62 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
end, and frequently it is highly vacuolated at its anterior end. Fi igure 
52 (Plate 4) represents a cross section of a Pyrsonympha through the 
nucleus, showing its position and that of the flagellum (/y.) in relation 
to the wall of the body. The nucleus varies considerably in shape ; 
sometimes it is very much attenuated (Fig. 50), and at others almost 
spherical (Fig. 53, nl). 
Within Pyrsonympha is a lash-like filament or flagellum (/9.), as 
shall call it, which is by far the most remarkable structure of this para- 
site. In the living specimen it is in constant motion, great waves pass- 
ing from the attached to the opposite end of the animal, often giving it 
the appearance of a revolving polygon inside the animal. Leidy says, 
“The motion of the indio cord and of the animal together im- 
pressed me with the idea of a snake in a bag, making its presence obvious 
in active contortions.” The effect of this fl: agellum is not unlike that of 
a churn, for it keeps the contents of the sac-like body, including the food, 
thoroughly stirred up. 
The flagellum is of nearly uniform thickness throughout the most of 
its length, and, as cross sections (Fig. 52) show, is oval in section ; its 
diameter varies with the state of contraction, but averages about 1.5 
by 2. Near its anterior end, where it joins the peduncle by means of 
the darkly staining tubercle, it sometimes becomes very much attenuated 
(Plate 4, Fig. 46). I am not certain, however, but that this may be 
due to abnormal tension induced by the increased activity of the flagel- 
lum when the parasite is put in normal salt solution. However this 
may be, the posterior portion of the flagellum tapers off very 8 rradually, 
finally becoming very fine at its posterior end (Migs. 45, 53, 5 55). 
Throughout the most of its length it is quite free from the body-wall, 
and moves about with vigorous strokes in the most unexpected manner. 
It is, however, attached to the body-wall at or near the posterior end of 
the parasite, as well as at the region of the peduncle. But quite fre- 
quently it seems to break loose from this posterior place of attachment, 
— perhaps owing to a too violent whipping about caused by the stimu- 
lating effect of the salt solution, — and then it sometimes projects pos- 
teriorly in the form of a tail-like appendage. This caudal appendage is 
surrounded by a layer of protoplasm, which has considerable thickness 
at its base, but becomes reduced to a condition of great tenuity (Plate 3, 
Fig. 34) at its tip. It flaps backward and forward very violently with 
every undulation of the flagellum, 
Leidy (77, p. 437) says of Pyrsonympha, “Sometimes too it appears 
terminated by a caudal appendage of variable form and length, but this 
