RAYS AND BILATERAL SYMMETRY 



249 



Brisinga corona has probably been studied more frequently 

 and exhaustively than any other many-rayed species, first of all 

 by G. O. Sars, '75, and last by Ludwig, '97 ; and these authors 

 agree in the conclusion that the arms are all present from the 

 outset; that, in the words of Ludwig, *' ein nachtraglicher 

 Einschub von Armen nicht stattfindet." 



But there is some evidence that Heliaster as well as La- 

 bidiaster (and Pycnopodia also, as we now show) formS' an 

 exception to this rule. Rathbun, '87, points out that in H. 

 cumingii Gray, while the full number of rays is from thirty-five 

 to thirty-seven, one small specimen examined by him had only 

 twenty-four. 



And Leitpoldt, '95, has made essentially the same observa- 

 tion with reference to the same species, but carries the evidence 

 a step farther by the remark that his twenty-four-rayed small 

 specimens " noch zwei sprossende Arme besitzen, so dass eigent- 

 lich deren schon 26 vorhanden sind." This last statement not 

 only furnishes almost conclusive proof that in Heliasier rays 

 are added until far into adult life (in Leitpoldt's smallest speci- 

 men r = 20 mm.), but it also makes it probable that they are 

 added in pairs as they are in Pycnopodta. Therefore, from the 

 little direct evidence at hand and from the affinities of the 

 genera concerned, we are warranted in believing \}i\dX Heliaster 

 will be found to follow the same law of ray multiplication that 

 « prevails in Pycnofodia} 



The only case definitely known, until now, of the multiplica- 

 tion of rays during adult life in star-fish, is, as already mentioned, 

 that of Labidiaster radiosiis. In this species Perrier, '88, '91, 

 and '93, has shown : (i) that new rays are added until far into 

 adult life ; (2) that as many as six of these may bud from the 

 disc practically simultaneously ; and (3) that these may be dis- 

 tributed at rather regular intervals around the entire circumfer- 

 ence of the disc. 



Our studies on Pycno^odia raise a number of interesting 

 questions concerning the ray multiplication in Labidiaster : Do 



1 There appears to be general agreement among authorities that Pycnopodia 

 an^ Heliaster are rather more closely related than are Heliaster and Labidi. 

 aster. A. Agassiz, '77 ; Perrier, '93 ; Ludwig, '97 ; Studer, '84 ; Vignier, '78, etc 



