114 



THREE CRUISES OF THE " BLAKE." 



Like other marine animals, ophiurans are distributed accord- 

 ing to the depth and temperature of the water. About one half 

 of the known species are confined to the zone between low-water 

 mark and 30 fathoms. These include the Medusa-heads ( Astro- 

 phyton). But not less than one tenth of the known living 

 forms are found entirely below 1,000 fathoms, and of these sev- 

 eral, such as OpMomusium Lymani,^ Ojihiocreas spinulosus, and 

 Ojjhiocamax hystrix, live in great colonies, just as some of the 



Fig. 399. — Sigsbeia murrhina. 1. 



shallow-water species do. They are found in various situations. 

 The localization of some of these is very marked. The stiff- 

 armed Ojyhiomusmm of deep water, with their swollen tubercu- 

 lous plates, naturally lie on the bottom, while other species with 

 supple or prehensile arms, such as Sigsbeia (Fig. 399) and As- 



1 All the way from Cape Hatteras to the extremity of George's Bank, OpMomu- 

 sium Lytnani was quite common in deep water. 



