94 PHYLUM PROTOZOA THE SIMPLEST ANIMALS. 



a fine striation. (A similar striated structure is seen in 

 some Amuebse, Gregarines, spermatozoa, etc., and above all 

 in striped muscle fibres. It seems to be some structural 

 adaptation to contractility.) The bell has a thickened 

 margin, and within this lies a disc-like lid ; in a depression 

 on the left side, between the margin and the disc, there is 



FIG. 40. Vorticella. After Butschli. 



1. Structure. A r ., Macronucleus ; ., micronucleus ; c.v., con- 



tractile vacuole ; ;//., mouth ',f.v., food vacuole ; v., vestibule. 



2. Encysted individual. 3. Division. 



4. Separation of a free-swimming unit the result of a division. 



5. Formation of eight minute units (;.). 



6. Conjugation of microzooid (mg.) with one of normal size. 



an opening, the mouth, which leads by a distinct passage 

 into the cell. On the side of this passage there is a weak 

 spot, the potential anus, by which useless debris is passed 

 out. The cilia are arranged so as to waft food particles 

 into the mouth and down the passage. There is a large 

 and horseshoe-shaped macronucleus, and a small micro- 

 nucleus. Food vacuoles and contractile vacuoles are 

 present as usual. 



