EXCRETORY SYSTEM IN DIGENEA. 325 



tubule and probably also along the posterior part of the sec- 

 ondary tubule. The exact n-umber of flame cells for each side of 

 the body is not clearly shown but somewhere between eighteen 

 and twenty may be counted (Looss, 1894: Fig. 191). 



A species showing relationships intermediate between these 

 two groups is Cercaria biflexa Faust, 1917. Both secondary 

 and tertiary portions of the tubule occur here (Faust, 1918, 

 Fig. 138), but they are'confined entirely to the head region of the 

 body. Numbers of tributaries to the primary tubule (Faust, 

 1918, Fig. 135) empty into that canal but no flame cells have 

 been found in connection with them. In the tail of this species 

 the main collecting tubule forks about two-fifths the way distad. 

 The forks continue to the end of the tail but have not been found 

 to end in pores to the exterior. Numerous branches empty into 

 the single and bifurcated portions of the canal. 



In the matter of origin the species with three flame cells in 

 the anterior and of the worm are undoubtedly the simpler. 

 It is not improbable that the three anteriormost flame cells of 

 the more complex type are homologous to the three cells of the 

 simpler type. This theory is supported by the evidence from 

 C. biflexa, which, with the flexures similar to C. complexa, has 

 the flame cells of the anteriormost portion of the tertiary tubule. 



In view of the relationship existing among these species it 

 seems feasible to classify them in groups on the basis of a flame- 

 cell formula. Thus the flame-cell formula of Cercaria chisolenata, 

 C. trisolenata and C. biflexa is 3, that of C. complexa is 15 = 

 (3 + 12), and that of C. acanthostoma is 16 = (3 + 13). Further 

 study of the flame cells in larval echinostomes will probably 

 furnish evidence of other types and many other points of funda- 

 mental interest. 



XlPHIDIOCERCARI/E. 



Anatomically the xiphidiocercariae have little in common. 

 Each species quite probably has a distinct type of stylet but no 

 satisfactory classification is likely to result on the basis of this 

 larval piercing organ. Likewise the genital and excretory 

 systems show very great differentiation. The writer (1918, 

 38-40) has shown and described some of the fundamental types 

 of bladder and main collecting tubules of this group. Ordinarily 



