498 



ECHINODERMATA OPHIUROIDEA 



tinguished from Amphiura by its shorter arms and smoother 

 arm -spines. It lives in the interstices of hard gravel. The 

 British species, 0. balli, presents no special features of interest, 

 but the .Neapolitan 0. virens is an extraordinary form. It has six 

 arms, three of which are usually larger than the other three, for 

 it is always undergoing a process of transverse division, each half 



Fig. 220. Oral view of Ophiomusium pulchellum. x 7. (From Wyville Thomson.) 



regenerating the missing part. It has from 1 to 5 stone-canals, 

 the number increasing with age ; numerous long-stalked Polian- 

 vesicles in each interradius, and in addition a number of long 

 tubular canals which spring from the ring-canal, and entwine 

 themselves amongst the viscera. 1 All the canals of the water- 

 vascular system, except the stone-canals, contain non-nucleated 



1 Simroth, "Anatomie und Schizogonie der Ophiactis virens" Zeitschr. wiss. 

 Zool. xxvii. 1876, p. 452. 



