephalus even though the flap of cuticle at the anterior extremity is suggestive of the "hood" of Rhipidocotyle. The ducts observed in the sucker musculature and their confluence with gaps between the groups of muscle fibres along the anterior margin of the sucker correspond in position with the three most dorsal tentacular ducts of adult specimens of Bucephalus longicornutus (Manter, 1954) and other species of Bucephalus. Furthermore, the nature of the sucker musculature is typical of many species of Bucephalus (vide Ziegler, 1883; Manter, 1940). In Rhipidocotyle, however, tentacular ducts are not present, and the musculature of the sucker differs from Bucephalus species (vide Nagaty, 1937).

Two other Bucephalus metacercariae are known. The metacercaria of B. elegans, figured but not described by Woodhead (1930), is insufficiently characterised for it to be closely compared with the present species. The metacercaria of the present species shows some resemblance to the metacercaria of B. polymorphus, described by Ziegler (1883) and Schurmans-Stekhoven (1934), with regard to the shape of the sucker and position of the gland cells, shape of the cyst, and granulation of the intestine and excretory vesicle. However, the cirrus sac is not as fully differentiated in the present species as indicated by Ziegler for B. polymorphus. It is not clear from Schurmans-Stekhoven's description or figures whether the longitudinal ridges in the sucker concavity of B. polymorphus are comparable with the tentacular ducts of the present species.