56 bulletin: museum of comparative zoology. 



The Number of Rays and the Order of their Succession. 



The large number of rays in Heliaster is one of the most interesting 

 features of the genus, but owing to the scarcity of material almost noth- 

 ing has been done in the way of investigating the amount of variabil- 

 ity in this character or the order in which the successively new rays 

 appear. In 1872, Liitken showed that there is no correlation between 

 size and the number of the rays in Heliaster, after a certain size (about 

 100 mm. in diameter), which we may call that of maturity, is reached ; 

 that is to say, very small specimens have a relatively small number of 

 rays and this number increases with increasing size, only until the ani- 

 mal is approximately mature, after which there may or may not be a 

 continued addition of new rays. Having only 15 specimens (H. helian- 

 thus) for comparison and only one of those less than 75 mm. in diameter, 

 Liitken did not attempt to discuss the original number or the sequence 

 of the rays, but it is hard to understand how any one could examine his 

 data and not see that the number of rays certainly does increase after 

 larval life and even after the starfish is 50 mm. across. Rathbun (1887) 

 in his report on Heliaster makes statements in regard to cumingii which 

 indicate his belief that the rays increase in number with increasing age 

 (see p. 441, line 8). In spite of these writers, however, Perrier, as late 

 as 1893, states that Labidiaster is the only starfish in w T hich additional 

 rays develop after the larval period is passed and the adult form as- 

 sumed. In 1895, Leipoldt referred to the presence of two young rays in 

 a specimen of H. cumingii (—polybrachius), about 50 mm. in diameter, 

 which had otherwise only 24 rays. In 1900, Ritter and Crocker showed 

 conclusively that Pycnopodia begins its post-larval life with only six 

 rays, and that the additional 14-18 rays are in process of appearance, nor- 

 mally in pairs, until well into adult life. There can no longer be any 

 question therefore that starfishes with twenty or more rays begin their 

 post-larval life with a much smaller number and continue to add new 

 rays for an undetermined period. Consequently specimens of Heliaster 

 with fewer than twenty rays are sure to be met with and if age and size 

 are disregarded, we cannot assign on a priori grounds the minimum 

 number which a starfish of this genus may show. The smallest speci- 

 men among the 346 examined measures only 20 mm. in diameter, and I 

 can find no published record of any specimen nearly as small. It is a 

 young individual of hubiniji (U. S. 1ST. M. No. 21950) from Lower Califor- 

 nia and 'has 12 rays, eight well developed, three much smaller and a 

 twelfth barely started. With it are two other specimens, 25 mm. in 



