Here, for the first time in an ascaridoid (Hysterothylacium auctum), we present structural features of the phasmids, paired sense organs, positioned in a bilateral manner close to the point of the tail; the features were obtained using scanning and transmission electron microscopy. We found that each phasmid consists of a single ciliated dendritic process situated in a phasmidial canal surrounded by 2 supporting cells, a socket and a sheath cell. The socket cell contains clusters of electron-dense fibrous material in its apical region and covers the phasmidial canal along its whole length. The sheath cell is characterized by a well-developed endoplasmic reticulum. The phasmidial canal is lined with a thin layer of cuticle that becomes incomplete at the base of the ciliated dendritic process. In this region, the dendritic process consists primarily of a high number of microtubule singlets and some peripheral microtubule doublets. The base of the dendritic process, containing numerous striated rootlets, gives off a large number of fingerlike offshoots, villi, invading the surrounding sheath cell. The systematic significance and functional implication of the phasmid in nematodes are also discussed.

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