ULTRASTRUCTURE OF THE DIGESTIVE-SYSTEM OF THE CERCARIA OF SANGUINICOLA-INERMIS PLEHN, 1905 (DIGENEA, SANGUINICOLIDAE)

The digenean blood fluke Sanguinicola inermis and the family Sanguinic olidae have previously been described as apharyngeate and sucker-less. Examination of the digestive tract of the cercaria of S. inermis with transmission electron microscopy (TEM), however, has confirmed the pr esence of a well-developed and complex muscular region immediately adj acent to the sub-terminal mouth. The presence of this muscular region, which may be an oral sucker, emphasises the need for further ultrastr uctural studies on other members of the family. The adjacent oesophagu s, composed of modified tegument, has distinct anterior and posterior regions. TEM shows that the surface of the posterior oesophagus is dee ply folded and the apical cytoplasm and sunken cytons become filled wi th secretory granules, eventually forming a ''secretory region'' adjac ent to the intestine. The dorsal, sac-like intestine is composed of a single layer of epithelial cells, the apical surface of which is drawn into shallow folds. Each cell contains numerous secretory granules, r ough ER, and a single basal nucleus. The intestinal lumen; which conta ins electron-dense striated bodies, is filled with fibrous material. O nly two sensory oral papillae are associated with the digestive tract, positioned either side of the mouth opening and possibly acting as ta ngo-, stretch- or pressure receptors. The papillae comprise several mi crotubules lying beneath the apical cytoplasm of the tegument within a nerve process, but there is no connection with the external body surf ace. The morphology of the alimentary canal of the S. inermis cercaria is discussed in relation to the functioning of the digestive system i n preparation for establishment in the final host.