40 MOVEMENTS OF SBIPLE 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 (Pleurosigma, Pinmdaria, 

 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 Volvocineae; 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 Diatomacese 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 periodic 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 capacity 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; 



