276 University of California Publications in Zoology [Vol. 19 



Each of these collecting tubes receives three accessory collecting tubes 

 (acct) making a total of twelve accessory collecting tubes. Each 

 accessory collecting tube, except the posterior on each side, is joined 

 by a group of three capillaries. The formula 2 X 6 X 3 = 36 has 

 been used by Looss (1894, p. 68) for this type of excretory system, 

 the "2" representing the sides of the system, the "6" the number 

 of accessory collecting tubes on each side, and the "3" the number 

 of capillaries in each group, making a total of 36 flame cells. The 

 arrangement of the capillaries of each group is significant. Each 

 accessory collecting tube divides into two subdivisions, one of which 

 forms directly a capillary ending in a flame cell while the other 

 subdivides into two capillaries. The paired capillaries of each group 

 go to one surface of the body and the single capillary to the other. 

 The dorso-ventral arrangement of the capillaries of any particular 

 group varied in different individuals and no regularity in this respect 

 could be discovered. The positions of the flame cells of the various 

 groups were modified greatly by the extension and contraction of the 

 body of the cercaria. Whether the finding of only two flame cells in 

 the posterior groups in Cercaria polyadena was due to failure to 

 locate the other member of each group or whether only two are 

 present cannot be determined at this time. It is possible that these 

 two posterior groups had lagged behind the others in division. Looss 

 in his figure of this same type of system in Anchitrema sangidncum 

 (Sonsino) (Looss, 1896, p. 110, pi. 8, fig. 77) shows not only the 

 posterior but the anterior groups on each side with two flame cells. 



The method of division of the capillary groups of Cercaria pol/y- 

 adena supports the hypothesis that each capillary group is formed in 

 the development of the system by longitudinal divisions of a single 

 flame cell (Cort, 1918a). According to this hypothesis an excretory 

 system of the type found in Cercaria polyadena passed through a 

 stage in development when it was composed only of the bladder, the 

 common collecting tubes and the anterior and posterior collecting 

 tubes, each of which received three capillaries from three flame cells, 

 making a total of twelve flame cells. The capillaries of these flame 

 cells would correspond to the accessory collecting tubes of the fully 

 developed system. In further development each of these flame cells 

 with its capillary first divided into two flame cells and capillaries, 

 one going to the dorsal, and one to the ventral side of the body. One 

 of these two flame cells again divided making the groups of three 

 found in the fully developed system. 



