Structure and function of the cerebral organs in 'Paranemertes peregrina', 'Tetrastemma candidum' and 'Amphiporus lactifloreus' (Hoplonemertea : Monostilifera)

The histology and ultrastructure of the cerebral organs have been studied in three species of monostiliferous hoplonemertean: Paranemertes penegnina Coe, Amphiporus lactifloreus (Johnston) and Tetrastemma candidum (O.F. Muller). The role of the cerebral organs in osmoregulation and behaviour has been investigated in Paranemertes. Based on the information obtained, it is concluded that the cerebral organs in these species are chemoreceptors. The structure of the cerebral organs is essentially the same in the three species studied. The cerebral organs consist of two groups of sensory cells, two groups of gland cells, and two groups of endocytic/lysosomal cells (vesicular cells), as well as ciliated cells and support cells, surrounding a ciliated, blind-ending canal. The canal is functionally divided into two channels, designated the major and minor canals. According to the orientation of ciliary basal feet in cells of the canal epithelium, the minor canal is an incurrent channel, and the major canal is an excurrent channel. The organization of cell types with respect to the direction of flow in the canal is such that along the minor canal; Type A gland cell processes are upstream from Type 2 sensory cell dendrites, and Type 2 vesicular cells are downstream from the dendrites. Similarly, in the major canal. Type B gland cell processes are upstream, and Type 1 vesicular cells are downstream, from Type 1 sensory cell dendrites. Based on this organization, and on the interpretation of cellular fine structure in the cerebral organs, it is proposed that the function of gland cells is to secrete a mucous coating over the sensory epithelium, and the function of vesicular cells is to remove this coating from the canal as the mucus is carried downstream from the dendrites by ciliary action. In gland cells, the amount of secretion product present may be regulated by autophagic breakdown of secretion granules (crinophagy), according to a variable demand for secretion in the canal. Crinophagy contributes to the amount of vesicular material (degraded secretion product) present in the cerebral organs. Although the dendrites are not innervated, dendrite sensitivity may be modulated by variation of the rate of flow through the canal, and the rate of mucous turnover across the two sensory epithelia. An efferent nerve fibre is present among the ciliated cells of the minor canal. The fibre is rare and its synapse has not been observed. It is thought that the fibre innervates a few cells which act as pacemakers, their cilia mechanically entraining the beat frequency of other cilia, thus determining the rate of flow through the canal. There is no indication that vesicular material is disposed of outside the cerebral organs. In Paranemertes and Amphiporus, but not in Tetrastemma, the cephalic blood vessel lies adjacent to the posterior glandular part of the cerebral organ, however, this association is not reflected in the internal structure of the cerebral organs. It is, therefore, unlikely that the cerebral organs in these species have an endocrine function. The function of the cerebral organs in Paranemertes has been investigated by comparing the behaviour of intact worms with the behaviour of worms from which the cerebral organs have been surgically removed. Cerebral organ removal did not affect trail following behaviour, which is associated with homing, but it abolished the response of Paranemertes to prey trails. It is concluded that the cerebral organs of Paranemertes are chemoreceptors responsible for the detection of prey. The behavioural physiology of Paranemertes has been investigated, using extracellular suction electrodes to record from the lateral nerve cords and the cerebral organ nerves. The results indicate that the cerebral organs are sensitive to prey extract and distilled water, but not to mechanical, thermal or photic stimuli. The role of the cerebral organs of Paranemertes in salinity stress tolerance has been investigated by measuring the effect of cerebral organ removal on volume regulation, and by observing the effects of hypo-osmotic media on the cytology of the cerebral organs. Removal of the cerebral organs decreases volume regulatory capacity, however, a similar change is seen in sham-operated worms, indicating that the decreased capacity for regulation is due to the operation itself and not to interference with a physiological role of the cerebral organs. Cytological changes caused by exposure to dilute sea water are similar to those seen in worms fixed in hypo-osmotic fixative. It is unlikely, therefore, that these represent a co-ordinated response of the organs to salinity stress. No exchange of material between the cerebral organs and the vascular system was observed. It is concluded that in Paranemertes, the cerebral organs are not involved in osmoregulation