Ultrastructure of multiciliated collar cells in the pilidium larva of Lineus bilineatus (Nemertini)

Summary The ultrastructure of the apical plate of the free-swimming pilidium larva of Lineus bilineatus (Renier 1804) is described with particular reference to the multiciliated collar cells. In the multiciliary collar cells there are several, up to 12, cilia surrounded by a collar of about 20 microvilli extending from the cells' apical surface. The cilia have the typical 9+2 axoneme arrangement and are equipped with striated caudal rootlets extending from the basal bodies. No accessary centriole or rostral rootlet were observed. Microvilli surrounding the cilia are joined in a cylindrical manner by a mucus-like substance to form a collar. In comparison with many sensory receptor cells built on a collar cell plan the multiciliary collar cells of the pilidium larva apical plate are rather simple and unspecialized. In other pilidium larvae monociliated collar cells are found in the apical plate. The possible function and phylogenetic implications of multiciliated collar cells in Nemertini are briefly discussed.List of Abbreviations a axoneme - b basal body - c cilia or flagella - d desmosome - G Golgi apparatus - m mitochondria - mf microfilaments - mu mucus - mv microvilli - n nucleus - nt neurotubules - pm plasma membrane - r rootlet - ri ribosomes - v secretory vesicles