MOVEMENTS OF SIMPLE ORGANISMS. 37 



face of the cell-wall and shrinks together so as in a short time to present the 

 appearance of a sphere occupying 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 short time in the middle of the cell-cavity. It leans almost 

 immediately 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 without, 

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

 from causes inherent in the protoplasm. 



MOVEMENTS OF VOLVOCINE^E, DIATOMACE^E, OSCILLAELE 



AND BACTEEIA. 



Very remarkable is the movement of those wonderful organisms which are 

 comprised under the name of Volvocinese. 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 firm 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 oarsmen, as soon as the protoplasts, which form the crew 

 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 Volvox-sphere rolling along can fail to be 

 impressed and delighted. 



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 



