3264 



ECHINODERMS 



irregular and small plates, it is difficult to understand how such a stem was evolved; 

 but the mystery is elucidated by some of the cystids and older crinoids. First, it 

 may be noted, that in those cystids possessing a crinoid-like stem, as well as in 

 many of the older crinoids, the axial canal of the stem is much larger than it is in 

 later forms. Secondly, that in many 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 conspicuous toward 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 

 toward the root end, the five portions of each ossicle 

 do not lie regularly above the five portions of the 

 underlying ossicle, but alternate with them to a cer- 

 tain 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 inclose 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 regu- 

 lar in its structure. The curiously elongate 

 and irregularly plated form called Pilocystis may 

 represent the earliest stage in its evolution, be- 



LOFODEN ROOT "CRINOIDS. ^ ore one can even sav t ^ iat a stem * s differenti- 

 (One and one-half natural size. ) ated at all. 



