Fine structure of the cerebral organs in hoplonemerteans (Nemertini), with a discussion of their function

Summary The fine structure of the cerebral organs is described in three species of monostiliferous hoplonemerteans. Amphiporus lactifloreus, Paranemertes peregrina and Tetrastemma candidum. There are two distinct groups of sensory cells in the cerebral organs of all three species. The ultrastructure of the sensory elements in these species is consistent with a chemoreceptive function of the dendrites. Incurrent and excurrent channels of the canal are postulated, based on the fine structure of the ciliary axonemes. Flow through the canal is such that each of the two groups of dendrites is downstream from a group of glandular cell outlets and upstream from a group of vesicular cells. It is suggested that the glandular, sensory and vesicular cells form a functional unit in which glandular cells secrete a coating material over the dendrites and vesicular cells actively remove this coating by endocytosis. Vesicular material is also found in glandular cells, where it probably arises in situ through crinophagy. There is no ultrastructural evidence that vesicular material is transferred to the vascular system. Small fibres containing dense vesicles are present among the ciliated cells and may represent an efferent nerve supply controlling the rate of flow through the canal.