102 
F. Cavers. 
have arisen, while from them lines may be traced which lead to the 
Proteomyxa, the Rhizopoda, the Foraminifera, the Heliozoa, and 
in fact to the whole of the remaining groups of Protozoa. The 
other colourless Flagellata are comprised in Senn’s groups Proto¬ 
mastigineae and Distomatineae, which differ from the Panto- 
stomatineae in that ingestion is possible only at certain points of 
the body and which are easily derived from Pantostomatineae along 
two diverging lines. The Distomatineae are a small group, differing 
from all other Flagellata in having a bilateral but asymmetrical 
body with two mouth-spots (oral grooves) placed on two opposite 
surfaces and 4 to 8 flagella arranged in two groups in or close to 
the mouth-spots. The periplast is very thin and the body capable 
of some change of form, as is also the case in the large group 
Protomastigineae which includes the great majority of the colourless 
Flagellata and is fairly sharply marked off from the Pantostomatineae 
by having in the holozoic forms a single mouth-spot, usually anterior 
and terminal but in a few cases lateral, and from the Distomatineae 
by never having the flagella in paired groups. 
The Protomastigineae are of great importance as including 
forms from which, according to the view usually taken, there may 
have arisen the coloured Flagellata groups—the Brown Flagellata 
(Chrysomonads in the wide sense, including Cryptomonads) and 
the Yellow-green Flagellata (Chloromonads in the wide sense, 
including Euglenineae)—which have in turn given rise to the Brown 
and Yellow-green Algal series. The relationships of the transitional 
and algal members of these coloured series were discussed in the 
paper dealing with the Flagellata and Primitive Algse. On the 
other hand, there are among the Protomastigineae forms which may 
well serve as the starting-points of lines leading to the Chytridiales 
and to the Myxomycetes, as well as to the Infusoria. 
Semi divides the Protomastigineae into nine families, each of 
which begins with a simple form essentially resembling a Panto- 
stomatinean genus like Mastigamceba or Cercobodo in structure but 
showing an advance in organisation in some special feature which 
is further elaborated in the higher members of the family. According 
to the number and arrangement of the flagella five main groups can 
be distinguished, to which may be added the heterogeneous forms 
which in addition to 1 or 2 flagella have short cilium-like appendages 
and which probably form a transition to the Infusoria. Of these 
five larger groups the first, marked by the possession of a single 
flagellum, begins with Oikomonas (Fig. 2, A) which is practically a 
Mastigamceba with ingestion limited to a mouth-spot or “protrusible 
vacuole ” at the base of the single anterior flagellum; this region 
thus forms a more or less permanent projecting organ of ingestion, 
though the vacuole subsequently migrates to the hinder part of the 
body, which is capable of amoeboid movements and can also project 
to form an attaching stalk. A notable line arising from Oikomonas 
apparently leads through elongated spindle-shaped forms like 
Leptomonas (parasitic) and Arcyromonas and flattened forms like 
Phyllomonas, the two latter with the flagellum directed backwards, 
to the parasitic Herpetomonas and the uniflagellate Trypanosomes 
(family Trypanomorphidae, including Trypanomorpha) which have 
an undulating membrane and a dual nuclear apparatus. Two other 
lines, the interpretation of which appears to have led to some 
