CRINOIDEA. 



261 



which is permanently rooted to some foreign object by the 

 base of a stalk which is composed of a number of calcareous 

 pieces or articulations. In some cases (as in Apiocrinus) the 

 base of the stem or " column " 

 is considerably expanded. In 

 other cases the column is 

 simply "rooted by a whorl 

 of terminal cirri in soft 

 mud" (Wyville Thomson). 

 The joints of the column 

 are movably articulated to 

 one another, the joint -sur- 

 faces often having a very 

 elaborate structure, so that 

 the entire stem possesses in 

 the living state a greater 

 or lesser amount of flexi- 

 bility. Each joint is per- 

 forated centrally by a canal, 

 which has been very inap- 

 propriately termed the " ali- 

 mentary canal," but which 

 in truth has nothing to do 

 with the digestive system of 

 the animal. At the summit 



of the stem is placed the 



body, which is termed the 



" calyx," and which is usually 



more or less cup - shaped, 



pyriform, bursiform, or dis- 



coidal. The calyx exhibits 



two surfaces, a dorsal and a 



ventral, of which the dorsal 



is composed, wholly or in 



part, of calcareous plates articulated by their margins, whilst 



the former is, in the living forms, composed of a more or 



less leathery integument, strengthened by the deposition in 



it of numerous small plates of carbonate of lime ; whereas 



in many extinct forms it too is composed of articulated cal- 



Fig. 150. Crinoidea. Rhizocrinvs Lofo- 

 tensis, a living Crinoid (after Wyville Thom- 

 son), four times the natural size, a, Stem ; 

 b, Calyx ; c, c, Arms. 



