CYSTIDS AND CRINOIDS, 



2 97 



older crinoids, the ossicles of the stem, instead of being simple rings, are generally 

 composed of five equal parts. In other words, there are five radial sutures or 

 joint-surfaces, running the whole length of the stem and dividing each ossicle 

 into five parts. These sutures are more con- 

 spicuous towards the -root end of the stem, which 

 was of course the first to be formed in each 

 individual. Thirdly, examination has shown that 

 in some of these stems, especially towards the root 

 end, the five portions of each ossicle do not lie 

 regularly above the five portions of the under- 

 lying ossicle, but alternate with them to a certain 

 extent, just in the same way as the circlets of 

 plates that make up the cup of a crinoid alternate 

 with one another. These facts alone would lead 

 us to suppose that the stem was originally com- 

 posed, like the cup still is, of a series of circlets of 

 small plates, five in each circlet, and alternating 

 with one another; that the stem was, in fact, 

 nothing more than a continuation of the cup, 

 with essentially similar structure. Turning to the 

 cystids, we may see how this view is confirmed and 

 extended. In certain forms, such as Trochocystis, 

 that part of the stem next the body consists of a 

 double series of alternating plates, which are thin 

 and enclose a large hollow. In Arachnocystis the 

 whole stem consists of four or five series of alter- 

 nating plates. In Dendrocystis, the plates forming 

 the upper part of the stem can only be distinguished 

 by their smaller size from those forming the cup ; 

 below they merge into the normal series of single 

 ossicles. Cigara is the name given to a stem 

 .entirely composed of small irregular plates. We 

 may, therefore, conclude that the stem originated 

 as a portion of the body of the animal, elongated, 

 and gradually becoming more and more regular 

 in its structure. The curiously elongate and 

 irregularly plated form called Pilocystis may repre- 

 sent the earliest stage in its evolution, before one 

 can even say that a stem is differentiated at all. 



THE STONE-LILIES OR CRINOIDS, 

 Class Crinoidea. 



LOFODEN KOOT-CRINOIDS (f liat. Size). 



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 



