34 



BULLETIN 



UNITED STATES NATIONAL MUSEUM. 



The rays terminated in a single ossicle with an unpaired tube-foot 

 that later developed at its outer end a pigment spot and finally an 

 "eye." 



At last the animal lost the stalk and moved about freely as the 

 primitive asterozoon. The flooring plates changed into ambulacraUa 

 and the roofing plates into adambulacraha, while the tube-feet were 

 used not for grasping and crawhng but for passing small particles of 

 food to the mouth. 



Figs. 1 and 2.— Ventral and dorsal views of theoretic phylembryo of Stelleroidea, indicating 

 how the rays are introduced. based on hudsonaster. j4(f, adambulacrals; am, ambula- 

 crals; ^i, marginal axillaries; ci),centro-dorsal; 7n/,inframarginals; jri, primordial radials: 

 r2 to rll, subsequent radials; sml, dorsal interradials or primordl^l supramarginals; sms 

 to smlo, subsequent supramarginals. 



The asterid radicle. — Hudsonaster is held to be very near the radicle 

 that gave rise through modification and inheritance to all subsequent 

 Stelleroidea. A study of the various species and specimens of Hud- 

 sonaster, reinforced by the chronogenesis of this genus and the de- 

 velopmental stages in the individuals, which are discernible in the 

 youthful distal ends of the rays Avhen contrasted with the mature 

 proximal region of the same, has led the writer to speculate on the 

 probable skeletal characters of the radicle of the Stelleroidea. An 

 analysis of mature H. matutinus and of half-grown H. incomptus 

 shows that their ancestors must have been devoid of all disk accessory 

 pieces, or for that matter of all secondary ossicles, since it appears 

 that it is on the disk that these plates first arise. Further, the 

 same material indicates that the younger the individual specimen, 

 or the older the species geologically, not only the smaller is the 

 specimen, but the fewer plates has it in any column. As the 



