The nervous system of the earliest functional stage of the actinotroch larva of
Phoronis vancouverensis
is described based on ultrastructural surveys and three-dimensional reconstructions, including serial reconstructions of selected parts of the system. The central element and main source of fibres in the system is the apical organ. Nerve cell bodies were found here and in the surrounding apical epithelium, but nowhere else in the body. Given the limitations of the methods used, the presence of nerve cell bodies elsewhere in the body cannot be ruled out, but based on this work and a recent study by A. Hay-Schmidt of whole larvae, it seems unlikely they occur in any numbers. The larval nervous system is thus highly centralized, an advanced and rather specialized feature in comparison with some other larval types, specifically those of primitive spiralia and echinoderms, in which nerve cell bodies are more widely distributed in the larval epithelium. The largest single nerve in the body is the primary hood nerve, which runs around the pre-oral hood slightly back from its margin. The nerve is a compact, well-defined tract of approximately 40 fibres, with an investment of glial-like accessory cells. A second set of smaller, accessory nerves run parallel to the primary nerve between it and the hood margin. The hood nerves all join at the base of the hood on either side of the mouth to form a pair of adoral nerve centres. A number of small nerves cross the hood from the apical organ to the hood nerves. Three of these are large enough to be considered major nerves: one is medial and connects to the centre of the hood margin, the other two are dorsolateral and connect to the adoral nerve centres. Fibre tracings, which show the distribution of vesicle-filled terminals and varicosities, suggest the hood nerves are mainly involved in neuromuscular control, specifically, in lifting the hood. This involves the stimulation, in sequence, of the radial and circular hood muscles by the primary and accessory hood nerves, respectively. Cells at the hood margin are organized somewhat in the fashion of a conventional ciliary band, but there is no obvious morphological evidence that any of the hood nerves are involved in neurociliary control. A diffuse plexus of small nerves connects the above apical structures to the nerves supplying the tentacles. There are two main tentacle nerves, the primary tentacle nerve, which runs along the upper, oral margin of the tentacular ciliary band, and a smaller accessory nerve, which arises as a branch from the primary nerve, and runs along the lower, aboral margin of the band. There is also a row of uniciliate sensory receptor cells at the oral margin of the band. Each cell has a basal process ending in a vesicle-filled terminal that abuts fibres in the upper tentacle nerve, and forms junctions with them. The cells themselves produce no other fibres. They appear to be mechanosensory, and are probably involved in initiating the hood lift response, which can be triggered by touching the top surface of the tentacles. Additional large, vesicle-filled terminals branch from the fibres in the primary tentacle nerve. Their positions suggest a neurociliary function. The accessory tentacle nerve is associated mainly with muscle cells. A series of small nerves, which probably arise as branches from the larger tentacle nerves, supply the region below the tentacles, later the site of the telotroch. The comparative and phylogenetic implications of the above are discussed. Phoronids are generally interpreted as being intermediate between deuterostomes and protostomes, with a curious mixture of characteristics of both groups. Phoronids are probably only distantly related to spiralian protostomes, but they are, strictly speaking, protostomes, and their larvae resemble the trochophore-type larvae of spiralia in many respects. Regarding ciliary band substructure and patterns of innervation, the actinotroch possesses too few features that are clearly primitive to support a detailed comparison with spiralian larvae, but the pre-oral hood band shows a sufficient number of prototroch-like features, to suggest the hood band and prototroch could be homologous. There is evidence for parallel evolution, in the two groups, of an increasingly centralized nervous system that provides improved effector control via nerve cells located in and around the apical organ. No evidence was obtained to support suggested homologies between the post-oral band of the actinotroch and circumoral or post-oral feeding bands in deuterostome larvae. The two appear, in fact, to be quite dissimilar in terms of their innervation. The results thus support conventional interpretations of the relationship between phoronids and other major groups.
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