ECHINODERMATA 
to 
1os) 
Oo 
Crass II. OPHIUROIDEA (Brittle Stars), 
CHARACTERISTICS.—Lchinodermata with a central disk bearing 
long slender arms, into the cavity of which no part of the 
alimentary canal is prolonged. There is no anus. The 
madreporic plate is ventral, and usually is an oral plate. 
There is no ambulacral groove, and the tube-feet are lateral in 
position. 
This class is allied to the Asteroidea, and is sometimes 
included with the latter in a single class. The Ophiuroids, 
however, differ from the 
Asteroids in the sharp dis- ie 
tinction between disk and germany 
arms,a condition approached ‘% % 
by Brisinga, m the absence 
of any digestive diverticula 
in the arms, in the ventral 
position of the madreporic 
‘ 
e WU AAda gant, MN aes 
plate, and in the almost khnantiiiee 
universal absence of pedi- 
cellariae. In the adult also 
the ectoderm is absent ex- 
cept on the tube-feet. FS 
The arms are long and 
slender, in most cases they 
are protected by four rows 
of plates, a ventral, a dorsal, and two lateral, the tube-feet 
protrude between the ventral and lateral; they have no 
ampullae. The nervous system has sunk under the skin, 
and is protected by the ventral plates. Dorsal to it is the 
radial blood-vessel, and dorsal to that the water-vascular 
vessel. In a transverse section of the arm, the greater part 
of the space 1s occupied by the ambulacral ossicles. Originally 
paired, these have fused and become single; they are grooved 
dorsally and ventrally. The dorsal groove lodges part of the 
coelom, the ventral the above-mentioned vessels and nerve 
cord. 
The mouth is armed with certain modified ossicles; it is 
central in position, and leads into a spacious stomach, which 
Fic. 135.—Ophiopholis bellis (upper surface). 
