42 Clark — Origin of Crinoidal Muscular Articulations. 



the outer (dorsal) ligament mass, which remains unchanged. 

 The muscles, unable to penetrate into the area occupied by the 

 two resultants from the originally single inner (ventral) liga- 

 ment mass, would gradually creep inward along their outer 

 edge toward the center of the joint face. When the encroach- 

 ment of the "soft parts" had progressed so far as to cause the 

 axial nerve cord to lie upon the transverse ridge, we would 

 find a condition as follows: first a large undivided dorsal 

 ligament fossa occupied by more or less scattered contractile 

 ligament, and bounded ventrally by the transverse ridge ; just 

 ventral to the transverse ridge would be found two triangular 

 fossae, one on either side, lying with their apices just under 

 the center of the axial cord and one of their sides coinciding 

 with the transverse ridge ; these fossae would lodge ligament 

 masses similar to the dorsal ligament mass, but much more 

 dense ; on the outer distal corners of each of these two trian- 

 gular fossae would be found muscular fossae, which would 

 extend more or less inward toward the center of the joint 

 face, but which would never meet in the median line. Thus 

 we have arrived at the type of muscular articulation charac- 

 teristic of the crinoid arm, comprising a single dorsal ligament 

 fossa, two interarticular muscular fossae, and two ventral 

 muscular fossae. This theory of the genesis of the crinoidal 

 muscular articulations (1) explains the peculiar denseness of 

 the interarticular ligament masses, and (2) does away with 

 the necessity of assuming that the muscles are the specialized 

 connective tissue of an original loose suture. 



I have already shown how the peculiarly modified non- 

 muscular articulations, the syzygy and the synarthry, are 

 derived from the muscular type of articulation ; so that now 

 we are enabled to derive every known type of union between 

 the ossicles of the crinoid crown from the simple connective 

 tissue union known as the loose suture. 



A few words in regard to the syzygy may not be out of 

 place. Minckert, like all his predecessors except Sars, believes 

 that the syzygies are joints of especial weakness in the crinoid 

 arm where fracture takes place in case the arm is seized, due 

 to this especial weakness. He also believes that in adolescent 

 autotomy the arm is voluntarily cast off by the animal at the 

 first syzygy. My experience with living crinoids has led me 

 to agree with Sars ; the syzygy is at least as strong as the mus- 

 cular articulations, as anyone may prove for himself by break- 

 ing up crinoid arms. Under certain conditions, however, the 

 syzygy becomes peculiarly weak, and often breaks of itself. I 

 believe that this is susceptible of ready physiological explana- 

 tion. Bosshard has shown that the fibers of the dorsal liga- 

 ment and of the syzygy are histologically the same, the only 



