VOETICELLA 



9 



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 become everted, 

 and the ciliary movement is resumed. 



It 



FIG. 3. A group of Vorticellae ; x 200. (c. H. H.) 



I. In the fully expanded condition : the arrows indicate the direction of 

 the currents of water. II. Expanded, and seen from the vestibular or ventral 

 surface. III. Seen obliquely from below ; the arrows indicate the direction of 

 movement of the endosarc. IV. Seen from above ; arrows as in I. V. In the 

 contracted condition. VI., VII., and VIII. Three successive stages of fission 

 VI., early stage of fission ; VII., later stage of fission, showing the posterior 

 circle of cilia on the individual about to separate ; VIII., last stage of fission, 

 the free-swimming individual just separating. 



CF, contractile fibre of the stalk. D, disc. P, food vacuole. M, myophan 

 striation. N, nucleus. PH, pharynx. PV, pulsating vacuole. V, vestibule. 



B. Structure. 



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

 also lines the vestibule and pharynx. The stalk is 

 a prolongation of the ectosarc only. 



