276 VORTICELLA chap. 



by a cyclone, the wave would travel round the circle, which 

 would then appear to revolve. 



Naturally the movement of the circlet of cilia produces a 

 small whirlpool in the neighbourhood of the Vorticella, as 

 can be seen by introducing finely-powdered carmine into the 

 water. It is through the agency of this whirlpool that food 

 particles are swept into the mouth, surrounded, as in 

 Paramcecium, by a globule of water : the food-vacuoles 

 (c) thus constituted circulate in the medullary protoplasm, 

 and the non-nutritive parts are finally egested at an anal 

 spot {an) situated near the base of the gullet. 



The stalk (a, si) consists of a very delicate, transparent, 

 outer substance, which is continuous with the cuticle of the 

 body and contains a delicate axial fibre {ax. f) running along 

 it from end to end in a somewhat spiral direction. This 

 fibre is a prolongation of the cortex of the body (c) : under 

 a very high power it appears granular or delicately striated, 

 the striae being continued into the cortex of the proximal 

 part of the body. 



A striking characteristic of Vorticella is its extreme 

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

 external stimulus. 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 so as to have 

 a mere fraction of its original length, and the body from 

 being bell-shaped becomes globular, the disc being with- 

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



The coiling of the stalk leads us to the consideration of 

 the particular form of contractility called muscular, which is 

 met with in multicellular animals, e.g. the frog (p. 60). It 



