20 THE SEA SCUM AND ITS NATURE 
tubules, with a clear investment, lying parallel, and in 
juxtaposition in one plane. Each tubule is septate, and the 
intervals between the septa, or cells, are filled with a fluid 
coloured by a soluble pigment which also obscures the natural 
hue of the chlorophyl granules which the cells contain in large 
quantity. 
The Trichodesmium erythreum, Ehrenb., as defined by its 
author [Pogg. Annalen, 1830. p. 506] in 1830, has been found 
by Montagne to include two distinct forms which he has named 
T. Ehrenbergii and T. Hindsii [Ann. des Sciences Nat. 3 Ser. 
vol. ii, pg. 360, pl. xix., and “ Comptes Rendus,” 1844, vol. xix., 
pg- 172] and this interpretation has been followed by Kitzing 
[Species Algarum et Tabule Phycologice I. pl. 9., fig. 3 and 4]. 
More recent authors however have not retained these species, 
and I am of opinion that the differential features stated to be 
afforded by the width of the filaments and the length of the 
intervals between successive septa cannot be relied on, as in 
every example of the Moreton Bay scum examined hy me these 
variants were present. I have not, however, been able to refer 
to either Montagne’s or Kiitzing’s figures of these species, and 
that given by Berkeley [Introd. to Crypt. Botany, London, 1857, 
fig. 36,] as representing T. erythreum does not present suffi- 
cient details to be of use for comparative purposes. 
On keeping a portion of the scum in sea water, decomposition 
of the nature of fermentation with the evolution of gas sets in 
after a few hours, and the pigment being discharged from the 
finid contents of the cells and from their contained chlorophyl, 
the scum itself changes from brown to green, and ultimately toa 
darker colour, meanwhile the filaments become ruptured or not 
at the septa, by the dissolution of their investing hyaline 
sheath. 
Although I have never heard of such an occurrence in 
Queensland waters, it is very evident that under certain natural 
conditions in other quarters—yielded by the degree of the 
