io6 PROTOZOA. 



containing some of them is held between the eye and the 

 light. Their food consists of small vegetable particles. 



In form, Faramcecium is a long oval ; the cell-substance is 

 bounded by a definite rind ; the cilia, which occur all over in 

 regular rows, serve for locomotion and for driving food- 

 particles into an opening or " mouth " on one side. Among 

 the cilia on the rind, there are small cavities in which Ue fine 

 protrusible threads (" trichocysts "), believed to be of the 

 nature of weapons. Internally, there are two nuclei, the 

 smaller "micro-nucleus" lying by the side of the larger 

 " macro-nucleus." There are food vacuoles as usual, and two 

 contractile vacuoles drain the surrounding cell-substance 

 and burst to the exterior. 



Life-History. — Growth is followed as usual by division 

 into two. One-half includes the " mouth," the other has to 

 make one. In conjugation, which is essential to the continued 

 fitness of the species, two individuals interchange certain 

 micro-nuclear elements, and both nuclei are entirely recon- 

 stituted. 



Vorticella or the bell-animalcule, — a type of those ciliated 

 Infusorians in which the cilia are restricted to a region around 

 the mouth. 



Groups of Vorticella grow on the stems of freshwater plants, 

 and are readily visible to the unaided eye as white fringes. 

 Each individual suggests a bell with a long flexible handle. 

 The base of the stalk is moored to the water-weed, the bell 

 swings in the water, now jerking out to the full length of its 

 tether, and again cowering down with the stalk contracted 

 into a close and delicate spiral. Up the centre of the stalk 

 runs a contractile filament, which in shortening gives the 

 non-contractile sheath a spiral form. The " mouth " is at one 

 side of the upper margin of the bell, and the cilia round 

 about this margin and close to the mouth, are arranged in 

 a manner which secures the inwafting of food-particles. 

 The nucleus is of horse-shoe shape. Food vacuoles and 

 contractile vacuoles are present as usual. 



Sometimes a Vorticella bell jerks itself off its stalk and 

 swims about ; in other conditions it may be passive and 

 somewhat encysted ; normally the cilia of the bell are very 

 active, and the contractions and expansions of the stalk 



