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 tlie 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 difi'ereutiation 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 

 imdulatiou of the muscular cords, the animal is constantly changing its 

 shape. Figures 59, 60, and 65 (Plate 5) represent difi"erent 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. 



