SHALLOW-WATER STARFISHES 15 



Variations from the five-rayed condition occur not very rarely 

 among living Crinoids, and were not uncommon among the ancient 

 fossil pentremites and cystideans ; ' but on the whole the crinoids 

 have been pretty constantly five-rayed through all the geologic ages. 



We 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, otherwise similar 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 number 

 of ambulacral sucker-feet, it might well be the form preserved by 

 natural selection, unless for some other important but unknown 

 reason, the five-rayed condition has certain other more important 

 advantages. 



It is certainly true that most of the shallow-water species with 

 multiple rays live among rocks in situations exposed to the surf. 

 This is true of the seven species of Heliaster, with very numerous 

 rays; and of Pycnopodia with twenty to twenty-four rays, and of 

 the various shallow-water and littoral species of Solaster and Cross- 

 aster, which usually have nine to fifteen rays (rarely eight or less). 

 It is also true of the numerous six-rayed species of Asterias, 

 Pisaster and allied genera.' 



However, the ability to cling tenaciously to rocks may be perfected 

 in other ways, involving equally an increased number of 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 

 some large species of Pisaster; and by increasing the size and 

 strength of the suckers. 



Although it seems probable that the added protection gained 

 against the violence of the waves by the evolution of more numerous 

 rays and suckers is a real cause for the retention of this feature, it 

 may not be the principal one. The attainment of four rows of sucker- 



' Some of the paleozoic Cystidea were tri-radial. Hence it has been thought 

 by some writers that this was the more primitive condition and that the 

 pentamerous condition was acquired by the addition of another pair of rays. 

 See Bather, op. cit., 1901. 



'The family Brisingidae, however, is mostly confined to deep water. All the 

 species are multirayed, with long rays. Some of them climb over gorgonians, 

 as do many ophiuroids. The genus Labidiaster, of the Patagonian region, is 

 found in shallow water among rocks and sea-weeds. It has numerous long 

 rays. 



