140 AMEBOID MOVEMENT 



organisms of which this holds true include, as far as known, all 

 the free-swimming unicellulars, swarm spores of algae and fungi, 

 uni- and multinucleate zoospores, rotifers, a large number of 

 worms and worm larvae of all classes (excepting the nematodes) 

 and the larvae of many molluscs, echinoderms and copepods as 

 well as some adult copepods. Organisms restricted to two dimen- 

 sions of space in their movements, in which orderly paths have 

 been recorded, are ameba and man and perhaps we may include 

 the horse and the dog. This is indeed only a small number of 

 organisms compared with all that can move; but there are rep-, 

 resentatives in the list of all the large groups excepting the higher 

 plants, and without doubt observation will greatly extend the 

 list, for there are mentioned here only such organisms whose 

 movements have been definitely recorded or personally observed. 

 As far as now known, no organism lacking orienting organs 

 moves in a straight line. Many spermatozoa with flagellate tails 

 seem, however, to do so, but no careful studies of their paths have 

 yet been made. 8 



The orderliness of the paths of these organisms when moving 

 under such conditions as described above, is itself orderly; that 

 is, the path of all these organisms is a spiral of one kind or 

 another: (i) a helical spiral, as in the free-swimming unicel- 

 lulars; (2) a true spiral in one plane, as in man; (3) a helical 

 spiral projected on a plane surface, as in ameba. 



These facts point inevitably to the hypothesis that the move- 

 ments of these and all other moving organisms are controlled 

 by an automatic regulating mechanism, which is of essentially 



8 Since this was written I have been able to examine the movement of 

 live sperm cells in a number .of representative animals, including the 

 jellyfish Aurelia; the molluscs Ostrea, Solemya, Pandora; the arthropods 

 Limulus and Anisolabia, and the vertbrates frog, turtle, snake, cat, dog 

 and man, with the result that all these spermatozoa revolve on their long 

 axes and swim in spiral paths resembling those of flagellates. Owing to 

 their minute size their movements are made out only with great difficulty, 

 but so far as could be determined all the sperms of any one species turn 

 on their axes in the same way, that is, -either right-handed or left-handed. 

 Recently there has also come to my notice the very informing paper of 

 W. D. Hoyt, 1910, in the Botanical Gazette, in which it is stated that 

 fern sperms of various species swim in spiral paths. 



