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 Ophiomusium Lymani,' Ophiocreas spinulosus, and 
Ophiocamax 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 Ophiomusium 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, Ophiomu- 
sium Lymani was quite common in deep water. 
