14 VERRILL 



The following species, which are normally five-rayed, have been 

 found with six rays : 



Henricia leviuscula (St.), from Alaska. 



Henricia sanguinolenta (Mull.), Bering Sea. 



Pisaster ochraceus (Br.), Monterey, Calif., and British Columbia. 



Patiria miniata (Br.). Six-rayed common; four-rayed and seven-rayed rare; 



California. 

 Dermasterias imbricata (Gr.), Sitka. 



When we consider the great geological antiquity and remarkable 

 persistence of the five-rayed condition in echinoderms generally, 

 it is very remarkable that so many genera and species of existing 

 starfishes should have acquired the peculiarity of having higher 

 numbers. It seems natural to conclude that there must be some 

 .special advantage gained by this increase in rays, which does not 

 obtain in the other classes to the same extent. 



At the present time all known species of sea-urchins and holo- 

 thurians are normally five-rayed, and the same holds good through 

 all the geologic ages for the former class at least. Abnormal sea- 

 urchins with four and with six rays have been observed in a number 

 of species. Dr. Robert T. Jackson, in his recent extensive work,' 

 has discussed this and various other variations of sea-urchins very 

 fully. He states that he has personally studied seventy-one cases 

 of more or less complete variations from the perfect five-rayed condi- 

 tion. To find these required the special examination of 50,000 speci- 

 mens, including many species. 



In some species about one individual in a thousand shows varia- 

 tions of this kind.* 



The serpent-stars (Ophiuroidea) show more variation in the 

 number of rays than the sea-urchins do. Several genera contain 

 some species that are normally six-rayed. The common Ophiocoma 

 pumila of Bennuda and the West Indies has about equal numbers of 

 five-rayed and six-rayed individuals living together. Many of the 

 species of Ophiactis have six to eight rays while young, and divide 

 autotomously, but when mature they are usually either regularly 

 five- rayed or six-rayed. Yet some of the species of the same genus 

 are always five-rayed and do not divide ; so it does not seem to be a 

 matter of great importance in this genus. 



' Phylogeny of the Echini, Mem. Boston Soc. Nat. Hist., vol. x, 1912. 

 Variations from the pentamerous system are described on pp. 35-50. 



' See also A. E. Verrill, Amer. Naturalist, vol. xlui, p. 545, for a discussion 

 of the subject with figures. 



