THE MECHANISM OF LIFE 



499 



that are constantly changing their form, but, like Bacteria and 

 Infiosoria, possess an axially differentiated body of a constant 

 form, which moves through the water by means of special organoids 

 of motion, flagella and cilia. The bodies of these organisms are 

 driven through the water after the manner of a row-boat by the 

 rhythmic stroke of the flagella or cilia. The analogy of the motion 

 to that of a boat moved by oars is complete, and may be recognised 

 even in details. Exactly the same means are used to turn and 

 direct the movements of the rowed boat, and the movements of 

 the free-swimming ciliated cell, and we can represent the peculiar 

 behaviour of Bacteria and Infusoria in their axial orientation upon 

 unilateral stimulation no better than by means of this simile. 

 Among the various organisms that propel their extended bodies 

 through the water by means of flagella or cilia, three types can be 

 distinguished as most important, according as they move by means 

 of a single flagellum, two flagella, or several or 

 very many cilia, corresponding to a boat pro- 

 pelled by one, two, or many oars. 



We will examine, first, the forms that possess 

 one flagellum, such as many Bacteria and flagel- 

 late Infusoria, and will select as representative 

 the delicate, green, flagellate-infusorian^w^'^ewff, 

 which, in summer, by means of its countless 

 numbers, changes the water of standing pools 

 into a deep green. The flagellum of the 

 Flagellata is upon the anterior pole of the body 

 and moves through the water in a screw-like 

 path. For the sake of simplicity its motion 

 may be considered as taking place in a single 

 plane. It is then seen that it oscillates about 

 the straight middle position (Fig. 247, a) by 

 means of alternate rhythmic contractions toward the right (5) 

 and toward the left {h^ ; the swing out of the middle position 

 (a) into one of the two extreme positions (5 or \) represents 

 the phase of contraction, the return from one of the extreme 

 positions into the middle position the phase of expansion. The 

 flagellum works, therefore, like an oar that is moved alternately to 

 the right and to the left at the bow of a boat. It is evident 

 that, while undisturbed and having equal conditions upon allsides, 

 the infusorian body must move forward in a straight line, if the 

 flagellum beats equally strongly toward the right and toward the 

 left, i.e., if contraction and expansion occur with equal rapidity 

 toward the two sides. But if a contractile stimulus acts upon the 

 flagellate suddenly from one side, and if the long axis of the body 

 is not already turned in the direction of the stnnuius with the 

 posterior pole toward its source, such a position is assumed by 

 means of a few strokes of the flagellum ; for with every oblique or 



K K 2 



Fiu. 247. — Sclieme of the 

 contraction of the 

 flagellum of a flagel- 

 late-infusorian cell. 



