MOVEMEXTS OF SIMI'l.E ORGANISMS. 37 



face of the cell-wall ami shrinks together so as in a sliort time to present the 

 appearance of a sphere occupj-ing the middle of the cell-cavity. Again, just as 

 this contraction is an instance of a special form of protoplasmic motion, so also 

 the further change which the contracted protoplast in a cell of Spirogyra under- 

 goes is reducible to displacements in its substance, and must be mentioned as 

 a special kind of protoplasmic movement. For the conglomerated protoplast 

 remains but a sliort time in the middle of the cell-cavity. It leans almost 

 innnediately to one side, thrusting itself into a protuberance of the cell-mem- 

 brane, which is concurrently developed, and which, when further developed, forms 

 a passage leading over into another cell-cavity. Its body becomes longer and 

 narrower, and at last slips through the passage into the next cavity, where a 

 second protoplast awaits it; and the two then unite, fusing together into one 

 mass. It is not premature to remark that all these displacements and invest- 

 ments of the protoplasmic substance in cells of Spirogyra, including the pheno- 

 mena of contraction, as well as those of pushing forward, escape, and coalescence, 

 are not produced as the results of a shock, impulse, or stimulus from \\-ithout, 

 but are to be looked upon as movements proper to the protoplasm, and resulting 

 from causes inhei'ent in the protoplasm. 



MOVEMENTS OF VOLVOCINE^, DIATOMACE^, OSCILLAEI^ 



AND BACTERIA. 



Very remarkable is the movement of those wonderful organisms -nhich are 

 comprised under the name of Volvocinea3. One species, Volvox globator, was 

 known to so ancient an observer as Leeuwenhoek; but he, and after him Linnaeus, 

 took it to be an animal on account of its extraordinary power of locomotion, and it 

 was named the "globe-animalcule." A Volvox-sphere consists of a large number of 

 green protoplasts living together as a family and arranged with great regularity 

 within their common envelope. They appear to be disposed radially, and to be 

 linked together and held tirm by a net-work of tough threads, their poles being 

 directed towards the centre and the periphery of the sphere respectively. From 

 the peripheral extremity, which in each protoplast is marked out by a bright red 

 spot, proceed a pair of cilia, and these protrude through the soft gelatinous 

 envelope of the whole sphere, and move rhythmically in the surrounding water. 

 A Volvox-globe rolls along in the water propelled by regular strokes, like a boat 

 manned by a number of oai-smen, as soon as the protoplasts, which form the ci-ew 

 of this strange vessel, begin to manipulate their propellers. The effect is exceed- 

 ingly graceful, and has justly filled observers of all periods with astonishment; 

 indeed no one seeing for the first time a \'^olvox-sphere rolling along can fail to be 

 impressed and delighteil. 



Another plant allied to the foregoing, the so-called "red-snow," has always 

 excited wonder in no less degree from the remarkable phenomena of motion which 

 it exhibits, but also because of its characteristic occurrence in situations where one 



