40 MOVEMENTS OF SIMPLE ORGANISMS. 
Beggiatoa. As regards the Diatoms, some of them are firmly attached to a 
support, and are not generally capable of locomotion; but others are almost in- 
cessantly in motion, and these little unicellular organisms steer themselves about 
with great precision near the bottom of the pools of water in which they live. 
Their cell-membrane is transformed into a siliceous coat, and this coat, which is 
hyaline and transparent, but very hard, consists of two halves shutting together 
like the valves of a mussel. The entire cell thus coated has the form of a gondola 
or little boat, with a keel either straight or curved (Plewrosigma, Pinnularia, 
Navicula), and is provided with various bands, ribs, and sculpturings on its 
siliceous walls. Driven by inherent forces, these little protected cruisers pursue 
their way at the bottom of the water or over objects which happen to be in the 
water. They either glide evenly over the substratum, or else proceed by fits 
and starts at rather long intervals, and apparently with difficulty. For some 
time they may hold a straight course, but not infrequently they deviate side- 
ways without apparent cause, and after deviating return again. They double 
round projecting objects or push them out of the way with one of their hard 
points, which are often thickened into nodules, and cause the obstructing objects 
to slip by alongside the keel of the little vessel. Yet no paddles or cilia are to 
be seen projecting from it, as in the case already described of Volvocinex®; nor 
does the siliceous coat exhibit any sort of motile processes whereto the move- 
ments might be attributed. But the strong analogy between the structure of 
these Diatomaceze and that of mussels seems to justify the assumption that the 
two siliceous valves, which are fast shut during the period of rest of the Diatoms 
in question, move a little apart, so that the protoplast living within can push 
out one edge of its body and creep along over the substratum by means of it. 
The movements of the filaments of Beggiatoa, Oscillaria, and Zonotrichia 
are explained in a similar manner. ‘These filaments are made up of a number 
of short cylindrical or discoid cells, and are attached by one end, but with the 
other execute most striking movements. They stretch themselves and then 
contract again, coil up and straighten out like snakes, and, most characteristic 
of all, make periodie oscillations in the water. The belief is that the mechanism 
of this motion is similar to that of the preceding, that infinitesimally fine fila- 
ments of protoplasm inserted spirally penetrate the cell-walls, and that these act 
like the propeller of a ship. 
On looking back over the multifarious examples of movement that have been 
described, the conviction that the eapacity for motion is inherent in all living 
protoplasts is difficult to resist. In many cases, of course, the displacement and 
replacement of the substance no doubt takes place so slowly that it is scarcely 
possible to express its amount numerically. Movement may even entirely cease 
for a time; but, as necessity arises, and under favourable external circumstances, 
the protoplasmic mass always becomes mobile again—the direction of its motion 
being determined by inherent forces. There is still much to learn, no doubt, con- 
cerning the objects and significance of the different movements of protoplasm; 
