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 ancestors. " 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 established. 



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 ; v. p., 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 



