16 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



paranuclei uniting. Increased vigour (rejuvenescence) appears to 

 be imparted by this process, which generally shows itself in greater 

 reproductive energy, as displayed in fission. More rarely, encyst- 

 ment may follow, when the nucleus becomes larger and much 

 longer, ultimately breaking up into a number of spores, oval 

 bodies, each provided with a circlet of cilia at one end. These 

 are liberated by the rupture of the cyst, and becoming attached 

 by their ciliated ends, grow up into adult forms. Before this 

 they may, however, increase by a process of fission. 



4. Contractility. The presence of a firm cuticle prevents the 

 formation of pseudopodia, though protoplasmic currents are observ- 

 able in the endosarc, by which the food-vacuoles are carried round 

 the body, and such streamings are also seen in the firmer ectosarc, 

 though to a much less extent. Locomotion is effected in the free 

 forms by cilia, while the currents that bring food and oxygen, 

 and carry away waste, are also due to ciliary action. Each cilium 

 is a delicate thread of protoplasm, protruded from the ectosarc 

 through a pore in the cuticle. By means of the alternate con- 

 traction of its longitudinal halves, bending and straightening are 

 produced in turn. 



The retraction of the body is due to the ectosarc, and in this 

 case contraction takes place is the direction of the myophan 

 striation, whence it appears probable that the deeper layer is the 

 part mostly concerned in the process. The thread which traverses 

 the stalk contracts in such a definite way, getting shorter and 

 broader, that it deserves to be called a specially contractile or 

 muscle fibre. The spiral direction taken by the fibre causes the 

 stalk to be readily thrown into spiral folds. The much slower 

 process of expansion appears to be largely due to the elasticity of 

 the cuticle, and this is especially true of the stalk. 



5. Irritability and Spontaneity. These phenomena are much 

 more definitely exhibited than in the case of Amoeba. The 

 animals appear sensitive to the slightest touch, which causes 

 them to contract rapidly. The same effect may be produced 

 by irritant solutions, such as weak acetic acid, and by other 

 stimuli. Spontaneity is shown by the way in which the cilia 

 work together to a common end, instead of acting irregularly. 

 Spontaneity is further exemplified by free-swimming individuals 

 in fixing and conjugation. 



