THE STONE LILIES OR CRINOIDS 3265 



THE STONE LILIES OR CRINOIDS Class Orinoidea 



The crinoids differ from the more highly developed of the cystids in the greater 

 regularity of their structure the symmetry of which is nearly always governed by the 

 number five, in the greater development of the arms which are often much 

 branched, and in the absence of special breathing organs, jtvhich were no longer 

 needed. It is easy to see how an organism that is fixed, and equally affected on all sides 

 by the surrounding medium, whether air or water, develops a radiate symmetry; the 

 same result being obvious in the case of most flowering plants. Slight consideration 

 will also show why the number five has been favored by these particular animals. 

 The body of a crinoid is incased by a limited number of relatively large plates, united 

 by the skin in which they are developed, and it is clear that the sutures 

 between these plates are lines of weakness. Supposing that there were four plates 

 in each circle, then the four sutures would be in opposite pairs, and the lines of 

 weakness would run right across the body of the animal, which would eaisly be 

 broken; and the same result would follow if there were six plates and three pairs of 

 opposed sutures. Though the test might be more flexible, still there would be three 

 lines of weakness in each circle instead of two. But when there are five plates, each 

 suture lies opposite to the middle of a plate, and so the line of weakness does not 

 run right across the body. A few crinoids have essayed other forms of symmetry, 

 but none have had a long existence. The alternation of the plates in a crinoid may 

 be explained by similar mechanical considerations; for such an arrangement corre- 

 sponds to the bonding of successive courses of bricks in a wall. There is reason to 

 suppose that the ancestors of all crinoids, as well as most of the Paleozoic crinoids, 

 were attached to the sea floor or some other object throughout life by the stem. On 

 hard rocky bottoms the attachment was by means of an incrustation, as in the pear 

 encrinite {Apiocrinus} of the Bradford Clay; but on oozy bottoms the end of the 

 stem broke up into numerous branches called cirri, as in the Rhizocrinus of modern 

 seas. In course of time these cirri were developed, not only at the root end, but 

 also higher up the stem, and eventually they came in some genera to be arranged in 

 regular whorls of five, as we see them in the living Pentacrinus. Since crinoids 

 were at all times liable to be broken from their attachment, some of them gradually 

 acquired slight faculties of locomotion, although they prefer to renew their attach- 

 ment, even though this be transferred to some other object. Sometimes the whole 

 end of the stem coiled itself around the stem of another crinoid, but usually it became 

 anchored by such cirri as chanced to remain on the preserved portion. In those 

 crinoids that have the cirri in whorls, the ligaments that unite the joints of the stem 

 stop short just below each ossicle bearing a cirrus, so that the division between this 

 ossicle and the one below yields readily to a bending or blow; and thus the crinoid 

 can anchor itself easily by the whorl of cirri left at the end of its broken stem. In 

 some cases there is developed at the end of the stem a little ball of calcareous tissue, 

 serving as a weight to keep the animal right way up, as it moved slowly through 

 the water by the waving of its arms; and in certain forms this ball developed 

 spines, directed' upward like the flukes of an anchor, and serving the same 

 purpose. 

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