12 THE EROTOZOA. 



b. The contractile band running down inside the 

 cuticular sheath of the stalk is continuous above 

 with the deeper, ' myoneme,' layer of the ectosarc 

 of the body. The spiral shortening of the stalk is 

 due to its contraction. In a specimen in which this 

 band had been broken, it was seen to be drawn up 

 suddenly towards the body at each inversion of 

 the disc, the stalk, however, remaining straight. 



C. Reproduction. 



1. Fission. The cell-body becomes broader from side to side, 



and a constriction appears across its distal surface. 

 This constriction extends downwards till it reaches 

 the base of the cone close to the stalk, dividing the 

 cell-body into halves. Of these, one is like the 

 parent, and remains attached to the stalk. The 

 other, while still attached to the base of the first, 

 develops an aboral circle of cilia near its base, 

 becomes detached, and swims away by means of the 

 aboral cilia. After a time it attaches itself by its 

 base to some submerged object, and forming a stalk 

 by modification of the aboral cilia and elongation 

 of the ectosarc it becomes a Vorticella similar to the 

 parent. The process of fission may be completed 

 in from one to two hours. 



2. Conjugation occurs sometimes, but it is unlike the con- 



jugation of Paramecium in two important points. 

 First, the conjugation is between two dissimilar 

 forms : an ordinary large-stalked form, and a much 

 smaller free-swimming form which has originated 

 by repeated division of a large form. Secondly, 

 the union of the two is a complete and permanent 

 fusion, the smaller being absorbed into the larger. 



This permanen b fusion of a small active cell with 

 a relatively large fixed cell, followed by a division of 

 the fused mass, presents a striking analogy to the 

 process of sexual reproduction occurring in higher 

 animals. 



