By H. Gee
The overall reader with a few heritage in comparative anatomy, will locate this reference an informative dialogue on old perspectives (eg, Garstang, Gislén, Romer, etc), now not so ancient perspectives (eg, Jefferies), and glossy anatomical and molecular effects bearing on how vertebrates arose. a large forms 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 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 likely, and the sessility of tunicates is perhaps a derived trait. whereas robust homologies among homeobox genes and organ structures in either arthropods and vertebrates are stated, it's famous that molecular equipment set deuterostome phyla basically except protostome phyla, suggesting the direct ancestry of the vertebrates isn't 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 positive aspects results in the emergence of the vertebrate head.
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Extra info for Before the Backbone: Views on the Origin of the Vertebrates
455) The carpoids, therefore, become the first offshoot from Smith's cladogram. This poses a problem, because carpoids have some decidedly unechinoderm features all their own. He continues: However, Jefferies (1981)^^ believes that Stylophora [cornutes + mitrates] show evidence of gill slits and a post-anal tail and should therefore be classified as stem chordates. If this proves to be correct then the other carpoids may be stem chordates, stem echinoderms, or stem (chordates plus echinoderms).
H) The enigmatic Eldonia ludwigi from the Burgess Shales, possibly a holothurian (photograph courtesy of Duncan Friend, University of Cambridge), (i) A tableau of stalked Palaeozoic echinoderms. The fourth from the left is a solute (from Wells et al, 1931). (j) Evolutionary diversification of early Palaeozoic echinoderms interpreted in terms of feeding strategy and habitat. Stipple indicates unconsolidated substratum, diagonal hatching, hard substratum (from Smith, 1990, by permission of the author and the Systematics Association).
Running along the floor of the pharynx is a groove filled with ciliated and glandular strips. This structure, called the endostyle, secretes mucus and is rich in iodine-containing compounds. Neither enteropneusts nor pterobranchs have an endostyle. These, then, are the main characteristics of chordates. Craniates and lancelets bear all these features as adults. Tunicates, though, have lost most of them by the time they reach sedentary adulthood. Although they have serially repeated pharyngeal slits, adult tunicates show no trace of somitic segmentation, and it is moot whether the groups of muscle cells in the microscopic tadpole larvae (Lankester, 1882; Crowther and Whittaker, 1994) can be regarded as true somites.
Before the Backbone: Views on the Origin of the Vertebrates by H. Gee