1 66 Practical Zoology. 



the disappearance of a vacuole. Do the two vacuoles contract 

 at the same time ? 



6. Means of Defense. Observe in the ectosarc many small, 

 oval sacs, with their ends toward the surface. These are the 

 thread cells. They contain a thread, which can be shot out, serv- 

 ing as a means of defense. 



7. Reproduction. Do you find a paramecium that is becom- 

 ing constricted in the middle like a dumb-bell or hourglass? 

 Paramecium divides into two by narrowing in the middle, pre- 

 ceding which is a division of the nuclei. If such a specimen 

 can be found, watch the process to completion, if possible. Draw 

 and describe all the stages of division, or fission. 



VORTICELLA, THE BELL ANIMALCULE. 



Vorticellae are often found on the stems and leaves of water 

 plants, or on stems and leaves that have fallen into the water. 

 Mount slender stems from water and examine wit^ a low power. 

 If you find a bell-shaped form attached by a slender, flexible 

 stalk, and especially if the stalk is suddenly thrown into a coil, 

 jerking the bell close to the stem to which it is attached, you 

 may be sure you have vorticella or a near relative. Some of 

 these forms occur in colonies, all growing from one main stalk. 

 Some have the power of coiling the stalk, while in others the 

 stalk is not contractile. Some of the colonies can be seen by the 

 naked eye, appearing like tiny spots of mold. If such a colony 

 is found, the part of the stem or leaf to which it is attached 

 should be carefully cut out and mounted on a slide. With a 

 higher power the details of form and structure may be studied. 



i. Form and Structure. The body is bell-shaped, with a 

 long, slender, flexible stalk in place of a handle. The outer 

 layer is a thin cuticle, next is the ectosarc, and the inner is the 

 endosarc. The extension which nearly fills the mouth of the 

 bell is the disk. The border of the bell is the peristome. On 

 one side find a groove between the disk and the peristome. This 



