60 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



is from I /x to 1.5 /u, in diameter, and may attain a length of at least 

 75 /x. It is deeply embedded in the epithelial wall of the host's intes- 

 tine. I am not certain that 1 have been able to trace it to its end, and 

 do not know if there is any specialized structui'e at the end ; conse- 

 quently I cannot state the possible maximum length. It is usually of 

 almost uniform calibre throughout the most of its length. The region 

 near the point of attachment, however, often shows a spindle-shaped 

 enlargement (Plate 4, Fig. 45). There is invariably a spheroidal struc- 

 ture that I have called the knob or tubercle (Figs. -16, 47, tub.), which 

 serves as a means of connecting the peduncle with the rest of the body ; 

 when an artificial separation between body and peduncle takes place, 

 the tubercle may remain attached to either part, but it usually separates 

 from the body (compare, however, Fig. 47). The tubercle is somewhat 

 thicker, and becomes much more deeply stained, than the peduncle ; it 

 is homogeneous and highly refractive. 



The attached end of Pyrsonympha is that which Leidy has called the 

 anterior end. Accepting this designation simply as a convenience in 

 description, it may be said that the free posterior part of the parasite 

 projects with a rounded end into the lumen of the host's intestine. 

 The body of the parasite, from the narrow tuberculate part to the free 

 rounded end, looks like a thin sac. 



The body-wall itself is frequently excessively thin, and in places 

 the body seems to be almost naked and amoeboid in its nature. The 

 slightest pressure of an object on its surface would cause it to enter 

 the substance of the body. I believe that something of this kind is 

 represented in Figure 40 (Plate 3) : at any rate, in this case a particle 

 of wood fibre was found about three fourths engulfed by the animal ; 

 whether the fragment was accidentally forced into the body, or whether 

 the animal was taken in the act of ingesting it for food, I am unable to 

 say. The parasites, of whicli this was one, had been removed from the 

 host for about an hour when I discovered this individual. During the 

 three or four minutes that I watched this one no change took place, and 

 at the end of that time it was dead. So it is still only a matter of con- 

 jecture that Pyrsonympha engulfs its food by the exceedingly mobile 

 posterior portion of the body. 



However that may be, the body-wall is so thin in all regions as to 

 make the condition noteworthy. There is no place where its thickness 

 is sufficient to allow it to be readily measured. In fully grown indi- 

 viduals the body is only sparsely covered with cilia, and portions of the 

 surface seem to be entirely destitute of them. When present they are 



