By H. Gee
The overall reader with a few historical past in comparative anatomy, will locate this reference an informative dialogue on ancient perspectives (eg, Garstang, Gislén, Romer, etc), now not so ancient perspectives (eg, Jefferies), and sleek anatomical and molecular effects referring to how vertebrates arose. a large different types 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 likely, 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 sincerely 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 gains ends up in the emergence of the vertebrate head.
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Additional resources for Before the Backbone: Views on the Origin of the Vertebrates
Echinoderms generally have an internal skeleton of calcite plates. The shapes and sizes of these plates are quite variable: in sea-urchins or echinoids they meet to form a complete test, but in most sea-cucumbers or holothurians they have all but disappeared, reduced to small needle-like spicules. But whatever its final shape, each plate is made from a single crystal of calcite. 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.
Neural crest-derived and epidermal placode tissues have been regarded as the embodiment of most of the things that are characteristically craniate (Cans and Northcutt, 1983). The pharyngeal arches are modified for ventilation rather than filterfeeding^^ Unlike other chordates, craniate pharyngeal clefts lack ciliated epithelia (that is, they have been secondarily lost). There is a well-developed and regionated brain, housed in its own cartilaginous box, the cranium. The braincase is essentially derived from mesoderm, but much of the outside of the skull, derived as it is from neural crest, is ectodermal in origin, or derives from the interactions between ectoderm and mesoderm (see Hanken and Thorogood, 1993, for a recent review).
The anus migrates forwards along the ventral midline, so the gut becomes a Ushape, but the mouth sidesteps leftwards, so that it emerges somewhat on the left-hand side above but slightly behind the anus, in front of the left somatocoel and close to the left hydrocoel. The coelomic compartments stay as they were, except that the left hydrocoel snakes itself round the oesophagus until it joins itself to form a hollow ring around it, the water ring - the germ of the entire water-vascular system. The connection between the left hydrocoel through the left axocoel to the hydropore becomes the stone canal.
Before the Backbone: Views on the Origin of the Vertebrates by H. Gee