26 ILLINOIS BIOLOGICAL MONOGRAPHS [26 



The tail arises as a median posterior protuberance, bilaterally symmetrical, 

 and is, according to the views of Ssinitzin, phylogenetically a paired organ. 

 This thesis is supported, in part at least, by the fact that the excretory trunks 

 arise as paired organs in both the body and the tail. In the furcocercariae 

 the caudal tubules remain separate in the rami of the tail and also in the 

 "eyelet anastomosis" at the junction of the body and the tail. There devel- 

 ops in the tail the usual complement of muscles, a transverse layer externally 

 and a longitudinal group more median. Within the cylinder of muscles is the 

 group of parenchyma cells surrounding the excretory tubule. 



In the tails of distome cercariae (Figs. 99, 133) the excretory vessel is a 

 paired structure, separated in the middle by a parenchymatous partition with 

 one or two nuclei in each section oil jx thickness. Some schistosomatid larvae 

 have, in addition, eleven or twelve pairs of oblong cells just lateral to the 

 excretory vessel. The tail of the monostome is characterized by extra large 

 longitudinal muscles with prominent nuclei. The portion within the longi- 

 tudinal muscle cylinder differs in structure in individual species. In C. pellu- 

 cida there is one ring of very large parenchyma cells situated around the 

 excretory vessel. There are eight to ten cells to each transverse plane of 7ju. 

 In C. konadensis and C. urbanensis there are glandular cells within the paren- 

 chyma ring; they are large and crowded with granules. In both of these species 

 (Figs. 25, 32) these cells are arranged in six paired groups. In C. konadensis 

 there are many cells to each member of the group, arranged in pyramidal 

 fashion with the apex directed distally. Thus the largest cells in each group 

 (Fig. 27) are proximal. These cells lie next to the excretory vessel. Cort 

 has described the cells of C. urbanensis thus: "extending the length of the 

 tail and forming a core are two rows of long cells v/hich are close together and 

 have their long axes parallel with the length of the tail. . . . They are full 

 of heavy staining granules. . . There is nothing suggestive of the possible 

 function of these cells. " He has failed to observe the exact number of these 

 cells (six pairs) and is in error in considering them as a core extending the 

 whole length of the tail, for they alternate with non-glandular tissue in about 

 half of the extent of the organ. Their structure is probably glandular. In 

 C. urbanensis these cells arise from undifferentiated parenchyma cells (Fig. 33). 

 They soon appear as falciform cells in trans-section (Fig. 34), separated in 

 a median sagittal plane by a partition arising between two intermediate 

 parenchyma cells, which soon differentiate into a muscular lamina. The 

 lamina arises before the excretory tubules differentiate as distinct lumina 

 among the parenchyma cells. Thus the bilateral symmetry along the median 

 sagittal plane is well shown. The excretory vessel is single in the mature 

 C. pellucida and C. konadensis, but remains paired in C. urbanensis. 



Looss (1893:24-28) cites the epithelial cells of the tail of cercariae as good 

 examples of " Blasenzellen, " where all cell elements of the mesenchyme usually 

 become " Blasenzellen ", and where no true glands take their place. The study 

 of C. konadensis, C. urbanensis, and C. gracillima, shows that axial cell glands 

 are present, and that they are derived from the parenchyma. Moreover 



