29 8 ECHINODERMS. 



of the arms — which are often much branched, — and in the absence of special breathing- 

 organs, which 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 favoured by these particular animals. The body of a crinoid is encased by 

 a limited number of relatively large plates, united together 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 easily be broken ; and the same result would 

 follow if there were six plates and three pairs of opposed sutures. Though the 

 test misfht 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 corresponds 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 Palaeozoic 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 (Apiocrinws) 

 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 r 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 

 attachment, 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 upwards like the flukes of an anchor, 

 and serving the same purpose. 



The stems of most crinoids have no great power of bending or coiling, since 

 the ridges and ligaments are distributed equally all over the joint surfaces. 

 Writing of pentacrinids, dredged in the Caribbean Sea, Professor Agassiz says: 

 " They move the cirri more rapidly than the arms, and use them as hooks to catch 

 hold of the neighbouring objects, and on account of their sharp extremities they 



