XVI 



ECHINODERMATA 



557 



given off just below the tip which become the radial canals of the 

 daughter arms. In the first dichotomy both arms are equal in size, 

 but in each dichotomy subsequently one branch remains short and 

 forms a pinnule but the other grows on and forks again, when the 

 same process is repeated, so that the apparently single arm of the 

 Crinoid is really a sympodium formed of a succession of the stronger 

 members of successive dichotomies. As the arms grow they become 

 more and more directed outwards, till from their original vertical 

 they pass into a horizontal position. 



Whilst these changes are proceeding secondary stone-canals are 

 formed, at first one in each of the four interradii not occupied by 

 the primary stone-canal. Each 

 of them arises as two solid 

 buds, one bud being formed. in 

 the outer wall of the coelom 

 and the other on the hydro- 

 coele ring. Each of these buds 

 becomes hollowed out to form 

 a vesicle. The external one 

 forms a pore and a thin- 



ch.o 



B l 



FIG. 412. Map showing the mutual relations 

 of the ossicles in the base of the calyx of 

 a fixed larva of Antedon rosacea when the 

 adult condition has been nearly attained. 

 (After Seeliger. ) 



Bl-B 5 , the live basal ossicles ; SB, the sub-basals ; 

 ax, central canal containing the axial organ ; c.d 

 (dotted line), the outline of the adult centro-dorsal 

 cell the plates within this become fused together ; 

 ch.o, the five canals of the chambered organ ; col, 

 the uppermost columnal. 



a pore ana a 

 walled sac communicating 

 therewith, a replica of the 

 anterior coeloni in fact. The 

 internal bud forms the new 

 stone-canal which reaches and 

 fuses with the vesicle. These 

 new anterior coeloms soon fuse 

 with the general coelom, as 

 did the original one, and so the 

 secondary stone-canals and the 

 secondary madreporic pores 

 both open into the general 

 coelom and are disconnected 

 from each other. 



In later stages the new pores and stone-canals do not even reach 

 one another, and eventually are formed irregularly and independently 

 of one another, and so the adult condition is reached. The first 

 branches of the stem, or cirri, are formed just when the first secondary 

 stone-canals make their appearance. The canals of the chambered 

 organ expand to form ampullae at the bases of these, repeating the 

 primitive ampullae at the base of the stem ; and a branch from the 

 axial organ and the central canal grows out and traverses each 

 ampulla, and forms a new central canal with its axial organ in the 

 centre of each cirrus. 



These ampullae are really the brain- vesicles of the all-important 

 aboral nervous system which dominates the movements of the adult. 

 The cells surrounding them, which are mesenchyme cells budded 

 from their walls, become converted into the ganglion cells of this 



