MOVEMENTS 2O/ 



the animal is not fixed by its foot. In some cases, as in Bdelloida, 

 the cilia lash downwards successively in the longitudinal plane of 

 the body (Fig. 109, C, D); this motion during fixation produces 

 a hollow vortex ring, like the rings of a skilled cigarette-smoker, 

 but when the animal is free it determines a simple forward pro- 

 gression through the water. In other cases the animal rotates 

 on its long axis, or may even turn somersaults (Synchaeta). The 

 appearance of the spokes of a wheel is a pure illusion due to the 

 greater visibility of the cilia in their slow recovery than in 

 their instantaneous down-lash. The finer cilia of the groove and 

 cingulum play a very minor part in the act of swimming, and in 

 the production of the great vortices at the edge of the disc when 

 the animal is fixed ; they serve to direct the particles brought by 

 the vortices to the edge of the disc onwards towards the mouth. 

 It is easy to see that the stream must be in opposite directions 

 on opposite sides of the groove ; its prolongation across the dorsal 

 median line would be useless, which explains the existence of 

 the dorsal median gap. At the ventral side we usually find a 

 prominent ciliated lip, whose cilia work outwards, and carry off 

 the excess of food particles as by an overflow spout. In many 

 cases among the Notommatidae, Coluridae, etc., the disc serves as 

 much for creeping over organic debris as for swimming. 



We have already noticed the springing bristles and limbs 

 of the Triarthridae and Scirtopoda respectively ; the great foot 

 of Scandium is also used for leaping. The Bdelloida have the 

 power of retracting their disc and progressing in loops like a 

 leech or looper (Geometrid) caterpillar. 



Baker, in a letter addressed to Martin Folkes, Esq., President 

 of the Eoyal Society, dated London, 16th January 1744-5, 1 gives 

 the following lively account of the aspect and movements of 

 Philodina roseola belonging to this group, with figures, some of 

 which we reproduce from the original copper-plate engraving : 

 " I call it a Water Animal, because its Appearance as a living 

 Creature is only in that Element. I give it also for Distinction 

 Sake the Name of Wlieeler, Wlieel Insect or Animal ; from its 

 being furnished with a Pair of Instruments, which in Figure and 

 Motion appear much to resemble Wheels. It can, however, con- 

 tinue many Months out of Water, and dry as Dust ; in which 

 Condition its Shape is globular, its Bigness exceeds not a Grain of 

 1 Reprinted in Baker's Employment for the Microscope, 1785, pp. 267 f. 



