16 ECHINODERMS OF THE BRITISH ISLES 



Key to the families of Crinoids known from or likely to he 

 represented in the British seas. 



1. Stalked ; upper joint of stalk not widened so as to form a centro- 



dorsal ; calyx externally visible ..... 2 

 No stalk, except in the young (Pentacrinoid) stage ; upper stalk - 

 joint widened so as to form a centro -dorsal, which covers the 

 calyx -plates more or less completely. Articulate cirri on the 

 centro -dorsal, or on the upper stalk-joint in the Pentacrinoid- 

 stage. (Comatulids) ....... 4 



2. Whorls of true, articulate cirri (5 in each whorl) at regular intervals 



on the stalk ..... Fam. Isocrinidce 



No true, articulate cirri, only rootlike, branching appendages 



may be present in the lower part of the stalk ... 3 



3. Stalk - joints short, with circular articular faces ; no root - like 



appendages ; ventral side of disk distinctly plated ; large 

 orals ....... Fam. Hyocrinidce 



Stalk- joints, except immediately below the calyx, much longer 

 than broad, with elliptical articular faces, root-like appendages 

 present in basal part of stalk ; ventral side of disk unplated ; 

 no orals ..... I. Fam. Bathycrinidoe 



4. Cirrus -sockets bordered above and on the sides by a horseshoe - 



shaped rim ; centro -dorsal elongate, conical 



II. Fam. Atelecrinidce 

 Cirrus -sockets not bordered by a horseshoe-shaped rim . 5 



5. Arms undivided (five in the British forms) 



III. Fam. PentametrocrinidcB 

 Arms divided on the second brachial ..... 6 



6. Oral pinnules with the outer edge of each of the distal joints 



produced into a thin, rounded, triangular process, forming a 

 conspicuous comb -like structure. Mouth usually excentric, 

 anal tube central. No sacculi . . Fam. Comasteridce 



No comb -like structure on the oral pinnules ; mouth central, 

 anal tube marginal ; sacculi present .... 7 



7. Pinnules stiffened, prismatic, sharply triangular in cross-section ; 



oral pinnules only feebly specialised ; ambulacral grooves on 

 pinnules and arms bordered by two rows of minute but distinct 

 plates. Cirri long, the distal joints with a prominent dorsal 

 spine ; cirrus -sockets arranged in 10 columns 



Fam. Thalassometridce 

 Pinnules not stiffened, not prismatic ; oral pinnules very special- 

 ised ; no, or only microscopical, plates along the ambulacral 

 grooves ..... IV. Fam. Antedonidce 



