64. BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
63). The superficial muscular cords exist, but take a longitudinal 
course as often as a spiral one (Figs. 57, 61). Sometimes they are 
distributed over only a part of the animal (Fig. 61). The nucleus of 
these small individuals resembles that of the adult, but is situated at 
the extreme anterior end. If, therefore, it were not for the presence of 
the flagellum and peduncle in some of these forms, we should be quite 
unwarranted in supposing them to be in any way related to Pyrsonympha. 
As it is, however, I think there can be no doubt of the relationship. 
The flagellum and peduncle are both absent in some forms (Figs. 
59, 60, 65) which I take to be the earliest known conditions of this 
parasite. 
I have come to the conclusion that the flagellum of Pyrsonympha is 
merely a differentiation of one of the superficial muscular cords. We 
may conceive that the cord which is destined to become a flagellum, 
after becoming larger, stronger, and more active than any of the other 
cords (Plate 5, Fig. 61), simply splits off from the inner surface of the 
body-wall, remaining fixed only at its two extremities; thus it enjoys 
free play for all its peculiar undulations. 
The peduncle is apparently a slow-growing structure. In sections of 
the intestine one often finds young parasites in closely packed masses, 
lining considerable portions of the intestinal wall (Plate 5, Fig. 56). 
Their anterior ends lie very close to the intestinal epithelium, thus 
showing that the peduncle is still very short. The young here referred 
to (Plate 5, Fig. 56) are not more than one third or one fourth as long 
as the adult parasites shown in Figure 45 (Plate 4). The deeply 
stained body which is to be seen at the anterior tip of even the youngest 
individuals (Fig. 56) is the knob-like structure or tubercle of the adult, 
from which the peduncle arises. 
In their activity the young far exceed the adults, for besides the 
undulation of the muscular cords, the animal is constantly changing its 
shape. Figures 59, 60, and 65 (Plate 5) represent different shapes, 
taken by the same individual in the course of a few minutes. 
It is of interest in this connection to note the effect of these para- 
sites on the intestine of the host (Plate 5, Fig. 56). The epithelial 
lining is very much indented. The peduncles force their way between 
the cells, reaching sometimes almost to the underlying muscular layer. 
The cells themselves are often reduced in size, but otherwise apparently 
perfectly healthy. It-is a mystery how the host can support such a vast 
number of parasites, unless they in turn are in some way of benefit 
to it. 
