154 



THE MICROSCOPE AND ITS REVELATIONS. 



son of Cork, in 1823, but originally supposed by him to be a perma- 

 nently attached Crinoid forms a most beautiful object for the lower 

 powers of the Microscope, when well preserved in fluid, and viewed by 

 a strong incident light (Plate xxi., fig. 3); and a series of specimens in 

 different stages of development, shows most curious modifications in the 

 form and arangement of the various component pieces of its calcareous 

 skeleton. In its earliest stage (Fig. 379, A), the body is inclosed in a 

 calyx composed of two circles of plates; namely, five basals t forming 

 a sort of pyramid whose apex points downward, and is attached to the 

 highest joint of the stem; and five orals superposed on these, forming 

 when closed a like pyramid whose apex points upwards, but usually 

 separating to give passage to the tentacles, of which a circlet surrounds 

 the mouth. In this condition there is no rudiment of arms. In the 

 more advanced stage shown at B, the arms have begun to make their 



FIG. 378. 



FIG. 379. 



Antedon (Comatula) or Feather-star, seen from its 

 under side. 



Crinoid Larva of Antedon: A, B, c, 

 successive stages of development. 



appearance; and the skeleton, when carefully examined, is found to con- 

 sist of the following pieces, as shown in Plate xxi., fig. 1: b, b, the cir- 

 clet of basals supported on the top of the stem: r l the circlet of first 

 radials, now interposed between the basals and the orals, and alternating 

 with both; between two of these is interposed the single anal plate, a; 

 whilst they support the second and the third radials (r 2 , r 3 ), from the 

 latter of which the bifurcating arms spring; finally, between the second 

 radials we see the five orals, lifted from the basals on which they origi- 

 nally rested, by the interposition of the first radials. In the more ad- 

 vanced stage shown in Fig. 379, c, and on a larger scale in Plate xxi., 

 figs. 2, 3, we find the highest joint of the stem beginning to enlarge, to 

 form the centro-dorsal plate (fig. 2, c d], from which are beginning to 

 spring the dorsal cirrhi (c i r), that serve to anchor the animal when it 

 drops from the stem; this supports the basals (b), on which rest the 



