ANNULOIDA I ECHINODERMATA. 



171 



and ciliated, and develops within itself a second larval form, 

 which becomes fixed by a peduncle " (Huxley). 



If we take such a living 

 Crinoid as Rhizocrinus (fig. 

 62), we shall be able to 

 arrive at a comprehension of 

 the leading characters of this 

 order. Rhizocrinus is one 

 of those Crinoids 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 Thom- 

 son). The joints of the 

 columns are rnovably articu- 

 lated to one another, the 

 joint-surfaces often having a 

 very elaborate structure, so 

 that the entire stem possesses 

 in the living state a greater 

 or less amount of flexibility. 

 Each joint is perforated 

 centrally by a canal, which is 

 very inappropriately termed 

 the " alimentary 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 discoidal. The calyx exhibits two surfaces, a dorsal and a 

 ventral, of which the dorsal is composed of calcareous plates 

 articulated by their margins, whilst the former is composed 

 of a more or less leathery integument strengthened by the 

 deposition in it of numerous small plates of carbonate of lime. 



Fig. 62 Crinoidea : Rhizocrimis Lofoten- 

 sis, a living Crinoid (after Wyville 

 Thomson), four times the natural size. 

 a Stem, b Calyx, c c Arms. 



