1865.] and Development of Antedon rosaceus. 377 



the calcareous skeleton of the calyx first shows itself, it consists only of 

 five oral plates arranged conformably upon five basal plates, as thus : 



O O O 

 B B B B B 



At a stage a little more advanced (which has been described by Prof. All- 

 man, Trans. Roy. Soc. Ed. vol . xxiii. p. 24 1 ), the rudiments of the first radial* 

 are found interposed between the orals and basals, alternating in position 

 with both, as in the margin ; and between two of these 

 first radials there appears a single small unsymmetrical O O O O 

 plate, which afterwards proves to be the anal. The a a a a a 

 first radials undergo a rapid increase in size, and B B B B B 

 soon become surmounted by second and third ra- 

 dials, which project between the orals ; whilst the orals and basals, under- 

 going no such increase, are relatively very much smaller ; the anal plate is 

 still found on the line of the first radials. But 



from this time the radials form the principal A 3 A 3 A 3 A 3 A 3 

 part of the calyx, which opens out widely in A* A 2 A 2 A 2 A* 

 conformity with the increase of space required O O O O O 

 for the digestive apparatus, the intestinal canal A ' A'f^A 1 A * ^ 

 being now developed around what was originally B B B B B 

 a simple stomach with one orifice. The highest 



joint of the stem also undergoes a remarkable increase in size, and begins 

 to acquire the form of a basin with an inflected rim, constituting what is 

 known in the adult as the centro-dorsal piece. When the calyx opens out, 

 the five oral plates which originally formed a circlet around the mouth, 

 retain that position, and detach themselves entirely from the divergent ra- 

 dials, nothing but the soft perisomatic membrane filling up the space be- 

 tween them. These oral plates never increase in size, and towards the 

 end of the Pentacrinoid stage they begin to undergo absorption. I can 

 still trace their basal portions in young specimens of the free Antedon ; 

 but as the creature advances towards maturity they are altogether lost sight 

 of. When the intestinal canal has been sufficiently developed to open on 

 the surface of the oral disk, the anal plate is lifted out of the position it 

 originally occupied, and is at last found on the anal funnel, far removed 

 from the radials. This, like the oral plates, begins to undergo absorption 

 towards the end of the crinoidal stage, and completely disappears in the 

 early part of the life of the free Antedon. The radial plates increase not 

 only in size but also in thickness ; and channels which are left on their in- 

 ternal surface by vacuities in the calcareous network, are converted into 

 canals by a further inward growth of this, which completely covers them 

 in. It is through these canals that the cords of sarcode pass to the arms. 

 The basal plates, like the oral, remain stationary in point of size, and pre- 

 sent no change in appearance or position until after they have been com- 

 pletely concealed externally by the centro-dorsal piece (the highest joint of 



