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NATURE 735again, threads were seen to pass from the tips of the parapodia to the wall of the tube. The animal then retreated slowly down the tube ; the anterior end moved repeatedly from side to side through a semicircle ; and the first four to fourteen pairs of parapodia move,d in circles, each slightly out of phase with its immediate neighbours, while the setre were alternately protruded and with.drawn. These movements sufficed to make a feeding funnel from the threads secreted by the parapodial glands.The funnel when completed was attached in front to the wall of the tube and behind was almost completely blocked by the animal's peristomium. Meanwhile, most of the more distal segments of the animal undulated violently, drawing water laden with suspended particles past the animal antero-posteriorly. These undulating movements continued when the funnel was complete, and particles were thus automatically sieved out of the water current. The worm then moved forward and swallowed the feeding funnel with its entrapped particles in several gulps.The numbers of parapodia involved in the various stages of the filter-feeding process and the time taken for each step vary considerably in one worm and from worm to worm. The following figures are summaries of observations from five worms : in a tube of 3 mm. internal diameter, worms ~7 cm. in length made feeding funnels 0•5-2•5 cm. long; at 16•5-20° C. a funnel was made in 30-140 sec., water was pumped through the tube for 30-257 sec. after the funnel was completed, and a feeding funnel with its entrapped particles was swallowed in 6-16 sec.The cycles of feeding may follow rapidly one after the other with a few short breaks, or they may occur with intervals of several minutes, or irregularly, for a total period of about two hours.Feeding mechanisms similar in principle to this in N ereis diversicolor have been described for C/w!,topterus variopedatus 6 , U rechis caupo 6 and some Chironomus larvre 7 • These investigations form part of a study of the feeding habits of N ereis.

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Electrical neurostimulation is effective in the treatment of neurological disorders, but associated recording artefacts generally limit its applications to open-loop stimuli. Real-time and continuous closed-loop control of brain activity can however be achieved by pairing concurrent electrical recordings and optogenetics. Here we show that closed-loop optogenetic stimulation with excitatory opsins enables the precise manipulation of neural dynamics in brain slices from transgenic mice and in anesthetized non-human primates. The approach generates oscillations in quiescent tissue, enhances or suppresses endogenous patterns in active tissue, and modulates seizure-like bursts elicited by the convulsant 4-aminopyridine. A nonlinear model of the phase-dependent effects of optical stimulation reproduced the modulation of cycles of local-field potentials associated with seizure oscillations, as evidenced by the systematic changes in the variability and entropy of the phase-space trajectories of seizures, which correlated with changes in their duration and intensity. We also show that closed-loop optogenetic neurostimulation could be delivered using intracortical optrodes incorporating light-emitting diodes. Closed-loop optogenetic approaches may have translational therapeutic applications.