STRUCTURE OF CRINOID 



3" 



The feather-stars or sea-HUes differ from other Echino- 

 derms in being fixed permanently or temporarily by a 

 jointed stalk. The modern Comatulids, e.g. the rosy 

 feather-star (Comatula or Antedon rosacea), leave their stalk 

 at a certain stage in life ; but the other Crinoids, e.g. 

 Pentacrinus; are permanently stalked, like almost all the 

 extinct stone-lilies or encrinites, once so abundant. Most 

 of them live in deep water, and many in the great abysses. 

 An anchorage is found on rocks and stones, or in the soft 



Fig. 1 68. — Diagrammatic vertical section through disc 

 and base of one of the arms of Antedon rosacea. — 

 After Milnes Marshall. 



The section is inter-radial on the left, radial on the right, t., Cili- 

 ated openings in body wall ; h., sub-epithelial ambulacral nerve ; 

 I., water vascular canal ; k., tentacle ; ;-., mouth ; s., intestine ; 

 g., central plexus, with " chambered organ " at its base ; /., 

 coelom; i2l.-/?3., radial plates ; Br., brachial plates ; m., muscle; 

 a., axial nerve-cord ; d., central capsule ; CD., centro-dorsal 

 plate ; p., cirri ; e., nerve branches from central capsule to cirri. 



mud, and great numbers grow together — a bed of sea- 

 lilies. 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 tem- 

 porarily. Small "'organisms — Diatoms, Protozoa, minute 

 Crustaceans — are wafted down ciliated grooves on the 

 arms to the central mouth, which is of course on the up- 

 turned surface. Some members of the class, e.g. Coma- 

 tula, are infested by minute parasitic " worms " (Myzo- 



