54 COLLEGE ZOOLOGY 



alcohol is added to a drop of water containing specimens. The 

 pellicle will then be raised as a blister, and will be seen to 



consist of many hexagonal areas 

 which produce striations on the 

 surface. 



The motile organs are thin 

 thread-like cilia, one of which pro- 

 jects from the center of each hex- 

 agonal area of the cuticle. The 

 beating of the cilia propels the 

 FIG. 34- Paramecium defend- animal forward or backward, and 



ing itself from an attack by a , ft - i *_ 



PROTOZoox,Didinium. The trich- draws food Particles into the 



ocysts are discharged and me- mouth, 



chanically force the enemy away. T . , .-, ,-, ,,. , 



(From Mast in Bioi. Bui.) J ust beneath the pellicle is a 



layer of spindle-shaped cavities in 



the ectoplasm filled with a semi-fluid substance. These are 

 called trichocysts (tr.} , and are probably weapons of offense and 

 defense. Under certain conditions the trichocysts may be ex- 

 ploded, for example when a little acetic acid is added to the 

 water, and long threads are discharged. 

 Figure 34 shows a Paramecium repelling 

 the attack of another PROTOZOON by the 

 explosion of its trichocysts. 



Two contractile vacuoles are present, 

 one near either end of the body. Each 

 communicates with a large portion of the 

 body by means of a system of radiating FlG 3S< _ 

 canals, six to ten in number. These swimming in a solution of 



1 . India ink, showing the dis- 



canals collect fluid irom the surround- c h a rge of the contractile 

 ing protoplasm and pour it into the vacuoles to the outside. 



(From Dahlgren and Kep- 



vacuole. The vacuoles contract alter- ner> a f ter Jennings.) 

 nately at intervals of about ten to 



twenty seconds. Their fluid contents are discharged to the 

 outside (Fig. 35). As in Ameba, they act as organs of excretion 

 and respiration. 



