CRINOIDS. 



299 



are well adapted to retain their hold. The stem itself passes slowly from a rigid 

 vertical attitude to a curved or even drooping position." Other crinoids, like 

 Herpetocrinus, have, however, a single fulcral ridge running across the surface of 

 the joints, allowing far more play between the latter. In Platycrinus the joints 

 had an elliptical outline, and the fulcral ridge formed the long diameter of the 

 ellipse. Such a structure would naturally give the stem great power of bending, 

 but only in one plane. This restriction was got over by giving the joints a slight 

 skew, so that the stem was twisted like a corkscrew and capable of movement in 

 every direction. In Rhizocrinus the same end is attained by each joint being so 

 twisted that the fulcral ridge at the top 

 is at right angles to the one at the 

 bottom. These types are, however, 

 merely side branches from the main 

 stem of crinoid evolution; the chief 

 advance having proceeded along the 

 lines of free locomotion. At various 

 periods forms have existed, which, 

 having once tasted liberty, have gradu- 

 ally dropped all traces of their former 

 attachment. Thus, Agassizocrinus of 

 the Coal-Measures is a crinoid that has 

 nothing left of its stem but a solid knob 

 at the base of the cup; Millericrinus 

 of the Great Oolite has been found at 

 all ages and stages of development, the 

 young individuals with a normal stem 

 which gradually withers as the animal 

 gets older, till in full-grown specimens 

 it is a mere tapering process. Uinta- 

 crinus and Marsupites from Cretaceous 

 beds, are two genera as unlike in 

 essential structure as crinoids well 

 could be, but resembling one another in 

 having thin-plated large cups, without 

 the smallest relic of a stem. A little 



crinoid of Jurassic age called Thiolliericrinus appears, however, most nearly 

 related to, if it be not the actual ancestor of, most of the free-moving unstalked living 

 forms. It seems to have been related to Bourgueticrinus and Rhizocrinus, but, 

 like Millericrinus, gradually dropped its stem, while the upper joint of the stem 

 coalesced and began to bear cirri. In the common feather-star (Antedon) of the 

 British seas this process has gone yet further ; the animal breaks away from its 

 stem when quite young, but retains the uppermost swollen and coalesced segments 

 of the stem, which form one solid mass bearing a number of cirri, while the two 

 lower circlets of cup-plates almost entirely disappear, so that only the upper circlet 

 of plates, from which the arms arise, remains. The Antedonidce, which have all 

 arisen since Jurassic times, include not only numerous species of Antedon, but at 



MEDUSA-HEAD PENTACEINID. 



a, Crown and part of stem (nat. size) ; b, Upper surface of 

 the body, the arms broken away, showing the food- 

 grooves passing to the central mouth. 



