ECHINODERMATA. CRINOIDEA. 



87 



ridges in the form of a rosette. From the stem small 

 branches known as cirri are sometimes given off, these 

 have a structure similar to that of the stem and are also 

 pierced by a central canal. The inferior part of the stem 

 often expands and branches in a root-like manner, and 

 serves to fix the animal. Sometimes the plates near the 

 top of the stem become very much larger than the others. 



FIG. 21. Diagram of the plates of Actinocrinus triacontadactylus, Carboni- 

 ferous Limestone. 6, basal plates ; rl, first cycle of radials; r2, r3, 

 second and third cycles of radials; br, brachial plates; ir, inter- 

 radials ; a, anal inter- radial. 



Resting on the topmost joint of the stem is the calyx 

 (figs. 21, 22), this consists at its base of a cycle of five 

 (sometimes fewer) plates, known as the basals (6), occasion- 

 ally these are not visible, being covered by the radials. In 

 some forms there is below the basals and alternating with 

 them another cycle of plates, termed the underbasals, and 

 the base is then said to be dicyclic ; when basals only are 

 present, it is monocydic. Above the basals and alternat- 

 ing with them, comes a cycle of five radial plates (ri); 

 often there are two or three cycles of radials (r2, ra), in 



