362 D. J. SCOURFIELD ON THE LOCOMOTION OF 



actual change of position that is taking place among the cilia. 

 The well-known ciliary organs of many Rotifers, looking just like 

 rotating cog-wheels, is a case in point. 



Many ciliated organisms rotate as they swim through the 

 water, and it is possible that this may be brought about by the 

 cilia beating somewhat obliquely to the direction of the long 

 axis of the body. In numerous cases, however, it would not be 

 necessary to make this assumption, as the shape of the body 

 is such (more or less spiral as a whole, or provided with twisted 

 ridges, grooves, etc.) that it would naturally rotate when pro- 

 pelled through the water. The result in either case is the same, 

 and it is of obvious advantage to the organisms, as already 

 pointed out. 



The presence of cilia for locomotive purposes is found to extend 

 to many very different groups of creatures. The ciliated In- 

 fusorians, of course, exhibit these organs in perfection, but they 

 also occur (either constantly or in certain stages of development) 

 in such diverse forms as Sponges, Hydrozoa, Echinoderms, 

 Worms, Rotifers, Gastrotricha, and Mollusca. I am inclined 

 also to include such a form as Volvox as a ciliated organism, 

 in spite of the fact that there are only two motile threads 

 attached to each cell in the colony, and that they are commonly 

 alluded to as flagella ; for the general result of the action of 

 all the threads seems to me to be of the ciliate and not of the 

 flagellate type, and it is extremely doubtful whether the in- 

 dividual threads act in the spiral manner described in connection 

 with flagella. 



Before going on to the consideration of locomotion produced 

 by specialised appendages, a few words may be said about 

 two or three kinds of progression which depend upon various 

 movements of the whole body. For instance, there is what 

 may be called the medusoid motion, which consists of the sudden 

 closing of a more or less umbrella-shaped organism. This type 

 of movement is shown among microscopic forms by the curious 

 Protozoan Leptodiscus and also by the tiny medusoids of some 

 Hydrozoa. Then there are a large number of small aquatic 

 creatures which progress by throwing their bodies into a series 

 of waves or even figures-of-eight. The Nematodes are excellent 

 examples of this, but many other worms, and also insect larvae, 

 propel themselves through the water in this way. Yet another 



