16 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



d. The stem is thrown into a spiral. 



VIII. Notice that this order is reversed during expan- 

 sion, which takes place much more slowly. 



IX. The Structure of the Body. 



As in ParamcBcium, the body-substance consists of 

 three layers, — the cuticle, the ectosarc, and the endosarc. 



a. The endosarc (Fig. 4, ^, Fig. 5, d) occupies the 

 central region of the body, but does not extend into the 

 stem. Its transparent, colorless sarcode contains numer- 

 ous minute, dark-colored granules, and it also contains 

 food vacuoles (Fig. 4, g), oil-drops, and foreign border 

 such as have been noticed in Paramoecium and Amoeba. 



1. Careful observation of a single vacuole or solid par- 

 ticle will show that the whole semi-fluid endosarc is in mo- 

 tion. The motion is most vigorous near the surface, and 

 least so in the centre. If the animal be placed with the 

 ciliated disc above, and the vestibule away from the ob- 

 server, the current will be found to flow 

 down the left side, across the bottom, and 

 up on the right side, as shown in Fig. 5, 

 by the arrows. 



Fig. 5. — Diagram of a vertical section of Vorti- 

 cella nebulifera, to show the arrangement of the 

 layers of the body (from Everts). 



a. Cuticle, b. Contractile layer of Ectosarc. 

 c. Inner lay-er of Ectosarc. d. Endosarc. e. Endo- 

 plast. /. Stem. 



2. Notice the movements of the semi-fluid endosarc 

 caused by changes in the shape of the body, and carefully 

 distinguish these movements from the constant circulation 

 of the endosarc. 



b. The ectosarc (Fig. 4, Tc and Fig. 5, c) is thin above ; 

 but it gradually thickens below, and it forms the entire 

 axis of the stem. The line separating it from the endo- 



PlG. 5. 



