226 Charles Lincoln Edwards 



and may characterize the young of the usual collection as Pourtales, 

 1851, notes. The ambulacral vessels of a number of the appendages, 

 particularly in the later stages, as may be seen in the sections, grow 

 within the body-wall for some distance from the radial canal and 

 consequently the appendage is superficially interradial in location. 

 Gn the other hand, the ambulacral vessels of some appendages turn 

 into the mid-line of the radius. Hence, through the varying lengths 

 of ambulacral vessels within the body-wall the fairly even distribu- 

 tion of appendages over the trivium, and in smaller numbers, over 

 the bivium, of the adult is established. 



From the larva of twenty-four days, with the first dorsal papillae, 

 to the eighty-seven day stage, there are eighteen cases detailed in 

 Table II, of which seventy-two per cent have the larger number of 

 appendages from the radial canals of the trivium. In making the 

 ratio of appendages in the trivium to those in the bivium, the ap- 

 pendages given off dorsad from the right and left ventral radial 

 canals were counted among those of the trivium, with which they 

 seem to function, albeit some of them later in the adult might be 

 placed in the bivium. The average of all the ratios gives one and 

 three-tenths as many appendages in the trivium as in the bivium. 

 The proportionate number ventrally increases with further growth, 

 for, as I have demonstrated elsewhere (1908), in the average young 

 of the ordinary collection series there are one and seven-tenths, and 

 in the average adult, one and nine-tenths, as many appendages per 

 square centimeter in the trivium as in the bivium. Since, on the 

 average, in this series, there are half again as many appendages 

 per square centimeter in the young as in the adult the increase in 

 the number of appendages does not keep pace with the general growth 

 of the body-wall. In every stage from larva to adult, all of the 

 ventral appendages are pedicels. Of the dorsal appendages of the 

 adult, eighty per cent are pedicels, and not all of the remaining 

 twenty per cent are true papillae. In each ambulacral appendage 

 a valve develops at the entrance by the time the appendage is func- 

 tional. The last stages of my series fail to exhibit any clearly 

 marked ampullae for the pedicels and papillae. The relatively large 

 Polian vesicle, therefore, functions as a general reservoir for the 

 lan^al ambulacral system. 



