CHARACTERISTIC DEEP-SEA TYPES. STARFISHES. 105 



In 1874, Sir Wyville Thomson described Zoroaster, discovered 

 by the " Challenger," a 

 genus remarkable for the 

 thickness and regularity of 

 the skeleton. The " Blake " 

 dredged two interesting 

 species of this genus ; the 

 one, Zoroaster Sigsbeei, 

 with large ossicles of the 

 disk and most distinct 

 arms ; the other, on the 

 contrary, Zoroaster Ack- 

 leyi (Fig. 382), with arms 

 and disk united, giving it 

 an external resemblance to 

 Chsetaster, the plates of 

 the actinal surface being 

 crowded with small flat- 

 tened spines, recalling 

 Luidia, the tentacles in four rows at the base and two rows 

 at the tip ending in a minute disk. 



Hymenodiscus Agassizii (Fig. 383) belongs to an interme- 

 diate type far more pronounced even than Brisinga. It recalls 

 the ophiurans by its round disk, distinctly separated from the 

 arms, which are long, slender, and mobile, furnished with a 

 lateral row of spines, as in the ophiurans, which may serve as 

 organs of locomotion. But there are twelve arms in these star- 

 fishes, while there are not more than six, or sometimes eight, in 

 ophiurans. The disk is membranous (Fig. 384), with a circle 

 of ossicles formed from the first joint of the arms. The skele- 

 ton of the arms is most simple, consisting of four longitudinal 

 series of pieces ; each piece carries a long lateral spine (Fig. 

 385), covered by a smooth sheath swollen at the extremity, and 

 a cluster of pedicellarise such as characterize the starfishes. The 

 true starfish ambulacral pieces are wanting in Hymenodiscus. 

 The dorsal skeleton of Brisinga may be considered as only a 

 shield of the genital glands, which are similar in their structure, 

 as is the digestive cavity, to the same organs of the ophiurans, 



Fig. 382. Zoroaster Ackleyi. j. (Perrier.) 



