A Critical Study of certain Unicellular Cyanophyceae 85 
the spindle-shape of Dactylococcopsis . The tapering extremities of the 
cell are a very constant feature of this genus, as of many species of 
the Oscillatoriaceae. In most species of Dactylococcopsis the cells 
show a tendency towards spiral coiling, but in the peculiar plank¬ 
tonic D. acicularis Lemm. the cell is straight and needle-like. The 
stiff character of the cells, which is seen to a less extent in other 
species of Dactylococcopsis affords a very clear systematic distinction 
from the Aphanothece type. In its cell-form Dactylococcopsis connects 
the Chroococcaceae with certain of the Oscillatoriaceae. By a slight 
elongation of the spiral which we may imagine as forming the axis 
of the cell we pass on to forms belonging to the genus Spirulina. 
These have a unicellular spiral filament 1 . It is worthy of note that 
there are no straight forms corresponding with the genus Spirulina. 
Here again the spiral form may be due to mechanical factors. The 
species are all characterised by rather slender cells, an average taken 
from the recorded measurements of seventeen well-defined species 
gives the diameter of the trichome as 2-0 [ jl . Except in size there 
appears to be no essential difference between Spirulina and 
Arthrospira, although the latter always has septate filaments. 
For an average of the width of trichome recorded for the seven 
recognised species of Arthrospira is found to be 5-1 fi and this suggests 
that the septate condition in Arthrospira may have been developed in 
relation to the more robust character of the plant. Where the septate 
condition obtains, i.e. in the majority of the Oscillatoriaceae, the 
straight form of the filament becomes mechanically efficient and is, of 
course, the predominant type in the family. There are reasons for sup¬ 
posing that the unbranched trichome found in many Oscillatoriaceae 
is homologous with the single cell of the Chroococcaceae. The observa¬ 
tions of Fischer (6), who found that the pigment-bearing region of 
the cell in Oscillatoria and Lyngbya has the form of a cylinder with 
open ends, so that, apart from the transverse septa the central region 
forms a continuous core throughout the trichome, support this view. 
The work of Fritsch(8) on the structure of the membrane, showing 
that in Oscillatoria the sheath is not split on the formation of a new 
transverse septum, is also in harmony with it, although this investi¬ 
gator suggests that the constrictions of the sheath opposite the septa 
in some species indicate a compound origin. These constrictions, 
however, were probably developed in the first place by the lower rate 
of mucilage secretion at the region of the transverse septa. At these 
1 Traces of septa may be observed in some species on staining by special 
methods, see Zuelzer, Archiv /. Protistenkunde, Bd. 28 , 1911. 
