DEVELOPMENT OF ECHINODERMS. 



261 



Class BLASTOIDEA. Wholly extinct. 



The Blastoids are first found in the Upper Silurian, later than Cystoids 

 and Crinoids ; they had their golden age in the Carboniferous and 

 Devonian times, but then disappeared. Their body was ovate, with 

 five ambulacral areas, with each groove of which jointed pinnules were 

 associated. 



Class CYSTIDEA. Wholly extinct. 



The Cystidea 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. 



DEVELOPMENT OF ECHINODERMS. 



The ovum undergoes total segmentation, and a hollow 

 hall of cells or blastosphere results. A typical gastrula is 

 formed by invagination. 



FIG. 123. Stages in development of Echinoderms. After Selenka. 



i. Section of blastula of Synapta digitata (Holothuroid), with a hint of 

 gastrnlation. 2. Section of gastrula of Toxopneitstes brevispinosu; (sea- 

 urchin) ; cc., ectoderm; en., endoderm ; ///., segmentation cavity with 

 mesenchyme cells in it. 3. Section of larva of Asterina gibbosa. (star- 

 fish) ; Bl., blastopore ; g: , archenteron; ~'.p., vaso-peritoneal vesicle; 

 r. and /., right and left sides. 



The mesoblast has a twofold origin : (a) from " mesen- 

 chyme " cells, which immigrate from the invaginated endo- 

 derm into the segmentation cavity ; (I)) from the outgrowing 

 of one or more ccelom pouches (vaso-peritoneal vesicles) 

 from the gastrula cavity or archenteron. From these 

 .vesicles the body cavity and the rudiments of the water 

 vascular system arise. 



The larva is, first of all, a slightly modified, diffusely 



