Clark — Origin of Crinoidal Muscular Articulations. 41 



ambulacral ossicles united by connective tissue with a longitu- 

 dinal muscle band running interiorly along either side of the 

 ambulacral series as a whole. It is easy to imagine that at 

 first the muscle bands were fanlike in arrangement and broad 

 as in the echinothurids, but later, with increased flexibility 

 of the test, became narrow, and finally resolved themselves 

 into longitudinal muscles more like those of the holothnrians. 

 "With increased scope of motion between the ambulacral 

 ossicles, beveling was induced, whereby the apposed faces of 

 adjacent ossicles were in close apposition in the median line 

 (perpendicular to the line between the center of the ossicle 

 and the center of the calyx) but sloped away from each other 

 both inwardly and outwardly. The connective tissue along 

 this median line, the prototype of the so-called transverse 

 ridge, now became very short and dense, forming what is 

 practically a very narrowly linear close suture; while the 

 connective tissue on the remainder of the articular surfaces 

 was more or less lengthened, and gradually became ligament- 

 ous in nature, at the same time, through the separation of the 

 internal edges of adjacent ossicles, the longitudinal muscle 

 bands were, by pressure from within the calyx, pushed in 

 between them, and certain of the fibers came to be inserted on 

 the apposed faces of the ossicles instead of only on the ventral 

 (interior) surface. This would give the ambulacral ossicles a 

 joint face consisting of two equal oblong ligament fossae, 

 separated by a narrow transverse ridge, with a small muscular 

 fossa at each outer corner of the inner ligament fossa. Now 

 the calyx of the crinoids is remarkable for its very small 

 size, and hence in the development of the race we may assume 

 that this decrease in size has caused it to press more or less 

 upon the internal organs. This pressure would not be equal 

 at all points on the inner surface of the ossicles because of the 

 existence of various radial vessels which run along the center 

 of the ambulacra, chief among which is the axial nerve cord. 

 These vessels would, by this pressure, encroach upon the area 

 of the inner ligament fossa, and would tend to excavate it in 

 the form of a more or less broad V, at the apex of which 

 would be the axial nerve cord ; at the same time the muscles 

 would encroach more and more upon the articular face, and 

 the part of the original muscular band which originally ran 

 along the inner side of each individual ossicle, having now 

 become useless, would disappear. The ventral ligament fibers, 

 as a result of the decrease in the area of the fossa occupied by 

 them through the encroachment of the " soft parts," not being 

 able to migrate past the close suture along the transverse ridge, 

 would come to lie more and more closely together, and would 

 form two masses each more dense and compact than that of 



