ULTRASTRUCTURE OF THE BUCCAL CAVITY AND THE CUTICLE OF 3 CEPHALOBIDAE(NEMATODA, RHABDITIDA)

The ultrastructure of the buccal cavity was elucidated in three specie s of cephalobids, two belonging to the genus Acrobeloides and one to t he genus Chiloplacus. The cuticular differentiations, as well as the i dentity of the surrounding structures or tissues are used to identify the consecutive buccal regions. Six distinct buccal regions are discer nible rather than the five parts generally assumed to be characteristi c for many nematodes. The first buccal region is surrounded by the lip cuticle in which three sclerotized plates (two in a dorsosublateral p osition and one in a medioventral position) are embedded. The second b uccal region is surrounded by two consecutive rings of arcade epidermi s. The posterior arcade epidermis contains an electron-dense ring that surrounds the buccal cuticle and partly penetrates the junction of th e cuticle and the anterior set of pharyngeal muscles. The basal lamina layer separating pharyngeal musculature from the other tissue compone nts in the labial region ends as it contacts the electron-dense ring. The fourth to the sixth buccal regions are surrounded by four consecut ive sets of pharyngeal muscles. The two anterior sets (m1 and m2) are composed of one muscle per pharyngeal sector, while the two posterior sets (m3 and m4) contain two cells per sector, interradially separated by cell membranes. Marginal cells, containing electron-dense radial f ibres, are situated in a perradial position, thus separating the muscl e cells. The anteriormost set of marginal cells spans the length of th e four anterior muscle sets. The dorsal gland opens into the buccal lu men by means of a narrow canal situated at the anterior margin of the dorsal m4 muscle set. A neuronal cell process is situated just anterio r to the canal. It is proposed that the observed buccal organisation d oes not fit the current concepts related to the homology of buccal par ts in nematodes.