PEOTOZOA 18 



may be, very distant localities. The recognised species are 

 widely distributed. 



Vorticella, the bell animalcule, is common in fresh water, in 

 marshes, and in the sea. It is similar to Paramecium essentially, 

 but it is fixed by a stalk. The cell is modified into a bell-shaped 

 expanded upper part or body, and a long stalk or peduncle 

 which is firmly attached to plant or animal in the water. The 

 bell consists of a granular fluid endoplasm containing a 

 horseshoe-shaped macronucleus and a small rounded micro- 

 nucleus, enclosed in a tough cortical ectoplasm, which is also 

 continued into the stalk. A closer study will show that the 

 bell ends in a rim or collar and that the free flattened end is 

 occupied by a rounded flat disc separated from the collar 

 by a groove, the whole forming the peristome. The cilia 

 are compounded by fusion into strong membranellae and 

 are restricted to the margin of the disc. The row overlaps 

 to form a spiral, the outer limb of which is continued into a 

 depression at one place. This is the outer opening of the 

 vestibule which leads downwards to open into the endoplasm. 

 It is provided with an undulating membrane produced by a 

 further fusion of the cilia. The vestibule also provides a 

 small opening for the discharge of undigested material remain- 

 ing after the digestion of the food. As in Paramecium, there 

 are therefore special openings for ingestion and egestion. The 

 membranellae by their action set up a whirlpool-like current 

 which attracts the particles of food in the neighbourhood 

 into the peristome, and these are swept into the vestibule, and 

 so to the endoplasm. The contractile vacuole discharges into 

 the vestibule. 



The cortical layer of cytoplasm is similar to that 

 of Paramecium, but there are no trichocysts. There is a 

 peripheral layer which covers the cell completely and the 

 peduncle. Internal to this are numerous myonemes which 

 are disposed in longitudinal direction, but there are besides 

 several circular myonemes, especially around the collar. The 

 whole cortical layer, in fact, is a highly modified part of the 

 cytoplasm to provide a sensory surface and intimately asso- 

 ciated contractile fibres, the myonemes. The longitudinal 



