CLARK: THE STARFISHES OF THE GENUS HELIASTER. 69 



Turning now to the external features of Heliaster, we find, as is well 

 known, that the abactinal skeleton, the papulae, the pedicellariae, and 

 the armature of the adambulacral plates are essentially the same as in 

 Asterias. It has commonly been stated also that the two genera are 

 alike in the quadriserial arrangement of the pedicels. As a matter of 

 fact, however, the real arrangement of the pedicels in Heliaster is quite 

 different from what is found in Asterias, for while a quadriserial arrange- 

 ment does occur in some species of Heliaster, it is virtually confined to 

 the middle portion of the ray, while in other species it is hardly correct 

 to speak of a quadriserial arrangement at all. These various conditions 

 are shown on Plate 7 from which it will be seen that although in the 

 middle of the ray there is a distinctly quadriserial arrangement in micro- 

 brachius (Fig. 11), in kubiniji (Fig. 9) that is scarcely the case. At the 

 base of the ray the arrangement is unqualifiedly biserial in all the species 

 (Fig. 10), at least for the first ten or twelve pairs of pedicels. In young 

 individuals (Fig 12), the biserial arrangement is marked even at the 

 middle of the ray. This condition is certainly perplexing if Heliaster is 

 merely an Asterias with numerous rays, for if that were the case, the spe- 

 cies with the fewest rays (kubiniji) ought to show most clearly the quad- 

 riserial arrangement, while a young individual with only 17 rays certainly 

 ought to have the same arrangement well marked. As we have just seen, 

 the reverse is the case. However, it seems probable that increase in the 

 number of rays, in a species having four rows of pedicels, with the conse- 

 quent lateral crowding, would lead to radial extension, which would re- 

 sult in the quadriserial arrangement gradually becoming irregularly, and 

 finally perfectly, biserial, as we find it at the base of the rays in Helias- 

 ter. That such a result does follow an increase in the number of rays in 

 a species with the quadriserial arrangement of the pedicels, is shown by 

 Coscinasterias calamaria (Gray) (Fig. 13), where the first two or three 

 pairs of pedicels of each ray are arranged in a single series on each side. 

 If, however, we are to assume that the change here first indicated in 

 Coscinasterias is continued in Heliaster to a far greater extent, we shall 

 have to admit that it is carried to different degrees of completeness in 

 the different species. It seems to have gone further in the species with 

 the narrower, freer, and more cylindrical rays, where the quadriserial ar- 

 rangement is nearly obliterated, than in those with broader and flatter 

 rays, where the pedicels still appear to be in four series at the middle of 

 the ray. Apparently, after there are 15-20 rays, the change to a biserial 

 arrangement of the pedicels is not promoted so much by the number or 

 degree of coalescence of the rays, as by their form and width. 



