210 
Excretory System in Digenea 
Between the third and seventh branches (tertiary tubules) there are several 
islands of protoplasm (Fig. 6). A study of the younger stages indicates that 
these were originally lines of separation for the tertiary tubules. 
Observation of the excretory system in consecutive periods of development 
in the fluke makes clear the process of successive differentiation. The young 
larva just beyond the germ-ball stage (Fig. 2) has a single drainage tubule for 
each side of the body, opening only at the posterior end and terminating 
anteriad in a single flame-cell. As the larva develops and the tail region is 
definitely marked off, the caudal portion of each tubule (Fig. 3) approaches 
the median line. Meanwhile the flame-cell at the anterior end of the body has 
divided and the main tubule has become recurrent, so that at the end of this 
stage there is a single flame-cell in the region of the pharynx and a second 
flame-cell in a postacetabular position. The next stage (Fig. 4) shows a merging 
of the two canals in the tail trunk with a separation only in the furcal buds. 
Each of the two flame-cells in the body has given rise to four cells and the 
canals have split far backward. This double series of four flame-cells I regard 
as the fundamental pattern of the excretory system of Cercaria pekinensis. 
Later these tertiary branches separate from one another, providing the eight 
main branches of the system. By a subsequent bifurcation and two succeeding 
quadruple splittings the pattern of the developed cercaria is achieved. 
In conformity with previous studies in this series (Faust 1919, 1919 a, 
1919 b), the pattern of the system may be expressed as 
[(2 x 4 x 4) + (2 x 4 x 4) + (2 x 4 x 4) + (2 x 4 x 4)] 
+ [(2 x 4 x 4) + (2 x 4 x 4) + (2 x 4 x 4) + (2 x 4 x 4)]. 
The primitive pattern may be designated by a + j8, while the fundamental 
pattern is 
[a 1 + a u + a iU + a iv ] + [)S i + + fi iU + j8 iv ]. 
A study of the sporocyst shows that exactly the same pattern obtains. 
There are two pores, one on each side of the body with a tubule emptying 
through it. Each main tubule in turn receives an anterior and a posterior 
branch. Tracing the system further distad each secondary tubule gives place 
to four tertiary ones, while each tertiary one is the confluence of four capillaries 
with a flame-cell at each terminus (Fig. 5). It is seen that this pattern is precise, 
a replica, indeed, of the double fourfold division in each of the quaternary 
tubules of the cercaria. Taking the entire excretory system on each side of 
the sporocyst, one sees that it corresponds part by part with a single main 
(tertiary) tubule of the cercaria. Phylogenetically considered, the system 
in this species is, therefore, more conservative than is usual in Digenea. 
Since this same pattern probably obtains in Cercaria stephanocauda , it seems 
possible that the entire group of cystocercous cercariae may have the same 
fundamental common denominator, 
[a 4 + a u + a iU + a iv ] + [0* + + \8 iU + 
It likewise seems significant that in the cercaria the pattern has been impressed 
