Before the Backbone: Views on the Origin of the Vertebrates by H. Gee

By H. Gee

The final reader with a few history in comparative anatomy, will locate this reference an informative dialogue on ancient perspectives (eg, Garstang, Gislén, Romer, etc), now not so historic perspectives (eg, Jefferies), and sleek anatomical and molecular effects bearing on how vertebrates arose. a large kinds of perspectives are supported, yet a couple of conclusions are still formulated. Gislén's view of the carpoid as an echinoderm with chordate affinities is taken into account extra right than Jefferies' view of the carpoid as a chordate with echinoderm affinities. contemporary molecular proof helps chordates diverging from (echinoderms and hemichordates), and in the chordates, urochordates diverging from (cephalochordates and craniates). Larval paedomorphosis because the mechanism originating the vertebrates is not going, and the sessility of tunicates is maybe a derived trait. whereas powerful homologies among homeobox genes and organ platforms in either arthropods and vertebrates are stated, it really is famous that molecular equipment set deuterostome phyla truly except protostome phyla, suggesting the direct ancestry of the vertebrates isn't from the arthropods. Molecular equipment additionally point out that the amphioxus isn't a degenerate vertebrate, yet basically a primitive one, and elaboration of its beneficial properties ends up in the emergence of the vertebrate head.

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The aboral ganglion also supplies nerves to all the arms and pinnules. Like most of the other modern echinoderm groups, crinoids came to prominence in the Ordovician. There have been four major groups of crinoid: the camerates and flexibilians, which became extinct in the Permian; the inadunates, which just made it into the Triassic before they too became extinct, and the articulates, which survive today, and include both the stalked and stalkless forms. Although most modern articulates are stalkless, the unattached mode of life is not a modern innovation.

Again, the position of enteropneusts with respect to the nerve cord is under review). The body continues backwards dorsally, past the anus, into a tail that consists almost entirely of muscle, nerves, notochord and other structures connected with movement, but no viscera. In other animals, the body terminates with the anus or, if there are postanal parts to the body, they bear a variety of structures not necessarily connected with movement. The throat region or 'pharynx' of a chordate is, at some part of its life cycle, perforated by a number of paired gill-slits.

10). Although the heritage of the crinoids is no more ancient than that of any other modern group of echinoderm, the relative paucity of living forms set against the richness and diversity of fossil crinoids fosters the view that the living forms are somehow 'living fossils' themselves, primitive relics of times gone by. This impression is misleading. Although some extant genera date back to the Mesozoic, no living species of crinoid is known as a fossil from before the Pleistocene, and the unfamiliarity may be a symptom of the fact that the approximately 80 sea-lilies known today live at depths greater than 100 metres.

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