THE PROTOZOA 



spiral for a considerable distance before resuming its forward progression. 

 More commonly, in response to weaker chemical stimulation or to such a 

 stimulus as contact with an obstacle, the animal exhibits a characteristic 

 avoiding reaction. This involves stopping, reversing for a short distance, 

 rolling in a cone-shaped path, and moving forward in a new direction. If 

 this new path brings the animal again into contact with the obstacle, the 

 pattern may be repeated indefinitely until the obstacle is cleared (Fig. 8.21). 

 Such trial-and-error methods constitute a large proportion of the behavior 

 of Paramecium: but by their infinite repetition, no matter how blindly, a suit- 

 able adjustment to environmental conditions can be effected. By thus demon- 

 strating its ability to react to changes in its surroundings, the paramecium 

 gives evidence of a capacity of responsiveness comparable with that found in 

 other organisms. 



In connection with the behavior of ciliates, the question arises whether 

 these animals can learn from experience. There are accounts of what is 

 claimed to be "learning," and reports of what appears to be the exercise of 

 "choice" in the acceptance or rejection of food particles, among ciliates. To 

 the extent that these reports are valid, the behavior of ciliates may be more 

 complex than its usual trial-and-error features indicate. 



Feeding and Metabolism, In feeding, the cilia of Paramecium draw a cur- 

 rent of water against the oral surface, so that particles like bacteria, smaller 

 protozoans, algae, and organic debris enter the gullet. By means of the cilia, 

 and by movements of the gullet, masses of this food included in a drop of 

 water pass into the cytoplasm and are thus ingested. The food vacuoles so 

 formed move along a definite course within the cytosome carried passively 

 by currents in a process termed cyclosis. As in Amoeba, it is assumed that 

 enzymes are secreted into the vacuoles and bring about digestion. The 

 products of digestion are evidently transferred into the surrounding endo- 

 plasm, since the vacuole finally contains only material to be egested at the 

 anal spot. The observations which can be made on Paramecium are similar 

 to those described for Amoeba, and we reason similarly from them with the 

 aid of knowledge concerning other animals. The products of digestion, 

 passing out of the food vacuoles, are utilized during metabolism. Cellular 

 respiration corresponds to the process in vertebrates; oxygen enters the cell 

 directly from the surrounding fluid, and final breakdown of the cellular 

 constituents occurs, with transformation of energy and formation of waste 

 products. Excretion of the wastes of metabolism occurs chiefly by diffusion 

 over the entire surface of the cell and to some extent by means of the con- 

 tractile vacuoles. Under suitable conditions the storage of reserves such as 

 glycogen and fat occurs in the cytoplasm. The nutrition of Paramecium is, 

 therefore, holozoic; and its metabolism is fundamentally like that of higher 

 animals. 



Life Cycle and Reproduction. The life cycle of Paramecium consists of an 

 active phase, which may continue indefinitely in a suitable medium. There is 

 no encysted pha.se that may be commonly observed in the laboratory, al- 



253 



