(Trichodesmium erythraeum) with that found over large spaces
in the Red Sea, and whence its name of Red Sea is derived. [8]
Their numbers must be infinite: the ship passed through
several bands of them, one of which was about ten yards
wide, and, judging from the mud-like colour of the water,
at least two and a half miles long. In almost every long
voyage some account is given of these confervae. They appear
especially common in the sea near Australia; and off
Cape Leeuwin I found an allied but smaller and apparently
different species. Captain Cook, in his third voyage, remarks,
that the sailors gave to this appearance the name of
sea-sawdust.

Near Keeling Atoll, in the Indian Ocean, I observed
many little masses of confervae a few inches square, consisting
of long cylindrical threads of excessive thinness, so as
to be barely visible to the naked eye, mingled with other
rather larger bodies, finely conical at both ends. Two of
these are shown in the woodcut united together. They vary
in length from .04 to .06, and even to .08 of an inch in
length; and in diameter from .006 to .008 of an inch. Near
one extremity of the cylindrical part, a green septum, formed
of granular matter, and thickest in the middle, may generally
be seen. This, I believe, is the bottom of a most delicate,
colourless sac, composed of a pulpy substance, which lines
the exterior case, but does not extend within the extreme
conical points. In some specimens, small but perfect spheres
of brownish granular matter supplied the
places of the septa; and I observed the curious process by
which they were produced. The pulpy matter of the internal