62 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



end, and frequently it is highly vacuolated at its anterior end. Figure 

 52 (Plate 4) represents a cross section of a Pyrsonympha through the 

 nucleus, showing its position and that of the flagellum {fg.) 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, nQ. 



Within Pyrsonympha is a lash-like filament or flagellum {fg.), as I 

 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 undulating 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 eifect of this flagellum is not unlike that of 

 a chura, 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 yu. 

 by 2 II. Near its anterior end, where it joins tlie 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 gradually, 

 finally becoming very fine at its posterior end (Figs. 45, 53, 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 

 tlie 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 efi'ect 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 



