664 



ECHINOnERMA— CLASS CRINOIDEA. 



together and pass up the ossicles of the arms. These are motor nerves, and 

 the central organ controls the movements of the arms, the stem, and all 

 appendages of the orinoid. 



Although crinoids are typically fixed by this stem, and although the 

 ancestors, at least, of all crinoids have been fixed by such a stem, still 

 there are some, both living and extinct, in which the stem has entirely 

 or almost entirely disappeared, leaving the crinoid with powers of free 

 locomotion. There are some four or five living genera in which this 

 has taken place; the most familiar instance is the rosy feather- star 

 of our own seas, shown in Fig. 15. In its younger stages, however (see Fig. 

 14), the animal is provided with a stem. From this it ultimately breaks off, 

 retaining but a few of the ossicles. These ossicles become fused together, 

 and form a thick knob at the base of the cup. From this knob, which is 

 called the centrodorsal, there pass small branches called cirri (C, Fig. 

 15) ; by these cirri the animal can hold on to stones or other objects at 

 the bottom of the sea, so that, although able to move at will, it can save 

 itself from being swept away by storms or currents. Fig. 15 shows one 

 of these creatures in the act of crawling away frcm the stone to 

 which it wrs attached. The arm towards the spectator is stretched out and 

 slightly curved back, the pinnules at its extremity being bent right back like 

 the barbs of a fish-hook. By these it will hook on to the rough surface of 

 the sea-floor, and the arms will then bend and the animal be dragged along. 



At the same time 



Fig. 15. — A FBATnBR-STAR CRAWLING (^Aniedon rosacea). Natural size. 



the opposite arms 

 are bent down- 

 wards and their 

 pinnules are 

 pushed in the op- 

 posite direction. 

 This shortening 

 of the stem and 

 power of locomo- 

 tion has been a 

 gradual develop- 

 ment. The Pen- 

 tacriiii, which are 

 among the com- 

 monest of living 

 stalked crinoids, 

 have their stem 

 beset at intervals 

 with whorls of 

 cirri. The joints 

 of the stem just 

 below each whorl 

 are not muscular, 

 but brittle, so 

 that the crinoid 



can easily break its stem at those joints, and after movinc away a short 

 space, can again aflix itself by the whorl of cirri remaining at the broken 

 end. In this way Pcntacriiii have been found attached to telegraph cables 

 but a short time after they had been laid down. 



