266 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS 



of common occurrence, since this mode of progression is usually adopted 

 by the spermatozoa of animals 1 . In general, there is a definite relation 

 between the direction of movement and the direction of the main axis. 

 Thus in Volvox 2 the vegetative or trophic pole goes first, while in the 

 ellipsoid zoospores of Vaucheria^ as well as in equipolar ellipsoid individuals 

 of Pandorina 3 , the long axis is parallel to the direction of movement and in 

 the same line. In all these cases the continually active cilia must work 

 in harmony, since if they all acted in different directions, no definite 

 locomotion could be produced. 



The same applies to the diffusely ciliated as well as to the bipolar 

 bacteria. Among the latter Spirillum undula is included, and it moves 

 alternately with one end first, and then with the other after a period of 

 rest 4 . Intermittent movement is in fact shown by many motile organisms 6 . 

 In the case of Spirillum it is not known whether the cilia at each end 

 undergo a periodic reversal in their mode of action, or whether only one 

 set acts at a time, and whether the two groups produce movement in 

 opposed directions. The organism may either follow a spiral path around 

 an ideal axis or may move along a straight or curved line parallel to 

 the long axis of the body. In the former case the ideal axis may either 

 be parallel or inclined to the long axis of the body 6 . 



The movements of the cilia are autogenic in character and either 

 pursue the same rhythm under constant external conditions or may be 

 subject to self-regulatory periodic inhibition or reversal 7 . As in the case 

 of other forms of movement, the external conditions may modify the 

 ciliary activity, and may under special circumstances produce a reversal of 

 the movement. The same result may on occasion be caused by an auto- 

 genic or aitiogenic modification of the orientation of the cilia in regard 

 to the body. Possibly it is in this way that Paramaecium is induced to 

 move in the opposite direction to the normal one when placed in 0-4 

 to 07 per cent, solutions of sodium chloride 8 . According to Putter the 

 backward direction of movement continues until the organisms have accom- 

 modated themselves to the salt solution. 



In many cases the impact against a foreign body causes the organism 



1 Hertwig, Zelle u. Gewebe, 1893, p. 65. 



3 Overton, Bot. Centralbl., 1889, Bd. xxxix, p. 68. 



8 Pfeffer, Unters. a. d. hot. Inst. zu Tubingen, 1884, Bd. I, p. 443. The same applies to Gonium, 

 in which the long axis of the body is the shorter axis of the colony. On Stephanosphaera see Cohn, 

 Zeitschr. f. wiss. Zoologie, 1853, Bd. iv, p. 84. 



4 Pfeffer, Unters. a. d. bot. Inst. zu Tubingen, 1888, Bd. II, p. 591 ; Migula, System d. Bacterien, 

 1897, Bd. I, p. 108. 



5 Biitschli, Die Protozoen, 1880-9, p. 850. 



6 Nageli, 1. c. Cf. also Biitschli, 1. c., p. 850. On the importance of the rotation of the body 

 in asymmetric organisms cf. Jennings, The American Naturalist, 1901, Vol. xxxv, p. 369. 



7 Loeb, Pfliiger's Archiv f. Physiologic, 1897, Bd. LXVI, p. 533. 



8 Putter, Arch. f. Anat. u. Physiol., physiol. Abth., Supplementband, 1900, p. 397. 



