The Nervous System Amphioxus: structure, development and evolutionary significance (NRC) http://pubs.nrc-cnrc.gc.ca/rp/rppdf/z04-163.pdf Also considered in Amphioxus Nervous System and Amphioxus Motor Nerves. A 29 page PDF with more than a dozen diagrams. Since the entire PDF is available online for free, I will use it as a major reference. This simplified diagram only shows the sensory nerves. The original diagram, which shows all the nerves can be found in Amphioxus Nervous System. Cross Section of an Amphioxus Spinal Cord Showing Only the Sensory Nerves  Note that the sensory nerves project to the dorsal (top) of the spinal cord, whereas, as depicted in Amphioxus Motor Nerves, the motor neurons emanate from a more ventral (lower) part of the cord. This configuration remains consistent throughout the chordate lineage, up to and including we humans. Exploring developmental, functional, and evolutionary aspects of amphioxus sensory cells (Wiki) http://www.biolsci.org/v02p0142.htm Full text available online. "Amphioxus has neither elaborated brains nor definitive sensory organs, so that the two may have evolved in a mutually affecting manner and given rise to the forms seen in extant vertebrates. ...sensory organs and the central nervous system may have evolved in a mutually affecting manner, both functionally and embryologically." Lacalli: 2002 Sensory pathways in amphioxus larvae I. Constituent fibres of the rostral and anterodorsal nerves, their targets and evolutionary significance Acta Zoologica http://www3.interscience.wiley.com/journal/118935783/abstract Link to abstract only, but I have the PDF. Lacalli: 2002 Sensory pathways in amphioxus larvae II. Dorsal tracts and translumenal cells Acta Zoologica http://www3.interscience.wiley.com/journal/118861162/abstract http://www.ingentaconnect.com/content/bsc/azo/2003/00000084/00000001/art00001 Only abstract available online. I got the PDF through the library. Sensory systems in amphioxus: a window ... (PubMed) http://www.ncbi.nlm.nih.gov/pubmed/15353907 Entire online text: "Amphioxus has an assortment of cells and organs for sensing light and mechanical stimuli. Vertebrate counterparts of these structures are not always apparent, and a strong case can be made for homology in only a few instances. For example, amphioxus has anatomically simple but plausible homologs of both the pineal and paired eyes of vertebrates. Placodal and neural crest derivatives are, however, more problematic: the evidence for an olfactory system in amphioxus is only circumstantial and, despite the variety of secondary sensory cell types that occur on the body surface in amphioxus, none are obvious homologs of vertebrate taste buds, neuromasts or acoustic hair cells. A useful perspective can nevertheless be gained by examining differences in amphioxus and vertebrate development, specifically how each specifies and positions sensory precursors, controls their proliferation, and deploys them through the body. The much larger size of vertebrate embryos and the need to cope developmentally with increased scale and cell numbers may account for some key vertebrate innovations, including placodes and neural crest. The presence or absence of specific structural adaptations, like the latter, is therefore less useful for judging homology between amphioxus and vertebrates than shared features of specific cell types. It is also clear that the duration of embryogenesis in vertebrates has been significantly extended in comparison with ancestral chordates so as to incorporate events that would originally have occurred during the post-embryonic growth period, including events of neurogenesis. Consequently, no scenario for the origin of vertebrates can be considered complete unless it deals explicitly with the whole of the life history and changes to it." 2006 50<73 Anatomy of the Hesse photoreceptor cell axonal system in the central nervous system of amphioxus. CotA Amphioxus Sensory Nerves 160818 - 1219 |
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