By H. Gee
The final reader with a few historical past in comparative anatomy, will locate this reference an informative dialogue on historic perspectives (eg, Garstang, Gislén, Romer, etc), now not so historic perspectives (eg, Jefferies), and glossy anatomical and molecular effects pertaining to how vertebrates arose. a large kinds of perspectives are supported, yet a few 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. fresh molecular facts 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 likely, and the sessility of tunicates is perhaps a derived trait. whereas powerful homologies among homeobox genes and organ structures 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 really from the arthropods. Molecular tools additionally point out that the amphioxus isn't really a degenerate vertebrate, yet primarily a primitive one, and elaboration of its good points ends up in the emergence of the vertebrate head.
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Extra info for Before the Backbone: Views on the Origin of the Vertebrates
In this section, if in no other, the distinction between vertebrates and craniates becomes important. 8): Craniates have a true 'head'. Much of the head develops from a special tissue called neural crest, which as its name suggests develops at the edges of the developing dorsal nerve tube along its length. Associated with the neural crest at the head end of the embryonic craniate are knots of ectodermal tissue called epidermal placodes. These placodes mark the sites of the special sense organs: the ears, nose and eyes.
Ideally, each node should connect no more than two species distally, in which case the cladogram is described as fully resolved. 7), but the reason for this failure lies often in insufficiency of data rather than inadequacy of method (for example, see Philippe et al, 1994). 7 A diagram showing two cladograms (a) fully resolved, with dichotomous branching throughout, and (b) with unresolved tri- and polychotomies. 7 CHORDATE CHARACTERISTICS Bearing all this in mind, we can be fairly confident that the chordates form a monophyletic group nested within the larger monophyletic group of the deuterostomes.
In addition to a sensory epidermal plexus similar to the nervous system of other echinoderms, there is a separate system centred on a mass of nervous tissue, the aboral ganglion, in the base of the calyx. In stalked forms, the aboral ganglion gives off a neural 'sheath', the peduncular nerve, that surrounds the chambered organ like the skin of a sausage. The peduncular nerve supplies side branches to the cirri, if present. 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.
Before the Backbone: Views on the Origin of the Vertebrates by H. Gee