10 THE PROTOZOA 



5. The pharynx is the narrower continuation of the vesti- 



bule inwards. Its cilia drive water, bearing food- 

 particles, into the endosarc. 



6. The anus opens into the vestibule, but is only visible at 



the moment of extrusion of faecal matter. 



7. The contractility of the animal is exhibited when it is 



irritated, as by lightly touching the cover-glass with 

 a needle. The peristome is then suddenly turned in, 

 concealing the disc and cilia, and at the same moment 

 the stalk is suddenly coiled up into a corkscrew 

 spiral. If the irritation be discontinued, the stalk 

 gradually straightens out, the disc and peristome be- 

 come everted, and the ciliary movement is resumed. 



B. Structure. 

 1. The ectosarc is the external layer of the cell-body : it 

 also hnes the vestibule and pharynx. The stalk is 

 a prolongation of the ectosarc only. 



a. The cuticle forms a protective covering for the 



whole surface. It is thickest on the stalk, and 

 the straightening out of the stalk after contraction 

 probably depends upon its elasticity. 



b. CiUa are restricted to the margin of the disc, 



the vestibule, and the pharynx. They produce 

 a current which runs round the disc in the 

 direction opposite to that of the hands of a 

 watch, then down the vestibule near its ventral 

 surface, and, after a turn in the upper part of the 

 pharynx, up along the dorsal wall of the vestibule 

 to the exterior. The appearance of a long vibra- 

 tile ' vestibular seta ' in the dorsal part of the 

 vestibule is probably due to the movements of the 

 vestibular cilia. 



c. A longitudinal ' myophan ' striation is visible in the 



deeper layer of the ectosarc, especially in the 

 lower part of the body close to the stalk. 



d. The contractile band running down inside the 



cuticular sheath of the stalk is continuous above 



