60 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
is from 1 to 1.5 in diameter, and may attain a length of at least 
75 p. It is deeply embedded in the epithelial wall of the host's intes- 
tine. J am not certain that I have been able to trace it to its end, and 
do not know if there is any specialized structure 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. 46, 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 aliaost naked and ameeboid in its nature. The 
slightest pressure of an object on its surface would cause it to enter 
the substance of the body. 1 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 which 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 
