Opalina. 307 
Usually in higher Caliata the conjugating individuals are nearly 
or exactly equal in size. It might seem that Opalina, having gametes 
of unequal size, is in this regard less primitive, but the fact that 
in the Vorticellas, whose gametes completely fuse, the gametes 
are often very unequal in size, and also the conditions in Opalina, 
argue in favor of a belief that anisogametes are for the Ciliata 
more primitive than isogametes. Caukins & CuLu reach a similar 
conclusion. They say (1907, p. 405). “There is little evidence to 
indicate the lines of evolution that have been followed in the deve- 
lopment of the participants in conjugation. The view that is usually 
adopted, without supporting evidence, is that the isogamous type 
like that of Paramaecitum, was primitive and has developed into 
an anisogamous type with sexually differentiated gametes (e. g. 
Harroe 1906). It is our belief that the reverse has been the case 
and that the Paramaecium type of conjugation has arisen from a 
type with sexually differentiated gametes, with intermediate stages 
in forms like the Vortzcelldae, where the size difference is great in 
Lagenophrys ampulla, less marked in Epistylis, and still less in Vorti- 
cella; and in Trachylinidae, where in Lionotus fasciotas the two or- 
ganisms are alike save for a slight difference in size (CauKrns 1902). 
In the Vorticelhdae the macrogamete fuses with the microgamete and 
there is no mutual fertilization, but in Paramaecium and probably 
in Lionotus, mutual fertilization takes place. The case of Lionotus 
is to be interpreted as a reminiscence of anisogamy, and we would 
expect in this case, that the smaller conjugant, if fertilized, would 
have a reduced vitality. In Paramaecium, finally, there is no mor- 
phological evidence of the relation to an earlier anisogamous con- 
dition, but there is well-marked physiological evidence in the lesser 
vitality of one of the ex-conjugants, apparent in 72°/, of all con- 
jugations in which the history of both was followed (Cun 1907).” 
We can, I think, at least say that its anisogamous and slightly 
heterogamous copulation does not argue strongly, if at all, against 
the comparatively primitive character of Opalina.') 
Finally the character of the excretory organs in Opalina — 
merely enlarged and confluent vacuoles of the ordinary cytoplasmic 
1) The very interesting Pycnothrix monocystoides described by Scuusotz 
(1908), which is apparently a holotrochous Ciliate, seems to have unequal gametes, 
but the exact systematic position of this remarkable form cannot be determined 
until its life history is more fully known, so that we cannot now say whither its 
condition argues in favor of the primitive nature of anisogamy for the Ciliata, 
or not. . 
