EXCRETORY SYSTEM IN DIGENEA. 333 



to a rather large aggregate of such units (C. rhabdocceca) . C. 

 gigas is apparently not related to this series, since an analysis 

 of its flame cells fails to show any direct correlation with the 



established series. 



DISCUSSION. 



The data presented in this paper are of fundamental signifi- 

 cance in establishing the relationships of various larval flukes to 

 one another and of larvae to adults. Present knowledge not 

 only preponderates in favor of the view that the number of flame 

 cells in the species is constant, but establishes the belief also 

 that the same basic number and arrangement of flame cells 

 exist within families or subfamilies. When once the number 

 and disposition of flame cells has been established for a particular 

 group it will be possible to predict the flame-cell formula of the 

 larvae. Thus a larva to belong to the Schistosomatidse sensu 

 stricto should possess a flame-cell arrangement consistent with 

 the common denominator which the apharyngeal species of furco- 

 cercariae possess in common, namely 2 = (i -f- i) ; and a plagior- 

 chiine species should have the common formula of that family, 

 6 = ([i -f- i + i] + [i + i + i]) ; and brachycoeliine species 

 should follow the scheme 4 = ([i -f- i] -f- [i + i]). This accords 

 with Cort's thesis on the conservatism of the excretory system, 

 but it suggests, from the number of types already known, a 

 larger number of possibilities of flame-cell arrangement than 

 had been previously anticipated, and, accordingly, a larger 

 number of family groups than the present knowledge of adult 

 flukes postulates. 



Because the number of basic groups is the most significant 

 point in the analysis of the flame-cell arrangement of cercariae 

 and adults alike, there is justification for presenting general 

 formulae of flame-cell units which will apply equally well to 

 all species of the same group. These formulae are readily ob- 

 tained if the common denominator of the species of the same group 

 is taken and literal equivalents substituted for the numbers 

 involved. Thus for the species of Schistosomatidae the formula 

 would read a + |8; for Brachycceliidae a' + /3'. + a" + 0", 

 where the primed letters represent the anterior groups and the 

 double-primed letters represent the posterior groups. Under 



