CLARK: THE STARFISHES OF THE GENUS IIELIASTER. 



59 



specimens having 34-3G rays, and only 42 per cent have an odd 

 number. 



Turning now from the amount of variability to the method of forma- 

 tion of new rays and the order of their appearance, we are favored by 

 the fact that in Heliaster the stomach is provided with five pairs of 

 conspicuous muscles attached to the ambulacral plates of five of the 

 rays, as in Asterias, and comparison of numerous specimens of all ages 

 leaves no doubt that these five rays are, as one would naturally suppose, 

 the original rays of the starfish on first assuming the adult form. This 

 arrangement is strikingly different from that shown by Pycnopodia, 

 where Ritter and Crocker 

 (1900) found that the post- 

 larval life apparently starts 

 with six rays. The youngest 

 available Heliaster (Jcubiniji), 

 20 mm. in diameter, has 12 

 rays but only eight of these Jit) 

 are at all nearly equally de- 

 veloped and it is fair to as- 

 sume that their arrangement 

 represents the normal con- 

 dition in an 8-rayed young 

 Heliaster. Numbering the 

 five original rays clockwise 

 from the madreporite, as the 

 specimen is looked at from 

 above, we find there is an 



accessory ray between rays 1 and 2, 2 and 3, 3 and 4. (Plate 8, fig. 1). 

 Adding now the four very young rays, in the positions where they occur, 

 we find there are now three between 1 and 2, two between 2 and 3, two 

 between 3 and 4, but there are still no rays between 4 and 5 or between 

 5 and 1 (Plate 8, fig. 2). Iu another young individual (kubinij'i) with 

 15 rays, we have the condition shown in fig. 3 (Plate 8), where it may 

 be seen that although there is now a ray between 4 and 5, 5 and 1 are 

 still side by side. The youngest polybraehius has 20 rays, four of which 

 are, however, very small ; in this specimen there are three well-developed 

 rays between 1 and 2 and also between 3 and 4, and 4 and 5, while 

 there are only two between 2 and 3 and none between 5 and 1. On 

 adding the four rudimentary rays, it is rather surprising to find that the 

 conditions in the interradii 2 and 3 and 5 and 1 are not changed, but 



if 20 « 23 2% 2t %S 2.6 27 aS 



Diagram 3. 



To show the relative abundance per thousand, of 

 H. canqpus with 20-27 rays. Based on 27 

 individuals. 



