Opalina. 223 
In O. caudata the conditions are very nearly the same, but the 
posterior end of the organ is usually branched, one or two shorter 
branches being seen in addition to the chief branch which runs 
forward along the axis of the body. The relation to the nuclei is 
like that in O. intestinalis and the excretory granules are similar. 
In O. dimidiata (Fig. 17) the conditions resemble those in 
QO. intestinalis. In this species there are many nuclei. One often 
sees that most, if not all, of these nuclei are surrounded by narrow 
perinuclear vacuoles. Many of the posterior nuclei are enveloped 
by the excretory organ. Probably by no means all of the perinuclear 
vacuoles have any direct connection with the excretory organ, but 
their contained excreta probably reach the excretory vacuoles by 
dialysis through the intermediate alveoles of the endoplasm. In 
Fig. 17, Pl. XIV, which represents the posterior end of an unusually 
slender but nearly full grown O. dimidiata, the axial series of excretory 
vacuoles is very clearly seen to consist merely of enlarged and 
irregularly fused alveoles of the endoplasm. The most posterior 
nucleus, in mitosis, is shown entirely enveloped by the excretory 
vacuoles. One often sees individuals of O. dimidiata, especially 
small forms in the spring, dragging behind them masses of extruded 
excretory granules. 
The three species already mentioned are circular in cross section 
and have the excretory vacuoles along the axis of the body. 
O. obtrigona and O. ranarum are very flat. It is possibly because 
of this flattening that their excretory organs are so much less deve- 
loped. In O. ranarum I have found no trace of any excretory organ, 
though one often finds perinuclear vacuoles present. In O. obtrigona 
there is present only a slight rudiment of the posterior end of the 
excretory organ in the form of a small elipsoidal or semilunar 
vacuole at the extreme posterior tip of the body. This vacuole 
occasionally contracts and one sees the shrunken, depressed contour 
where the vacuole previously was. No excretory granules have been 
found in this vacuole, nor have I seen the living animals dragging 
a mass of extruded granules after them, as is so frequent in the 
three spindle shaped species above described. 
Excretory organs have not been seen before in Opalina, the 
genus being always described as unique among the Cilvata in having 
no excretory vacuoles. 
The condition of the excretory canals in the new Ciliate Pycnothrix 
monocystoides, described by Scuusotz (1908) is very interesting 
in comparison with the cylindrical Opalinae. This simple axial 
