358 D. J. SCOURFIELD ON THE LOCOMOTION OF 



at any but the slowest speeds the locomotive organs must either 

 possess very large surfaces so as to get a grip of the water, or, 

 if small, they must act in a very rapid manner. Numerous 

 instances of both of these arrangements occur among microscopic 

 aquatic creatures. 



On the other hand, as compared with air, the resistance offered 

 by water is very considerable, and although this implies that the 

 locomotive organs of aquatic organisms need not be so large nor 

 act as rapidly as those of flying creatures, it does mean that 

 for quick locomotion the organism should be fashioned so as to 

 offer the least amount of obstruction to its passage through the 

 water. It thus comes about that most of the good swimmers are 

 of an elongated form and especially of some modification of the 

 fusiform type. 



There is one other consideration in connection with locomotion 

 under water which may be referred to, for it is one which has 

 made itself very much felt in connection with the navigation of 

 submarine vessels. It is the difficulty of keeping a straight 

 course owing to the want of fixed points or datum lines, for both 

 horizontal and vertical directions, from which bearings may be 

 taken. Some automatic arrangement is necessary, and various 

 contrivances for this purpose are to be found among aquatic 

 creatures, but the most common, at least among the lower forms, 

 is the power of rotation on the long axis. This rotation 

 produces a compensating effect for any little irregularities in 

 shape or motive power, so that, although the actual path really 

 becomes a long spiral, its general direction is practically a 

 straight line. 



Coming now to some of the different kinds of locomotion found 

 among the minute inhabitants of water, we must commence by 

 glancing at the so-called amoeboid motion. This, as the name 

 suggests, is typically seen in the Amoeba, a very lowly form 

 of animal life, often described, though not quite accurately, 

 as a mere " speck of animated jelly." The locomotion is produced 

 in the simplest conceivable way, namely, by the flowing out 

 in various directions of portions of the actual body-substance 

 of the animal. One of these outflowing processes, or pseudopodia 

 as they are called, usually becomes bigger than the others, and 

 finally draws the rest of the body into itself, so that the whole 

 comes to occupy a new position ; and as the formation of new 



