CRINOIDEA. 273 
water, and many in the great abysses. An anchorage is 
found on-rocks and stones, or in the soft mud, and great 
numbers grow together—a bed of sea-lilies.s The free 
Comatulids swim gracefully by bending and straightening 
their arms, and they have grappling “cirri” on the aboral 
side, where the relinquished stalk was attached. By these 
cirri they moor themselves temporarily. Small organisms— 
Diatoms, Protozoa, minute Crustaceans—are wafted down 
ciliated grooves on the arms to the central mouth, which 
is of course on the upturned surface. Some members of 
“Il 
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« 
i om 
i Pye 
Fic. 140.—Diagrammatic vertical section through disc and 
base of one of the arms of Axmtedon rosacea.—After 
Milnes Marshall. 
The section is inter-radial on the left, radial on the right. 74., Cili- 
ated openings in body wall; %., sub-epithelial ambulacral nerve ; 
Z., water-vascular canal; %, tentac'e; ~, mouth; s., intestine ; 
&, central plexus, with ‘‘chambered organ” at its base; f, 
ceelom ; #1.-R3,, radial plates ; B., brachial plates ; ~., muscle ; 
a,, axial nerve-cord; @., central capsule; C.D., centro-dorsal 
plate; Z., cirri; ¢., nerve branches from central capsule to cirri. 
the class, eg. Comatula, are infested by minute parasitic 
“worms” (Myzostomata) allied to Chzetopods, which form 
galls on the arms. A lost arm can be replaced, and even 
the visceral mass may be regenerated completely within a 
few weeks after it has been lost. It has been suggested that 
the occasional expulsion of the visceral sac frees the Crinoid 
from parasites (Dendy). 
The animal consists of (I) a cup or calyx, (2) an oral disc forming the 
lid of this cup, (3) the radiating ‘‘ arms,” and (4) the stalk supporting 
the whole. The lowest part of the cup is supported by a pentagonal 
18 
