122 Papers from the Marine Biological Laboratory at Tortugas. 



their absence is a primitive or specialized condition. Perhaps it 

 may be either. In any case, the fusion of the margins of the slit 

 so as to form two openings on each side of the arm-base is un- 

 doubtedly a highly specialized condition. 



12. The arm-bones or vertebrcs arise by the fusion of two skeletal pieces 



lying side by side above the radial water-tube. They are at first 

 relatively elongated and low, with the two component halves 

 distinguishable and the articulation resembling a sirnple ball- 

 and-socket joint. Specialization is indicated by loss of indication 

 of the bilateral origin, by greatly increased height, and hence a 

 relatively short longitudinal axis, a greatly increased number of 

 elevations, projections, and ridges for the attachment of muscles, 

 and an articulation complicated by development of zygapophyses, 

 protapophysis, and epanapophysis. Owing to changes from com- 

 pression and other causes the vertebrae of the basal arm-segments 

 do not reveal the typical form for the species. At about 8 or lo 

 segments from the mouth the vertebrae of an adult ophiuran with 

 40 or more arm-joints are fairly typical, but after a dozen, or even 

 fewer (in less mature individuals) segments further out, youthful 

 characters begin to show and the vertebrae become less and less 

 typical as we approach the terminal segments of the arm. 



13. The terminal plate of the arm is the first formed. It is originally a flat 



or concave plate on the upper side of the tip of the water-tube. 

 It grows down and around the tube and by ventral union becomes 

 a cylinder. The distal margin of this cylinder may bear one to 

 several teeth or spinelets. It may become enlarged proxirnally 

 and even conspicuously cup-shaped, but such changes indicate 

 specialization of some kind. The most primitive condition is 

 undoubtedly a simple plate or a half-formed cylinder; a completed 

 cylinder shows at least some specialization. 



14. The side arm-plates are the first plates of an arm-segment to appear. 



They grow rapidly and soon meet each other in the mid-dorsal 

 and mid-ventral lines, where they are in close contact, although 

 they do not fuse. They inclose the rudiments of the vertebrae, 

 which do not arise until they are well begun. In later develop- 

 ment they may be more or less separated from each other ventrally 

 by the development of the under arm-plates and dorsally by the 

 upper arm-plates. At first they are low and longitudinally elon- 

 gated, the whole segment being long and flat. In their usual 

 development the height increases much more rapidly than length 

 and they consequently tend to become short and high. The distal 

 margin is somewhat thickened and bears one or more arm-spines. 

 It may become notably enlarged and a conspicuous spine-bearing 

 ridge may be developed, which in the course of growth comes to 

 lie in the median vertical region of the plate. The distal margin 

 of the lower end of each side arm-plate may be more or less deeply 

 grooved for the passage of the podium (tube-foot) and usually a 

 tentacle-scale arises at the same point. Evidently then, long, 

 low side arm-plates, meeting above and below and with no spine- 

 bearing ridge or tentacle-groove, show a primitive condition of 

 the arm-segment, while short, high side arm-plates separated above 

 and below, with a conspicuous spine-bearing ridge and a notice- 

 able tentacle-groove, indicate considerable specialization. All con- 

 ditions between these two extremes may be found. 



