IOO 
F. Cavers. 
The Rhizomastigaceae may he divided into uniflagellate and 
biflagellate genera, which present a parallel series. The uniflagellate 
forms include Mas tig amoeba, Mastigina, Mastigella, Actinomonas and 
Pteridomonas. In Mastigamceba (Fig. 1, B, C) the body is ovoid or 
elongated in the free-swimming phase, and even during this phase 
sometimes has pseudopodia, while in the amoeboid phase it creeps 
by means of pseudopodia which are often branched ; the flagellum, 
persistent through the amoeboid phase, arises directly from the 
nucleus which lies at the anterior end of the body. The same 
relation between flagellum and nucleus, which apparently does not 
obtain in other Flagellata and which recalls the relation of the axial 
filament of the pseudopodia in Heliozoa to the nucleus, occurs in 
the allied genus Mastigina which differs from Mastigamceba in having 
the body covered by a thick pellicle. In these two genera the only 
form of reproduction known is by longitudinal fission in the motile 
state, but Goldschmidt (1907) has observed a sexual process in 
Mastigella (Fig. 1, D, E), which differs in having the flagellum quite 
independent of the nucleus and arising from a flagellar reservoir as 
in various other Flagellates. During the vegetative life of Masti¬ 
gella vitrina binary fissions occur, preceded by withdrawal of the 
pseudopodia and flagellum and a mitotic division of the nucleus, 
the number of chromosomes seen in these divisions being about 40. 
Sexual reproduction is preceded by the formation of micro- and 
megagametocytes, in which minute granules of chromatin (chromidia) 
are extruded from the nucleus, increase in number and size, and 
form the nuclei of the gametes; in the formation of the mega- 
gainetes at least one mitotic division occurs which is interpreted as 
a polar division, and a similar division probably occurs also in that 
of the microgametes, but the nuclear elements are so small that a 
definite reduction in number of chromosomes was not established. 
In both kinds of gametocytes encystment occurs, accompanied by 
withdrawal of pseudopodia and flagellum, but the microgametocyte 
encysts sooner than the megagametocyte ; the gametes, both uni¬ 
flagellate, escape from the gametocytes and conjugate to form an 
active zygote which divides several times by fission and then grows 
and assumes the usual characters of the genus. In Actinomonas 
(Fig. 1, F) the body is ovoid or spherical, capable only of slight 
changes in form, and gives out numerous fine radiating pseudopodia ; 
it may either be free-swimming or become attached by a pseudo¬ 
podium-like stalk at the posterior end. The pseudopodia are, as in 
Mastigella, provided with a central axis which springs from the 
endoplasm—this appears to be the case in all those Rhizomastigaceae 
which have a heliozooid phase, or in which the pseudopodia become 
more or less permanent slender structures during part of the life 
of the organism—and in A. vernalis they are capable of being fully 
retracted. Pteridomonas (Fig. 1, G—I) is a remarkable genus, in 
which the single long stout flagellum is surrounded by a series of 
8—12 short fine flagella, or cilia, which are capable of becoming 
rolled inwards like a watchspring and then bent outwards, exerting 
a jerking action by which the organism hops backwards ; or these 
cilia may simply assist the flagellum in propelling the body forwards; 
while when the organism is attached by a stalk, formed as in Actino¬ 
monas, two of the cilia, somewhat thicker than the others, perform 
