PROTOZOA 



27 



Vorticella is attached by a stalk to submerged water-weeds, 

 etc. Up the centre of this stalk runs a muscle iibre, a differ- 

 entiation of the protoplasm, attached at iatervals to the 

 cuticular sheath of the stalk. The differentiation of cortical 

 and medullary protoplasm is well marked. The nucleus is a 

 coiled loop. The animal sometimes encysts, but this is prob- 



ably only the formation of a hypnocyst, and has no repro- 

 ductive significance. Binary longitudinal fission is the com- 

 monest form of reproduction, one half remaiaing on the stalk, 

 the other (macrozooid) acquiring a ring of cilia and swimming 

 away to settle elsewhere. At other times the Vorticella 

 divides into eight microzooids, which conjugate permanently 

 with the sessile iudividuals. These also occasionally produce 

 microzooids by budding, and the colonies are also increased by 

 the formation of buds. 



Order 11. Heterotricha. — The body is covered uniformly 

 with short cilia, and a circlet or spiral of long cilia is developed 

 in relation to the mouth. 



Stentor has a moniliform or beaded nucleus, and a consider- 

 able number of paranuclei. In some cases these latter 

 correspond in number with the beads of the nucleus. >S^. 

 folymorphus is one of the largest CUiata, reaching a length of 

 Jjr in. In this genus conjugation takes place by the oral face, 



