DE VELOPMENT OF ECHINODERMA TA. 219 



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 Cystoidea. Wholly extinct. 



The Cystoids are first found in Lower Silurian rocks, had 

 their golden age in Upper Silurian times, and died out in 

 the Carboniferous ages. Their body was ovate or globular, 

 sessile or shortly stalked, covered with polygonal plates often 

 irregularly arranged. They seem usually to have borne two 

 to five feeble, unbranched arms. 



Both Cystoids and Blastoids seem to have been half 

 smothered in lime, and perhaps this is in part the explana- 

 tion of their extinction. We cannot describe their structure 

 here, but it is important to recognise that the Cystoids seem 

 to occupy a central position in the group. 



Development of the Echinodermata. 



As the embryology of Echinodermata presents many 

 difficulties, it will be convenient to begin with the most 

 general results. 



The ovum undergoes total segmentation, and a hollow ball 

 of cells results. Apart from two alleged cases of delamination, 

 a gastrula is always formed by the invagination of this 

 blastosphere. Ectoderm and endoderm, or epiblast and 

 hypoblast, are thus established. 



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

 chyme" cells, which are separated from the invaginated 

 hypoblast, and migrate into the segmentation cavity ; {b) by 

 the outgrowing of one or more ccelome pouches from the 

 gastrula-cavity or archenteron. It is thus that the body-cavity 

 and the rudiments of the water-vascular system arise. 



