482 ECHINODERMATA — OPHIUROIDEA chap. 



a median ventral pit and two dorso-lateral pits, and on the 

 distal surface there are pits corresponding to the knobs on the 

 proximal side and vice versa (Fig. 210, C). These knobs and 

 pits restrict the movement of one vertebra on the next, so that 

 although the arms can undergo an unlimited amount of flexion 

 from side to side, they cannot be rolled up in the vertical plane. 

 When the under surface of the vertebra is examined there is 

 seen on each side of the central groove two round holes, a 

 distal and a proximal. The distal pair are for the passage of the 

 canals connecting the radial water-vessel with tlie tentacles, 

 these canals traversing the substance of the vertebra for a part of 

 their course ; the proximal pair are for nerves going to the 

 longitudinal muscles, which likewise perforate part of the 

 ventral border of the vertebra. 



In order to understand the anomalous circumstance that the 

 canals going to the tentacles actually perforate the vertebrae, it 

 must be clearly borne in mind that the basis of the body-wall 

 in all Echinoderms is a mass of jelly with amoebocytes in it, to 

 which we must assign the power of secreting carbonate of lime, 

 and all we have to assume in the case of Ophiuroids is that 

 calcification spread outwards from the original ambulacral 

 ossicles into the surrounding jelly, enclosing any organs that 

 happened to traverse it. 



When the ossicles of the arm are followed inwards towards 

 the mouth, they are seen to undergo a profound modification, so 

 as to form, by union with the corresponding ossicles of adjacent 

 arms, a structure called the mouth-frame. The general character 

 of this modification is similar to that affecting the first ambulacral 

 and adambulacral ossicles in the arms of an Asteroid, but in 

 the Ophiuroid the change is much more profound. The first 

 apparent vertebra consists of two separated halves, and each is 

 fused with the first adambulacral (lateral) plate, which in turn is 

 firmly united with the corresponding plate in the adjoining arm. 

 Thus is formed the "jaw," as the projection is called. The 

 extensions of the mouth-cavity between adjacent jaws are termed 

 " mouth-angles." To the apex of each jaw is attached a plate 

 bearing a vertical row of seven short blunt spines called " teeth " 

 (Fig. 212,^). The plate is called the " torus angularis " (Fig. 211, 

 T), and on its ventral edge there is a tuft of spines which are termed 

 " tooth-papillae " (Fig. 208, t.jp). On the upper aspect of the jaw 



