MOI^OGRAPH OF THE EXISTING CRICOIDS. 479 



The infrabasals eventually become fused to the proximal (most posterior) 

 columnal, which is cup shaped, with a considerable central cavity for the reception 

 of the chambered organ. Their anterior borders now reach to the aboral ends of 

 the basals, which they begin to overlap. 



In a 110-hour larva the centrodorsal was similarly formed, but somewhat 

 more extensive ; it measured fi-om 160 [x to 180 jj. in diameter, and was composed of 

 a somewhat finer meshwork than the basals. The suture between the infrabasals 

 could no longer be recognized, but as the outermost border showed four low and 

 broad projections it is probable that there originally had been four. 



The number of the skeletal elements in the column may be as high as 17 or 18, 

 including the centrodorsal and the terminal stem plate, though in many cases it is 

 three or four under this number. The distance between them gradually decreases 

 toward the calyx. In the vicinity of the infrabasals they lie quite closely 

 together, and in younger larvae the proximal end of the series is in part overlapped 

 by the aboral ends of the latter. 



As from the first the terminal stem plate considerably exceeds all the other 

 coliminals in size, and now lies entirely in the disklike broadening of the attached 

 end of the body. 



The progressive decrease in the size of the following pieces becomes gradually 

 less and less marked, but for a long time very small plates, apparently newly 

 formed, lie just beneath the calyx, the last of these eventually fusing with the 

 infrabasals to form the centrodorsal. When this occurs the number of coliminals 

 has risen to at the most 19. 



Meanwhile the columnals have become greatly altered. Those lying at the 

 aboral end outstrip the others in their development, though the latter gradually 

 overtake them so that finally all are cssentiallj' similar. But the thickness of the 

 individual columnals decreases toward the calyx up to the last stage studied by 

 Seeliger. 



In the younger fixed stage the crescentic form of the plates occurs only at the 

 oral end. The crescents are open ventrally, and the regularly curved inner borders 

 are smooth. From the outer borders single and dichotomously branched processes 

 arise, of which those in the dorsal median line are the most developed, the others 

 gradually decreasing in size toward the horns of the crescents. These last 

 gradually grow together until they meet and a plate is formed which shows a large 

 circular opening displaced excentrically toward the ventral side. Meanwhile 

 an active growth of the calcareous processes takes place all about the periphery, 

 which is still a considerable distance from the ectoderm of the stem, while peglike 

 processes arise abundantly on the meshwork parallel to the longitudinal axis of the 

 stem and at right angles to the plane of the columnals. These peglike processes, 

 which occur very numerously even on the borders of the plates, grow very rapidly. 

 They are mostly somewliat longer and thicker centrally than peripherally, though 

 toward the center of the plate .small (mes may be found. In cross section they 

 appear as glistening circular or oval structures forming an irregular broad band 

 of dots. 



