208 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



adults have the full complement of rays characteristic of a species. 

 In some forms there are individuals with smaller immature arms 

 but it is not certain that these are newly introduced rays, or that 

 "this disparity in size is due to the regeneration of the halves of 

 automatically bisected animals" (p. 248). It is true that certain 

 species start with the full complement of rays and introduce none 

 in later growth. Others certainly deviate from this rule, and a 

 species with as many as 37 rays has one small specimen with only 

 24 plus 2 young sprouting rays. It is therefore plain that ''in 

 Heliaster rays are added untU far in adult hfe," and it is also probable 

 that ''they are added in pairs" (p. 249). In Lahidiaster as many as 

 six new rays may bud simultaneously at mtervals around the entire 

 circumference of the disk. 



An adult Pycnopodia has from 20 to 24 rays, and "this variation 

 does not depend upon the size of the specimen. * * * The 

 number is generally even, * * * but a few specimens with 21 

 have been found" (p. 250). The smallest specimen had six equal 

 arms and two much smaller budding rays, one larger than the other. 

 Between these two extremes all intervening stages occur. At least 

 the original five and probably six rays are of larval origin, all the 

 others being postlarval. When the sixth ray is introduced is un- 

 known; the rest as a rule appear in bilateral order — that is, in pairs. 

 They are all developed "in a budding area adjacent to I and II" 

 on the side toward the sixth ray until late in adult life. When the 

 arms are of unequal number it is due to the suppression of a ray. 

 In Pycnopodia, therefore, ray multiphcation goes on tlirough a con- 

 siderable part of adult life, but not the whole of it, and is variable 

 with the individuals. All of the rays, whether of larval or post- 

 larval origin, have the same structural characters. 



Finally, it seems that the sixth ray in Pycnopodia has its origin 

 in, or at least "is in some intimate way related with the larval organ 

 of the embryo" (p. 268). Therefore multiradiation among asterids 

 has its origin in late larval hfe. 



In Heliaster, Clarke states,^ "the formation of new rays is funda- 

 mentally different from that in Pycnopodia.^' They do not develop 

 in distinct generations, but appear entirely independently of each 

 other. "A considerable number may develop at approximately the 

 same time, often as many as six or seven and sometimes eight or 

 nine in H. polyhracJiius, but they show no definite relation to each 

 other." 



The evidence seems "to show that they tend to arise in aU four 

 quadrants of the circumference of the starfish about equally, but 

 successively rather than simultaneously." 



1 H. L. Clarke, Bull. Mus. Comp. Zool., vol. 51, 1907, pp. 25-76. 



