ECHINODERMATA, ELEY 
tain any prolongations from the stomach, nor have their under 
surfaces excavated into grooves for the protrusion of ambu- 
lacral tube-feet. The arms, in fact, are not prolongations or 
lobes derived from the body itself, but are special appendages 
added for purposes .of locomotion and prehension. The arms 
are very much larger than the diameter of the disc, and are pro- 
tected by four rows of calcareous plates—one above, one below, 
and one on each side. Inthe centre of each arm is a row of 
en 
a 
KY me 
LINE 7a ae 
aE At 
IP ASS MIMSY Zam a ; 
Sy ® 
a 
i) 
y 
ca bi 
H Pay 
Uy sp i 
N Ki ZF r SS 
Gy a = 
: an x A A b 
Soo A 
Fig. 45.—Ophiuroidea. «a Ophiura texturata, the common sand-star; 
b Ophiocoma neglecta, the grey brittle-star (after Forbes). 
calcareous pieces which form a kind of internal axis or skeleton, 
below which is placed the radiating ambulacral vessel. All the 
internal organs are contained within the disc, and none of them 
pass into the arms except the nerve-cords and ambulacral ves- 
sels. The mouth is placed in the centre of the under surface of 
the disc, and opens into a globular, simple stomach, which is 
not furnished with an anal aperture, all indigestible particles 
being got rid of through the mouth. In various points of their 
