No. 513] 



STAB-FISHES 



551 



common than in either of the other classes, as already in- 

 dicated. But why this is so is a perplexing problem in 

 evolution. 



No doubt there are some advantages in the five-rayed 

 condition, or else it would not have remained so constant 

 through all the geologic ages. But it is equally certain 

 that it is more advantageous for certain starfishes, in 

 their special environments, to have six or more rays, 

 otherwise they would not have retained this condition. 

 AVe must conclude that all these variations originated, at 

 first, as " sports," which have persisted by heredity and 

 natural selection, because they were advantageous. It is 

 easy to conjecture that, in the case of two starfishes, simi- 

 lar in size and structure, living together on a rocky shore 

 and exposed to violent surf, the one with six rays would 

 be able to cling more securely to the rocks than the one 

 with five rays. Therefore, because of the increased num- 

 ber of ambulacral sucker-feet it might well be the form 

 preserved by natural selection, unless for some other im- 

 portant but unknown reason, the five-rayed condition has 

 certain other more important advantages. 



It is certainly true that most of the species with mul- 

 tiple rays live among rocks in situations exposed to the 

 surf. 13 This is true of the various shallow water and 

 littoral species of Solaster and Crossaster, which usually 

 have nine to thirteen rays (rarely eight or less). It is also 

 true of the several species of Heliaster with very numer- 

 ous rays, and many other such species, as well as the 

 numerous five-rayed and six-rayed species of Astenas 

 and Pisaster. 



But the power of effectually clinging to rocks may be 

 perfected in other ways, involving an increased number o 

 sucker-feet. This is often attained by lengthening the 

 rays, as in many species of Asterias; by crowding the 

 suckers into more than four rows, as in Pycnopodia and 

 some large species of Pisaster ; and by increasing the size 

 and strength of the suckers. 



