VORTICELLA. 93 
pe opens into the inner protoplasm by a small aperture—the 
mouth. 
Under the upper surface there is a single contractile vacuole and 
deeper down there appears a horseshoe-shaped macro-nucleus with a 
small micro-nucleus. The feeding processes are very like those of 
Paramecium as regards the course of the food and the temporary 
anus. 
Vorticella is, like most sedentary animals, axo-symmetric. The 
stalk is straight when expanded, but on stimulation it contracts into 
Fig. 30.—VORTICELLA NEBULIFERA 
(Entire Colony Magnified). 
C.V. Contractile Vacuole. A free-swimming individual 
with two rings of cilia is seen on the right. 
a spiral coil. The peristome is contracted and the whole bell becomes 
spherical. As in Paramecium, there are no excretory nor respiratory 
organs. 
Pee niacin is by binary fission, the bell dividing down the centre. 
In allied species the two fresh individuals remain on the same stalk and so 
on for several generations. In these instances a ‘‘ colony” is produced, 
but in Vorticella one of the individuals leaves the stalk soon after fission 
and settles elsewhere. In such a case we may regard the migrating half 
as the offspring and the other as the parent. 
