MONOGRAPH OF THE EXISTING CBINOIDS, 



163 



new plate formed ^vill develop along the same lines as the plates abeady present 



in that zone. . .• i r i 



The zones of similar skeletal potency of the echinoderm are not entire y radial 



as has commonly been assumed, but are chiefly concentric about the dorsal pole as 



FIO. 102. 



... m.m.-m, noK3...-.w or . spkc.m.. o. s.ko.o„.™. o..^;;^:;;^ rr™;r.TKor.xr:Hr 



a center a circumstance which is at once explained when we remember the homology 

 between the sick« of a crinoid from the doi.al aspect to the ventral perisome with 

 the sides of an insect or crustacean. 



