318 M. M. Mercaur 
O. zellert is slightly compressed to the form of a very thick wedge. 
If this compression could be carried much further until the whole 
body was thin and flat, its anterior end, and indeed its whole form, 
would resemble that of O. ranarum, for frequently O. ranarum has 
the contour of the posterior end of the body concave. 
At the posterior end of the body the longitudinal ridges show 
rounded ends, there being a terminal depression of considerable 
depth between their posterior ends. O. dimidiata is pointed posteriorly, 
often very sharply pointed and slender, differing most markedly 
from O. zellerz. It was the superficial furrows between the ridges at 
the posterior end of the body, rather poorly drawn by ZELLER, which 
Denace & Herrovuarpd interpreted as internal canals, remnants of a 
system of water canals. 
The broad rounded longitudinal ridges are usually five or six 
in number, though one finds individuals showing four or eight ridges. 
They are constant, not changing as the animal swims. They are 
slightly spiral, following the same general direction as the spiral 
lines of cilia. 
Opalinae of any cylindrical species may often, when living under 
adverse conditions, show a decided spiral twisting of the body, which 
is then raised into ridges. These ridges are constant, not changing 
as the animals swim. They may possibly be, in a general may, com- 
parable to those of O. zelleri. 
Always at least one and often two of the longitudinal ridges 
show at the posterior end a rounded protrusion, giving a more 
pointed appearance than that of the other ridges. It is possible that 
this rounded point marks the morphological posterior end of the 
body and that when two such points are present they indicate nascent 
longitudinal division of the body. These points remind one slightly 
of the short sharp protrusions from the posterior ends of very stocky 
Opalinae caudatae (Fig. 88, Pl. XX), though I have never seen two 
points upon an animal of the latter species. 
The minute structure of O. zelleri, as seen in sections, agrees 
so exactly with that of O. dimidiata, as to need no description. The 
anterior end of the body, as in all other species of Opalina, has 
denser endoplasma and more numerous endosarc spherules than the 
rest of the body. 
This form deserves further study, but it is rare. I found it 
only twice, once on the 22? of June and again on an unrecorded 
date at about the same time. No cysts were present with the 
Opalinas in the rectum. ZeELLER describes its reproduction as like 
