110 THREE CRUISES OF THE " BLAKE." 



Besides the peculiarity of branching arms (Astrophyton cce- 

 cilia, Fig. 388) which distinguishes some of the genera, the As- 

 trophytidse have characteristic joints in the arm-axis, which sepa- 

 rate them from the Ophiuridae. They are also usually covered, 

 not by conspicuous plates of lime carbonate, but by a leathery 

 skin (Ophiocreas spinulosus, Fig. 389). The typical Ophiu- 

 ridse have a well-marked central disk covered with plates or 

 scales, and from it radiate five arms encased in four longitudinal 

 rows of plates (Ophiozona nivea, Fig. 390). The side arm- 



Fig. 390. Ophiozona nivea. 



plates bear spines, which may lie close along the arm (Ophio- 

 phyllum petilum, Fig. 391), or stand out from it at a strong 



Fig 391. Ophiophyllum petilum. 



angle (Ophiocamax hystrix, Fig. 392). There is an almost 

 endless variety in the shape, consistency, number, and size of 



