306 



ERYLUS ROTUNDUS. 



DIMENSIONS OF ADULT .\SPIDASTERS. 



The youngest asjiidasters observed were oval or circular discs, about half 

 the size of the adult, and composed of numerous, long, exceedingly slender, 

 perfectly straight rays, lying nearly in the same plane, and radiating from an 

 irregular, lobose structure 2-4 ji in diameter. This lobose centrum is appar- 

 ently composed of short, terminallj^ rounded concentric rays, arising obliquely 

 to the plane occupied by the long rays. The rays, which in this young stage 

 appear to be isolated throughout, grow in length and in thickness. Their 

 longitudinal growth leads to an increase in the size of the aspidaster, their 

 transverse growth (thickening) to a coalescence of the rays themselves, which, 

 as the growth continues, progresses from the centre towards the margin of the 

 disc. Thus the aggregations of isolated, radial vsiys become larger, solid discs 

 with serrated margins. This goes on until the spicule has attained nearly its 

 full size. Then the longitudinal growth of the rays ceases, while their lateral 

 gro\\i;h continues. This leads to a fiUing up of the serrations and to the forma- 



