CLARK : THE STARFISHES OF THE GENUS HELIASTER. 



61 



The first indication of a new ray in Heliaster, which can he seen with- 

 out a microscope, is an internal one, simply the gradual separatidn of 

 the two halves of an interhrachial wall, close to the discohrachial wall. 



Diagram 5. 



To show the relative abundance per thousand, of II. cumin gii and //. polybracliius with 

 29-40 rays. Based on 83 individuals. 



There can be little reason to doubt that the actual first step in the new 

 ray formation is the pushing out of a bud from the outer side of the 

 circumoral, watervascular canal, and the growth of this bud with its 

 attendant tissues is the cause of the separation of the halves of the inter- 

 hrachial wall ; the bud itself becoming the radial water vessel. There is 

 no direct evidence in support of this hypothesis, but it is reasonable, in 

 line with the indirect evidence and open to no serious objection. After 



