240 M. M. Mercatr 
individuals would show a similar proportion of transverse divisions. 
Figs. 44, Pl. XVII, and 20, Pl. XV show individuals in transverse 
division. ZELLER describes and figures transverse divisions for 
QO. intestinalis, but does not speak definitely as to their relative fre- 
quency, though implying that they are numerous. 
In O. caudata transverse division is of the same character and 
about as frequent as in O. intestinalis. The longitudinal divisions 
are exactly similar in the two species. In O. dimidiata longitudinal 
division resembles that of the binucleated species, except that in 
this multinucleated form there is no apparent connection between 
nuclear division and the division of the body. I have not observed 
transverse division in this species, but ZELLER’s observations show 
clearly that it occurs. I have once seen longitudinal division of the 
body in O. zelleri: it resembled that of O. dimidiata. In O. ob- 
trigona and O. ranarum one finds longitudinal, transverse and 
irregular divisions, the latter in the spring when division is very 
rapid (Text Fig. III). In all species the longitudinal divisions follow 
the main axis of the body. As this is bent,) the really longitudinal 
divisions, especially in the flattened species, appear to be oblique, 
as ZELLER has described them. Conn (1904) and ScHouTEDEN (1905) 
have shown that the so-called oblique division of O. ranarum is 
morphologically longitudinal. I have studied O. ranarwm but little, 
but from observation of O. obtrigona I doubt if the longitudinal and 
transverse divisions are so definite in their sequence as ZELLER 
describes them. Reference to the figures of irregular division in 
O. obtrigona (Text Fig. III) shows that in the spring one may find 
almost any sort of irregularily, even two or three entirely irregular 
1) Ciliata and Flagellata have either the body form asymmetrical, or the 
organs of locomotion asymmetrically arranged, or both, so that the animals rotate 
on their main axes as they swim, producing spiral progression, Opalina is no 
exception to this rule. The spiral motion in Opalina is caused by two factors, 
first by the bend in the anterior end of the body, second by the spiral arrangement 
of the rows of cilia. The latter is not a result of the former, for, if one should 
straighten out the bend in the body of an Opalina, the rows of cilia would still 
be spiral. Dare (1901), Watrencren (1903), quoted by Jennines (1906), em- 
phasize also the direction of the beat of the individual cilia in O. ranarum in 
producing spiral progression, rows of cilia along the right side of the anterior 
end being said to beat forward and to the left, while the others beat backward 
(Text Fig. XVI, page 335). It seems to me these authors have failed to emphasize 
that the broad anterior end of the body in this species is bent “to the right” as 
is so evident in other more slender species, so that the morphological anterior end 
is not the actual anterior end. The cilia in all species seem to beat nearly if not 
quite in the morphologically posterior direction. 
