THREE CRUISES OF THE “BLAKE.” 
116 
further examination showed these tufts to be bunches of minute 
spines enclosed each in a thick skin bag, resembling long- 
stemmed parasols with 
small shades. T'is struc- 
ture differs radically from 
that of the spines of all 
other ophiurans hitherto 
Fig. 402. 4. Fig. 403, 40, known where there is no 
Ophiohelus umbella. (Lyman.) departure from the single 
row of articulated spines. 
A bunch of these umbrella-shaped spines of Ophiohelus um- 
bella 1s given in Fig. 403. 
CRINOIDS.! 
The stalked crinoids are among the most interesting of the 
deep-sea animals. Their paleontological relations run back in 
the case of the Pentacrinoidea and the Apiocrinide (Rhizocrinus) 
to the jurassic period ; while the relationship of Holopus may 
probably: extend to the silurian (Edriocrinus). 
The Pentacrinide, of which four species were known from 
the Caribbean district, are characterized by the verticillate ar- 
rangement of the cirri along the whole length of the stem, 
while in the Bourgueticrini the whole stem even may be free of 
cirri. Recent species of Pentacrinide have been found both in 
the Pacific and Atlantic, and they are common at depths of less 
than 100 fathoms. The species of the genus Metacrinus (Fig. 
404) replace in the Pacific, to a certain extent, the Atlantic Pen- 
taerini. Our first accurate knowledge of the type dates from 
Miller, who compared the structure of the fossil species with 
that of both Pentacrinus asterius (Fig. 405) and the free Coma- 
tule. This relationship was subsequently most satisfactorily 
proved by J. V. Thomson, who in 1836 discovered the penta- 
erinoid stage of a species of Comatula. (Fig. 406.) 
There seems to be no special order in the division of the sec- 
ondary and tertiary arms of the Pentacrinide, though the dif- 
1 The account here given of the Cri- collections of the “Challenger” and 
noids is drawn up from the Reports “Blake” expeditions. 
made by Dr. P. H. Carpenter on the 
