THEIR NATURE AND HABITS. 
203 
from observation made on Surirella gemma , a common marine form, 
from the shell of which he maintained that long delicate threads 
projected, which the “ creature ” voluntarily drew in or extended. 
Thread-like processes consisting of fungoid growth or parasitical 
algse are not unfrequently met with, similar to those here described, 
but that they have any connection with the movement of the 
frustule, beyond a tendency to decrease its speed, is not at all 
probable. The ‘ Journal of the Eoyal Microscopical Society ’ for 
June, 1884, reports one of the latest of these revivals, in which pro¬ 
jecting filaments are described as causing motion in foreign particles 
when the frustule passes near, but apparently without being in 
actual contact. A rather novel modification of the ciliary theory is 
suggested by Mr. J. C. Adams in the ‘ American Monthly 
Microscopical Journal’ for 1883, to the effect that the cilia exist 
within the frustule and act against the current of water passing 
through. 
The third hypothesis, which supposes the presence of pro¬ 
toplasmic material exterior to the siliceous envelope, has of late 
received the most supporters, who, however, differ greatly in respect 
to details. Probably the ablest contribution to this theory is a paper 
by Professor Schultze, published in 1865. Professor E. Itay 
Lankester makes special reference to this publication in the 
‘Popular Science Peview ’ for 1866, where he describes it as 
“ ending in a satisfactory and conclusive explanation of the method 
and nature of locomotion in these organisms.” This opinion 
unfortunately has not been universally adopted, and has not 
hindered the publication of numerous modifications and rival 
theories. Schultze concluded, after a long series of most careful 
and minute observations, that the motion was caused by the 
protrusion of hyaline protoplasm through the apertures which exist 
in the raphe, or line of suture connecting the two valves. The 
protoplasm thus extended is supposed to form a moving band or foot¬ 
like process, which, passing along the exterior of the shell, comes in 
contact with some solid body and thus forces the diatom in the 
opposite direction. In support of his theory, Schultze discovered, 
with the aid of very powerful objectives, considerable agitation 
amongst certain minute granules of protoplasm immediately within 
the apertures, but was unable to distinguish any such movement on 
the exterior of the frustule. This, however, he accounted for by 
supposing the apertures to be too minute to allow the exit of these 
granules, by which alone the protoplasm was visible, consequently 
the exterior band would be perfectly transparent. He states that in 
the specimens examined (Pleurosigma angulatum ) the diatom always 
moved with the raphe in contact with some foreign body. This, 
however, is not invariably the case with other species, some of 
which may frequently be observed moving on their secondary sides, 
on which the raphe is absent. Several later writers have suggested 
that free as well as stipitate diatoms are surrounded by an extensible 
membrane, and that the movements are in some way connected with 
this material. A confident statement is made by a writer 
