. Outlines of zoology. Zoology. 242 ECHINODERMA. Class Cystoidea. Wholly extinct. The Cystoids are first found in the Lower Silurian rocks, had their golden age in Upper Silurian times, and died out in the Carboniferous period. Their body was ovate or globular, sessile or shortly stalked, covered with polygonal plates often irregularly arranged. Some (according to Bell, the more primitive) types were "never fixed, and had not fixed ; They seem usually to have borne two to five feeble, unbranched arms. Development of Echinoderms. The ovum undergoes total segmentation, and a
. Outlines of zoology. Zoology. 242 ECHINODERMA. Class Cystoidea. Wholly extinct. The Cystoids are first found in the Lower Silurian rocks, had their golden age in Upper Silurian times, and died out in the Carboniferous period. Their body was ovate or globular, sessile or shortly stalked, covered with polygonal plates often irregularly arranged. Some (according to Bell, the more primitive) types were "never fixed, and had not fixed ; They seem usually to have borne two to five feeble, unbranched arms. Development of Echinoderms. The ovum undergoes total segmentation, and a hollow ball of cells or blastosphere results. Apart from two alleged cases of delamination, the gastrula is always formed by the invagination of this blastosphere. Ectoderm and endoderm, or epiblast and hypoblast, are thus Fig. 105.—Stages in development of Echinoderms.—After Selenka. 1. Section of blastula of Synapta digitata (Holothuroid), with a hint of gastrulation. 2. Section of Gastrula of Toxopneustes brevispinosus (sea- urchin) ; ec., ectoderm ; en., endoderm ; m., segmentation cavity with mesenchyme cells in it. 3. Section of larva of Asterina gibbosa (star- fish ) ; BL, blastopore ; g., archenteron ; , vaso-peritoneal vesicle ; r. and I., right and left sides. The mesoblast has a twofold origin: (a) from " mesen- chyme " cells, which immigrate from the invaginated hypo- blast into the segmentation cavity; (b) by the outgrowing of one or more coelom pouches from the gastrula cavity or archenteron. It is thus that the body cavity and the rudiments of the water vascular system arise. According to Hertwig's fundamental thesis, this double origin is a primitive condition, and the mesenchyme here, as always, is non-epithelial, and gives rise to the connective tissues and to the vascular system. On the other hand, it has been asserted that in Echinoderms the mesenchyme is. Please note that these images are extracted from scanned page images th
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