PKOTOZOA 21 



of food in saprophytic and parasitic conditions, and of freeing 

 energy by enzymotic action, even in the absence of oxygen. 1 



Psychology. The Protozoa are nervous. Vorticella in 

 the fixed state is irritable and contractile. In response to 

 waves communicated by the medium in which it lives a con- 

 traction at once takes place, involving the bell and the stalk. 

 Some part therefore of the cortex of the bell, or all of it, 

 is capable of receiving the waves, but evidently the peristome 

 is particularly sensitive, and the effect is transmitted to the 

 myonemes and the contractile filament of the peduncle. There 

 in are, other words, a sensory receptor surface and a primi- 

 tive neuromuscular system intimately associated. With respect 

 to Ciliates and Flagellates in general, we can point to modi- 

 fications of the ectoplasm concerned in the process. The 

 reflex in the case of Vorticella is protective and is automatic, 

 and may be said therefore to be instinctive. It results with 

 unfailing regularity when disturbance takes place, but it 

 varies in quantity. The contraction may not be quite com- 

 plete, and the length of the contraction varies. This means 

 that the cell is able to inhibit both the contraction and the 

 expansion. 



But this is not all. Vorticella is able to develop a special 

 band of cilia and to snap itself free from the peduncle when 

 conditions become difficult or impossible. This is a post- 

 poned reflex associated with the presence of, or the ability to 

 form, a breaking joint. It may be said that this postponed 

 reflex, like the other, is provoked in response to external stimu- 

 lus ; but it points also to volition, and its history to a gradual 

 modification of the cytoplasm in association with protection. 



It is difficult for us to appreciate the meaning of all this. 

 Experiment has shown that such free cells are attracted or 

 repelled by various stimuli. From the results of experiments 

 they are described as being positively or negatively photo- 

 tactic, chemiotactic that is, attracted or repelled by light or 

 chemical stimuli and so on. The impression conveyed is that 

 the cell is like a boat without men, directed and controlled 

 by wireless waves. Many Protozoa alternate between periods 

 of desiccation and a free and usually an aquatic life. In the 



1 1915. Bayliss, Principles of General Physiology. 



