PROTOZOA 27 
Vorticella is attached by a stalk to submerged water-weeds, 
etc. Up the centre of this stalk runs a muscle fibre, a differ- 
entiation of the protoplasm, attached at intervals 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- 
Fic. 20.—Trichodina pediculus, Ehrb. x 300. 
I. View of the base. 
1. Mouth. 
bo 
. Contractile vacuole. 
3. Corneous collar. 
commas | Il. View from the side. 
rl. 
i 
1. Corneous collar, 
a4) 2 and 3. Ciliated rings. 
eS 4. Nucleus. 
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 remaining 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 individuals. These also occasionally produce 
microzooids by budding, and the colonies are also increased by 
the formation of buds. 
Order II. Heterotricha— The body is covered uniformly 
with short cilia, and a cirelet 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. 
polymorphus is one of the largest Ciliata, reaching a length of 
35 in. In this genus conjugation takes place by the oral face, 
