WHEEL ANIMALCULES 



159 



or a leech — fixing itself by its tail, then stretching out the 

 head and fixing that, 

 whilst letting go the tail 

 and bringing it up by 

 " telescoping " it, near 

 to the head region. The 

 tail is forked, and in the 

 side view (Fig. 34, B) it 

 is seen to have a soft 

 branched process, which 

 helps it to cling. The 

 letter V in Fig. 34, A, 

 points to the vent or 

 opening of the gut at 

 the fork of the tail. 

 The mouth, marked 

 M, is seen between the 

 two " wheels." The two 

 "wheels" are really 

 two discs, the edges of 

 which are beset by 

 coarse "cilia," or vibrat- 

 ing hairs of protoplasm. 1 

 These cilia " lash " and 

 straighten again one 

 after the other, so that 

 the optical illusion is 

 produced of the toothed 

 edge of the disc being 

 in movement like a 

 wheel. They may be 

 "focused" with the 

 microscope so that the groups or 



Fig. 34. — Diagram of Rotifer vulgaris — 

 the common wheel animalcule — one 

 hundred and twenty times as long as 

 the creature itself. A, front view. 

 B, side view. C, head showing eyes S, 

 and retracted wheel apparatus W. 

 The letters in A and B have the 

 following signification : A/, mouth. 

 W, wheel or ciliated disc. S, eye 

 spots on head. T, spur or tentacle. 

 G, gizzard. St, stomach. Int, in- 

 testines. V, vent: aperture of intestine. 



bunches " of them 



1 For some account of "cilia," see "Science from an Easy Chair," 

 Figs. 29, 33, 40 and the accompanying text. 



