I30 VORTICELLA AND ZOOTHAMNIUM less. 



A striking characteristic of Vorticella is its extreme 

 irritability, i.e., the readiness with which it responds to any 

 external stimulus (see p. lo). The slightest jar of the 

 microscope, the contact of some other organism, or even a 

 current of water produced by some free-swimming form like 

 Paramcecium, is felt directly by the bell-animalcule, and is 

 followed by an instantaneous change in the relative position of 

 its parts. The stalk becomes coiled into a close spiral (d\ d^) 

 so as to have a mere fraction of its original length, and the 

 body from being bell-shaped becomes globular, the disc being 

 withdrawn and the peristome closed over it (d^, d^). 



The coiling of the stalk leads us to the consideration ot 

 the particular form of contractility called muscular, which 

 we have already met with in Stylonychia (p. ii8). It was 

 mentioned above that while the stalk in its fully expanded 

 condition is straight, the axial fibre is not straight, but forms 

 a very open spiral, i.e., it does not lie in the centre of 

 the stalk but at any transverse section is nearer the surface 

 at one spot than elsewhere, and this point as we ascend the 

 stalk is directed successively to all points of the compass. 



Now suppose that the axial fibre undergoes a sudden con- 

 traction, that is to say, a decrease in length accompanied by 

 an increase in diameter, since as we have already seen 

 (p. lo) there is no decrease in volume in protoplasmic 

 contraction. There will naturally follow a corresponding 

 shortening of the elastic cuticular substance which forms the 

 outer layer of the stalk. If the axial fibre were entirely 

 towards one side of the stalk, the result of the contraction 

 would be a flexure of the stalk towards that side, but, as its 

 direction is spiral, the stalk is bent successively in every 

 direction, that is, is thrown into a close spiral coil. 



The axial fibre is therefore a portion of the protoplasm 

 which possesses the property of contractility in a special 



