184 THE ESSEX NATURALIST.
But there is a more fundamental peculiarity of structure
which is not so easily observed in the living condition, About
all that can be seen of this in the living diatom are a couple of
curved lines running obliquely over the main part of the frustule
as shown in Fig. 1. In mounted specimens, however, these
curved lines are found to be parts of two narrow dotted bands,
with about 20 dots in 10u, running from end to end of the frustule
in about three turns, thus forming a double-threaded screw
pattern on the surface of the siliceous casing of the diatom
(Fig. 2). Careful examination of these dotted bands under a
high power of the microscope shows that they are similar to the
so-called "canal-raphe" of a typical Nitzschia. Whereas,
however, in the latter case there is one canal-raphe running
in a straight line from one end to the other of each of the two
valves, in Cylindrotheca the two are twisted in a spiral about the
apical axis. In all the specimens that I have examined the spiral
has been a right handed one. There appears to be no mention
of the direction of the spiral in any account of this species ;
Hustedt (1928), however, in his Fig. 21 shows a left handed
spiral, but in his later Fig. 746 (1930) it is a right handed one.
In addition to the two dotted bands there can also be seen,
under favourable conditions, two other very faint structureless
bands occupying intermediate positions with regard to the dotted
ones (Fig. 2). It is probable that these correspond to the
girdle bands of other diatoms. The conclusion therefore seems
to be that Cylindrotheca is essentially a twisted sort of Nitzschia.4
This complete twisting is unique in the whole range of the Dia-
tomaceae, for although cases occur in which there is a partial
twisting of the frustule, e.g. in Surirella spiralis and the species
of Amphiprora, in none of these is there more than half a turn
about the axis.
We come now to the characteristic movement of Cylindrotheca.
As the cell travels forwards it rotates on its long axis exactly as
a screw would do (Fig. 4). No other diatom does this.
Moreover it is easy to see that the rotation is right handed if
attention is fixed upon the chromatophores. As already stated
there are usually three or four of these on each side of the central
nucleus. Each of these consists of a comparatively large rounded
plate closely adpressed to the cell-wall, but without making a
complete ring (Fig. 1), so that there is a change of appearance
as the cell rotates. This enables one to be quite sure of the
direction in which rotation is taking place. The connection
between the right handed spiral structure and the right handed
rotation is so obvious that the conclusion that the one conditions
the other is inescapable.
4 W. Smith (1853-1856) was nearer the truth in this matter than many later authors, for
he actually named it Nitzschia taenia.