308 M. M. Mercaur 
foam — is very lowly. As parasitic life does not seem in general 
to produce degeneration of the excretory organs, the lowly character 
of the excretory organs of Opalina is probably primitive. Their re- 
semblance to the excretory organs of Hoplitophrya argues for the 
relationship of the two genera. 
It seems, therefore that Opalina is a member of the Ciliata and 
that in many respects it is quite primitive. 
Ltebr and Dusoscg (1904 6) suggest that, because of its habi- 
tat in a marine fish, O. saturnalis is probably the most primitive 
Opalina known, all others being parasitic in terrestrial or fresh- 
water forms. I have suggested above that the restriction of the 
rapid division and subsequent conjugation in Opalina to a brief time 
in the spring is an adaption to the habits of its hosts. It would 
be interesting to know of O. satwrnalis 1) if it shows a similar re- 
stricted period of rapid division followed by conjugation, and 2) if 
the habits of Box boops are such as to make such a restriction in 
the rapid reproduction period of its parasite a decided advantage. 
If the habits of Box boops are not such as to render infection much 
more easy at one period, and if, in spite of this fact, O. saturnalis 
still has a restricted period of rapid reproduction, then probably 
the adoption of Box boops as a host is recent, O. saturnalis showing 
a peculiarity in its reproduction, which was originally adopted in 
adaptation to conditions such as we find among the Amphibia. I do 
not know the habits of Box boops. Licrr & DusosQ found the cysts 
of O. saturnalis in the month of September, but do not say whether 
they are found only at that season. We must, therefore, leave un- 
answered this interesting question as to the comparatively recent 
adoption of Box boops as a host for Opalina. 
It seems probable that the Opalinas were originally uninucleated, 
that the binucleated condition was brought about by the suppresion 
of one division of the body when the nucleus divided, and that the 
multinucleated condition is still more secondary bring due to further 
suppression of divisions of the body. In Text Fig. H, page 206, I 
have grouped the several species of Opalina in three groups which 
seem to me to indicate their probable relationship: group 1) bi- 
nucleated species, all circular in cross section; group 2) multi- 
nucleated species, circular in cross section; group 3) multinucleated 
flattened species. Compare also the list of species on page 207 in which 
the same arrangement is followed. Opalina ranarum, the form which 
has been most studied, is probably one of the most highly modified 
species. 
