6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 72 



shoe, supports the animal on soft ooze or mud (fig. 35), and in one 

 family (Comasteridse) they are frequently quite absent, the centro- 

 dorsal being atrophied and sunken in so that its now flat surface does 

 not project beyond the dorsal surface of the crown (fig. 11). Penta- 

 crinites attach themselves to arborescent marine animals by their 

 cirri, like the comatulids, or lie upon the sea floor with the column 

 more or less coiled and the crown raised high above it. The other 

 crinoids, except Holopus, are either attached to solid objects by the 

 expanded and encrusting basal columnal or terminal stem plate 

 (Plicatocrinidse, Apiocrinidse, and Phrynocrinidse), or send out from 

 the distal portion of the column a mass of root-like processes by 

 which, plant-like, they are able to maintain an erect position on muddy 

 bottoms (Bourgueticrinidse). It is curious that in some of these if 

 the crown be lost before the stem dies a mass of roots is produced 

 from the upper end of the column similar to those at the lower end. 



The crown of a crinoid is composed primarily of three alternating 

 circlets of five plates, excepting in the Plicatocrinidse in which there 

 are only two, calling to mind the bracts, sepals and petals of a flower. 

 The plates of the lowest circlet (infrabasals) are small, often only 

 three in number, and frequently, or perhaps usually, absent in mature 

 animals; the plates of the second circlet (basals) are larger and more 

 constant, though in the feather-stars they become transformed into 

 an internal septmn (fig. 7, shown cut across just above the cavity 

 within the centrodorsal) with no hint of their original significance, 

 and in the Plicatocrinidse there are frequently only three of them; the 

 plates of the third circlet (radials) are always highly developed, and 

 are structurally the basal plates of the arms. In a few types each of 

 these is doubled so that there are ten of them instead of only five 

 (compare figs. 49 and 50). 



Following the radials there is a linear series of ossicles which 

 rarely remains undivided (figs. 42, 49, 50), almost invariably forking 

 on the second (figs. 39, 46-48) ; this gives ten arms, the commonest 

 number among the crinoids ; but each of these may again divide on the 

 second or fourth segment beyond the first division (very rarely on any 

 other) (figs. 40, 41) and this process may be repeated in extreme 

 cases as many as eight or nine times (fig. 17, left). Ordinarily all 

 of the five groups of arms are alike and all of the arms are of the 

 same length, but in the Comasteridae the arms arising from the left 

 posterior radial, sometimes from both posterior radials, are frequently 

 shorter than the others, sometimes scarcely more than a third as long 

 (fig. 44), and they are further peculiar in lacking the ambulacral 

 groove (fig. 36), in bearing more numerous and more developed 



