The evolutionary origin of vertebrate placodes remains controversial because divergent morphologies in urochordates, cephalochordates and vertebrates make it difficult to recognize organs that are clearly homologous to placode-derived features, including the olfactory organ, adenohypophysis, lens, inner ear, lateral line and cranial ganglia. The larvacean urochordate Oikopleura dioica possesses organs that morphologically resemble the vertebrate olfactory organ and adenohypophysis. We tested the hypothesis that orthologs of these vertebrate placodes exist in a larvacean urochordate by analyzing the developmental expression of larvacean homologs of the placode-marking gene families Eya, Pitx and Six. We conclude that extant chordates inherited olfactory and adenohypophyseal placodes from their last common ancestor, but additional independent proliferation and perhaps loss of placode types probably occurred among the three subphyla of Chordata.

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The evolutionary history of placodes: a molecular genetic investigation of the larvacean urochordate Oikopleura dioica

1. The locomotory cilia of Mangelia and Pneumoderma larvae undergo arrests spontaneously and in response to tactile stimulation. These events are often associated with muscular contractions in an overall response thought to be protective in nature.2. Isolation of the ciliated bands from the central nervous system abolishes the ability for coordinated ciliary arrests and the cilia show continuous metachronal beating.3. Recordings with suction electrodes attached to the surface show patterns of electrical signals during periods of ciliary arrest. Intracellular recordings with glass microelectrodes from single ciliated cells in Pneumoderma show rapidly rising, slowly decaying, all or none 50 mV spikes when the cilia undergo arrest. There are no fluctuations in membrane potential during metachronal beating.4. The existence of a rich motor innervation supplying the ciliated epithelium in Mangelia has been established using optical and electron microscopy. The nerve endings appear to derive from neurons whose c...

Nervous control of ciliary activity in gastropod larvae.

The cilia lining the stigmata of the branchial sac of an ascidian circulate water through the animal. These stigmatal cilia are under nervous control; when either siphon is stimulated, both siphons close by muscular contractions and at the same time the stigmatal cilia stop beating simultaneously in all parts of the branchial sac. Spontaneous ciliary arrests may also occur, with or without associated closure of the siphons. Elements of the branchial nervous system that run in the gill bars are assumed to be concerned in coordination of the ciliary arrests. The majority of the branchial nerve fibres emerge dorsally from the visceral nerves that form the posterior brain roots, although nerves are also believed to enter the branchial sac along its anterior margin. No cell bodies could be found in the branchial nerves or in the visceral nerves, so that the cell bodies of the branchial nerve fibres are assumed to lie in the central nervous system. The branchial nerve fibres form a peripheral conducting net extending throughout the branchial sac. Branches of these nerve fibres terminate in contact with some of the ciliated cells; cell-to-cell conduction (through close junctions?) probably spreads excitation to the other ciliated cells. Nerve-nerve junctions appear to be more sensitive to curare than those between nerves and ciliated cells. Electrical recordings from the branchial sac, obtained with suction electrodes, show that arrest of the cilia is accompanied by electrical activity, and that prolonged arrest is maintained by trains of regular pulses. Intracellular microelectrodes in the ciliated cells indicate that these cells have a negative resting potential of 30-40 mV, and that a ciliary arrest is associated with a positive-going spike of 45-50 mV. The externally recorded 'ciliary arrest potentials' probably represent the coordinated depolarization of many ciliated cells. The rhythmical character of the trains of pulses presumably depends on pacemaker activity; this is not localized, since intact organisms or isolated small portions of the branchial sac are capable of generating similar trains of pulses. During the arrest response the stigmatal cilia first perform a reverse beat, then maintain the reverse position for several seconds before slowly relxaing and after several more seconds recommencing to beat with progressively increasing amplitude. The duration of the arrest response varies in media with different concentrations of the common cations, and also varies in response to repetitive stimulation, in a manner which suggests that the depolarization of the ciliated cells is associated with an influx of Ca$^{2+}$, so that the ciliary control here may have some close parallels with that described for Paramecium.

Branchial innervation and ciliary control in the ascidian Corella.

Mucous-mesh grazers (pelagic tunicates and thecosome pteropods) are common in oceanic waters and efficiently capture, consume and repackage particles many orders of magnitude smaller than themselves. They feed using an adhesive mucous mesh to capture prey particles from ambient seawater. Historically, their grazing process has been characterized as non-selective, depending only on the size of the prey particle and the pore dimensions of the mesh. The purpose of this review is to reverse this assumption by reviewing recent evidence that shows mucous-mesh feeding can be selective. We focus on large planktonic microphages as a model of selective mucus feeding because of their important roles in the ocean food web: as bacterivores, prey for higher trophic levels, and exporters of carbon via mucous aggregates, faecal pellets and jelly-falls. We identify important functional variations in the filter mechanics and hydrodynamics of different taxa. We review evidence that shows this feeding strategy depends not only on the particle size and dimensions of the mesh pores, but also on particle shape and surface properties, filter mechanics, hydrodynamics and grazer behaviour. As many of these organisms remain critically understudied, we conclude by suggesting priorities for future research.

https://royalsocietypublishing.org/doi/pdf/10.1098/rspb.2018.0056

Mammoth grazers on the ocean's minuteness: a review of selective feeding using mucous meshes.

ALTHOUGH cilia have many important functions in the majority of animal phyla, our knowledge about the mechanism by which their activity is controlled has been fragmentary1. Extensive electrophysiological studies of the ciliated cell, with intracellular microelectrodes, have been made only with the relatively large protozoans, principally Paramecium, where the ciliary activity has been shown to be closely associated with the electrical properties of the cell membrane2,3. Among the metazoans, some examples of nervous control of ciliary activity have been reported4, but, mainly because of technical difficulties, intracellular recording of the membrane potential from the ciliated cells has been made in only a few cases5,6; in no case has nervous control of cilia been studied by recording simultaneously the electrical and the mechanical events. Such recording is essential to understanding the mechanism of nervous control at the cellular level.

Correlation of electrical and mechanical responses in nervous control of cilia.

1. Reversal of the water current through the pharynx of Oikopleura is brought about by a change in the action of the cilia of the two stigmatal ciliated rings. These ‘ciliary reversals’ occur synchronously in the two ciliated rings and can be evoked by the addition of particulate material to the incoming water or by tactile or electrical stimulation of the lips. 2. Nerves run from the lips via the brain to individual ciliated cells, and it is therefore likely that the ciliated cells are under nervous control. 3. At each ciliary reversal an electrical potential can be picked up on the body surface. The same events are recorded by microelectrodes inserted into the ciliated rings. The microelectrode recordings resemble intracellular recordings, and the reversal potentials are considered to represent depolarizations of the membranes of the ciliated cells. 4. Ciliary reversals continue after removal of the brain, suggesting the existence of a peripheral pacemaker.

C. P. Galt

Papers

Electrical Correlates of Ciliary Reversal in Oikopleura