VORTICELLA. 



93 



vestibule 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 

 ParajHoechim as regards the course of the food and the temporary 

 anus. 



Vorticella is, like most sedentary anmials, 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 Pararncecijim^ there are no excretory nor respiratory 

 organs. 



Reproduction 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. 



