EMBRYOLOGY. 



729 



Miiller and Metschnikoff, are remarkable for the long process {Fig. a,?) devel- 

 oped in very earl} 7 stages {Figs. 59, 60), and which seems, as far as known, 

 eminently character- fig. C6. 



istic of the Spatan- 

 gina. The embryos 

 of Schizaster {Figs. 

 59-61. copied from 

 Miiller) differ very 

 strikingly from the 

 pluteus of the regular 

 Echini. The small 

 size {Fig. 61) of the 

 arms and their uni- 

 form size are in 

 marked contrast to 

 the extraordinary de- 

 velopment of some 

 of the arms in Ar- 

 bacia {Fig. 66) and 

 Strongylocentrotus 

 ( Fig. 55 )■ This seems 

 to be the final condi- 

 tion of the pluteus 

 in the oldest known 

 Spatangoid embryo, 

 in the pluteus of Schizaster, as well as in another Spatangoid pluteus {Fig. 

 68), probably Echinocardium. 



Fig. 67. 



The pluteus of the Clypeastroids is not known with certainty. Miiller 

 iio-ures in bis seventh Memoir (Plate VIII., fig. 4) a larva which he considers 



