214 PROCEEDINGS OF 77//.' VATIONAL MUSEUM. vol.38. 



would again occur midway between the first articulation and the 

 calyx. Now, this fracture would almost certainly differ from the 

 original fracture in being formed at right angles to it; for any force 

 exerted in the same plane as that which caused the original fracture 

 would be taken up by the articulation which has formed; but, owing 

 to the definite direction of, and close union along, the fulcral ridge, 

 any force coining parallel to the fulcral ridge, that is, at right angles 

 to the original force, would meet with resistance, as for a force ex- 

 erted in this direction the original articulation would be practically 

 nonexistent, and a second fracture would occur in the weakest spot, 

 namely, halfway between the original articulation and the calyx. 

 developing into a second articulation in which the fulcral ridge would 

 run at right angles to the direction taken by that of the first. A 

 still further increase in stem length would mean a progressive in- 

 crease in the number of articulations, each of which would, in the 

 direction taken by its fulcral ridge, alternate with those on either side; 

 and thus would eventually be formed the primitive polycolumnar 

 crinoid stem, a stem exactly comparable to the stem of Rhizocrinus, 

 Baihycrinus, or the young of Antedon. 



Now, there is a definite limit to the possibilities of further growth 

 in a stem composed of long columnars fastened end to end by alter- 

 nating articulations consisting of two ligament masses separated by 

 a fulcra! ridge; if the animal remains small with a small light crown, 

 such a stem may attain a length of one hundred or more columnars 

 safely; but if the crown should become of large size and heavy, a stem 

 of this type would not be able to support it; the growing tendency to 

 "buckle" would therefore limit the available length of a stem of this 

 nature. 



There are four possible waysof escape from such a calamity: (1 ) Tin 1 

 stem may be discarded; (2) the individual columnars may become 

 greatly shortened, the motion lost through the great diminution of the 

 original beveling at the articulations being compensated by the greatly 

 increased number of articulations in a given section of stem, and 

 the columnars may become enlarged along these lines; (3) they may 

 alter the direction of their fulcral ridges, so that, instead of each being 

 at right angles to those preceding and succeeding, they may each lie 

 at only a slight angle lo the preceding (all diverging toward the same 

 side), thus mutually bracing each other and attaining a collective 

 rigidity like a pile of narrow boards built up spirally; or (4) the orig- 

 inal fulcral ridge may disintegrate, each half breaking up longitudi- 

 nally and spreading out fanlike, the two fanlike figures eventually 

 uniting to form an articular surface composed of numerous uniform 

 radiating lines. 



The Comatulida fulfill the conditions of the first possibility; before 

 the animal is large enough to cause any danger of ''buckling" the 

 stem i- discarded at the articulation between the topmosl columnar 



