260 



THE ZOOLOGIST. 



Ophiuroids. 



Ophiocoma nigra. — Three specimens with six rays each. Such 

 varieties are not uncommon {vide Bateson, loc. cit. p. 447). 



In numbering the rays of Asteroids it is proper to regard the 

 madrepore as being "between radii i and n, the former being 

 on its dextral and the latter on its sinistral side, while the 

 succeeding members of the series, viz. in, iv, v, &c, follow on 

 the sinistral side of n." (Gemmill, Proc. Eoy. Phys. Soc. Edinb. 

 vol. 18, p. 181, 1911.) 



Pig. 1, A. rubena ; 2, P. pulvillus ; 3, 8. papposus ; 4, S. endeca. 



This system has the further advantage of giving the develop- 

 mental order of sequence of the rays. Following this system, 

 we find that the anus is placed in interradius v/i in five-rayed 

 forms, or v/vi in many-rayed forms. On examining the seven- 

 rayed A. rubens and the six-rayed S. endeca, the anus was found 

 in both cases in interradius v, vi. This means that the variation 

 in the Asterias is towards the Solaster type, and the variation in 

 the Solaster, by omission of the latest developed rays, is a throw- 

 back towards the older Asteroid type. 



