194 THE ORGANIZATION OF TREPANGS. 



The blood circulating system, tenfold repeated in the Starfish, 

 significantly approximates in Caudina the singleness of charac- 

 ter of that of marine worms, and occupies an almost identica. 

 relative position along the median line of the body. Lastly, but 

 by no means less prominently, the reproductive system contrasts 

 its uniformity with the tenfold repetitions of the homologous 

 organs of the Starfish, and establishes itself, in conformity with 

 the rule which the other organs have followed, in the strictest 

 symmetrical relation to right and left, along the upper median 

 line. 



called odd ambulacrum being reduced first, and then the two on the htemal side. 

 In Holothurians the same result is arrived at by an inversion of the process as 

 seen in the Echini ; that is, the haemal pair disappear first, next the single anti- 

 haemal one, and finally, carrying the process farther than in the Echini, the 

 antihaemal pair vanish. The consequence in the latter case is the perfection of 

 nercottx cephalization, and therefore a ranking of the apodous Holothurians 

 above the pedicellate ones. 



