ON THE MOVEMENTS OF THE DIATOMACE2E . 401 
as to become either supported or jammed endways between two obstacles. In 
this case particles in contact with the sides are carried up and down from the 
extreme ends with a jerking movement and a strange tendency to adherence, 
the Diatom seeming unwilling to part with the captured particle. Under 
these circumstances I have distinctly perceived the undulating movement of 
an external membrane ; whether this envelops the whole surface of the 
siliceous valves I am not able to determine, nor do I know if the existence 
of such a membrane has yet been recognized. The movement that I refer to 
occupied the place at the junction of the two valves , and is caused by the 
undulation of what is known as the “connecting membrane.” This will 
account for the progressive motion of the Diatomaceae, which is performed in 
a manner analogous to that of the gasteropoda. The primary cause, however, 
is different, and not due to any property of animal vitality, but arises, in my 
opinion, from the effects of vegetable circulation. I have observed several 
corpuscles of uniform size travel to and fro, apparently within the membrane 
which is thus raised in waves by their passage. 
We now come to tlie researches of Professor Max Schultze, 
of Bonn, which were published in December, 1865. After 
a review of the history of the subject and some remarks on 
the external currents in Rhizosolenia and Coscinodiscus, he 
proceeds to describe his very minute and careful observations 
on a marine naviculoid species, the Pleurosigma angulatum , 
which is not unfrequently to be met with on the sea-coast. 
The P. angulatum is to be preferred for examination to the 
larger P. balticum, because the transverse markings on its 
frustule do not impede to so great an extent the observation 
of what is going on within. When you have a specimen of 
P. angulatum under the microscope, if crawling, it always 
has its broad side turned to view* with one long curved 
“ raphe 39 uppermost and the other in contact with the glass 
on which it is placed ; at the central part is seen the thick- 
ened “ umbilicus 99 (PI. XYIII., figs. 1, 3). Within the siliceous 
frustule is the yellow colouring matter, or “ endochrome,” 
which fills the cavity more or less completely, and is arranged 
in two longitudinal masses, to the right and left of the raphe. 
In the broader part of the frustule these bands of endo- 
chrome describe one or two complicated windings, as seen in 
the figure. The breadth of these bands varies in different 
individuals, according to the development of the endochrome. 
It is only possible in those specimens in which the bands 
are narrow properly to trace their foldings and ascertain that 
only two exist, since an examination of frustules richer in 
endochrome has led to the impression that there are four of 
these bands, and, indeed, is often hardly satisfactory as to 
their existence. Often a long searching* is required before 
the spots which are not occupied by the endochrome can be 
discovered, and then they are always found to he under the 
