116 THREE CRUISES OF THE "BLAKE." 



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. This struc- 

 ture differs radically from 

 that of the spines of all 

 other ophiurans hitherto 



Fig. 402. f. Fig. 403. .so. kn <>wn where there is no 



Ophioheius umbeiia. (Lyman.) departure from the single 



row of articulated spines. 



A bunch of these umbrella-shaped spines of Ophioheius urn- 



bella is given in Fig. 403. 



CRINOIDS. 1 



The stalked crinoids are among the most interesting of the 

 deep-sea animals. Their palseontological relations run back in 

 the case of the Pentacrinoidea and the Apiocrinidse (Rhizocrinus) 

 to the Jurassic period ; while the relationship of Holopus may 

 probably extend to the silurian (Edriocrinus). 



The Pentacrinidae, 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 Pentacrinida3 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- 

 tacrini. 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- 

 tulse. This relationship was subsequently most satisfactorily 

 proved by J. V. Thomson, who in 1836 discovered the penta- 

 crinoid 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 Pentacrinidse, 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 



