By H. Gee
The overall reader with a few history in comparative anatomy, will locate this reference an informative dialogue on old perspectives (eg, Garstang, Gislén, Romer, etc), now not so old perspectives (eg, Jefferies), and glossy anatomical and molecular effects referring to how vertebrates arose. a large forms 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 perhaps a derived trait. whereas robust homologies among homeobox genes and organ platforms in either arthropods and vertebrates are said, it's famous that molecular equipment set deuterostome phyla essentially 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 really a degenerate vertebrate, yet primarily a primitive one, and elaboration of its positive factors 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
Thus, the monophyly of deuterostomes is tested with reference to a creature that we know is not a deuterostome, such as a flatworm or cnidarian, which we can assume to be primitive with respect to deuterostomes in all the features in which we are interested. This 'reference' species is called an 'outgroup'. The process is called outgroup comparison, and is a vital part of reconstructing phylogenies on Hennigian principles. It is important to note that in cladistics, 'primitiveness' is relative, and does not imply simplicity of organization or crudeness of adaptation.
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. Most hemichordates have pharyngeal slits but lack the other components that make up the distinctive chordate pharynx, such as the endostyle.
Another distinctive feature of echinoderms is the water-vascular system, derived from the left mesocoel. A doughnut-shaped 'water ring' around the central mouth is connected by a tube called the stone canal (on account of its impregnation with calcite in some species) to the surface of the animal through an opening called the hydropore, which perforates a platelike structure, the madreporite. From the water-ring, tubes extend into (typically five) radial fields or ambulacra, which extend along the arms or, if the animal has no arms as such, over the surface of the body.
Before the Backbone: Views on the Origin of the Vertebrates by H. Gee