. The causes and course of organic evolution; . erteans, and so to rep-resent possibly the first origin of gill structures. The somewhat diagrammatic Figs. 19 a, b, c, d and e will aiddescriptions. The slight or deep paired grooves with enlargedciliated cells in various Rhabdocoela, and the paired depressionsof the nemerteans, with their internal projecting ciliated lamel- 446 Causes and Course of Organic Evolution IcP, end blindly against or in the mesoderm substance. In theCyclostomata the depressions are continuous by their innerfaces ^\^th the pharyngeal cavity, while the branchial lamella
. The causes and course of organic evolution; . erteans, and so to rep-resent possibly the first origin of gill structures. The somewhat diagrammatic Figs. 19 a, b, c, d and e will aiddescriptions. The slight or deep paired grooves with enlargedciliated cells in various Rhabdocoela, and the paired depressionsof the nemerteans, with their internal projecting ciliated lamel- 446 Causes and Course of Organic Evolution IcP, end blindly against or in the mesoderm substance. In theCyclostomata the depressions are continuous by their innerfaces ^\^th the pharyngeal cavity, while the branchial lamellaeare represented by a series of vascular horizontal and parallelridges, radiating outwards along the roof, floor, and lateralwalls of each gill sac, and invested by an epithelium that ispartially ciliated. In lower Amphibia the gill pouches ordepressions are also continuous from without into the pharynx(Fig. 19e). In those genera that develop external gills first,these arise as dorsal lamellae, which instead of merely projecting a b —^ -J ep.
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