444 



EMBRYOLOGY 



much as new segments (the stem-joints) are interpolated 

 between it and the basalia, it moves farther downward. The 

 stem-joints take their origin at the base of the calyx. The 

 youngest, therefore, lie next to this, the oldest, on the other 

 hand, next to the terminal plate. At first they form annular 

 plates, but soon alter their shape and become thick segments 

 by the secretion of rod-like calcareous concretions on both 

 their surfaces. 



Between the uppermost 

 stem-joints and the basalia 

 lies a larger skeletal piece, 

 which has been called the 

 centrodorsal plate (Fig. 223 

 cd, p. 451). It forms the 

 important foundation of the 

 basal plate of the calyx. 

 According to Bury, it arises 

 by the union of several skele- 

 tal pieces. For three sub- 

 basal plates make their ap- 

 pearance below the basalia ; 

 these at length fuse with one 

 another into a five-pointed 

 star and finally unite with 

 the uppermost joint of the 

 stem to form the centrodor- 

 sal plate. This condition is 

 impoi'tant, because certain 

 fossil Crinoids (Ichthyocri- 

 noidc'e) also possess three 

 sub-basal plates with the 



im 



Lm. 



\ 



Fir,. 217, — Larva of AixlfAon rosacea, 

 with ciliated bands and tuft of cilia, as 

 well as fundaments of the skeletal 

 plates inside. Qx, pit, by means of 

 which the larva attaches itself; L\>x, 

 the so-called larval mouth. 



same airangement. 



The series of plates of the calyx are at first not arranged in a closed 

 ring, but in the form of a horseshoe, the open side of which corresponds 

 to the position of the " larval mouth." 



Before the skeleton attains the development described, 

 the larva has already given up its free life. After about 

 twelve hours of swarming it attaches itself by means of the 



