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 
Chetaster, the plates of 
the actinal surface being 
erowded 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, distinetly 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 pedicellariz 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. 3. (Perrier.) 
