A fine structural analysis of the foregut of the tardigrade Milnesium tardigradum (Doyere)

The fine structure of the foregut of the tardigrade Milnesium tardigradum is presented. The analysis covers the structures of the anterior foregut, including the oral region, buccal tube and salivary gland, the pharynx and the esophagus. The oral region consists of a terminal mouth opening surrounded by six plate-like lips lying within a circlet of six prominent papillae. This pattern is remarkably similar to the arrangement of oral structures in certain nematodes. The buccal cavity is enclosed within a thick cuticular tube which possesses appendage-structures, the stylet sheaths, stylet supports and paired protrusible stylets. Again the presence of a cuticularized buccal capsule and protrusible stylets finds parallels among the nematodes. Finally, two large salivary glands envelope the buccal structures and contain voluminous amounts of secretory product which has access to the buccal cavity through the openings of the stylet sheaths. The pharynx of M. tardigradum is a large muscular structure which acts as a pumping or sucking organ during feeding. It has a triradiate, cuticle lined lumen surrounded by radially arranged muscle cells and special apical cells which cap each of the ventricles of the lumen. Among the Aschelminthes a strikingly similar pharynx is found in the nematodes, and perhaps also in the gastrotrichs which await more thorough examination. [...]