280 PROTOZOA 



Among the most interesting problems suggested by the Protozoa 

 are those relating to their apparently conscious activities. Conscious- 

 ness has often been ascribed to the Protozoa in order to account for 

 certain actions which appear to be voluntary. Thus they are often de- 

 scribed as " selecting " their food, of " choosing " building material 

 for their shells and tests, or of "voluntarily" moving around an 

 object, etc. Dujardin was one of the first to discredit con- 

 sciousness in the Protozoa, and, with a truly modern point of view, 

 he wrote as follows : "If one invokes the faculty which the 

 Protozoa have of directing themselves in the liquid, and of wilfully 

 pursuing their prey, at least will it be necessary first to verify the 

 reality of this faculty which I believe as fabulous as everything else 

 reported as instinct on the part of these animalcula." x Neverthe- 

 less, it is generally recognized that consciousness, like life itself, 

 could not have arisen at once in the higher animals, but must 

 have developed by a slow process of evolution from some property of 

 protoplasm, which, if the principle of genetic continuity involved in 

 the doctrine of evolution holds good for the lowest forms, must be 

 present in some form in all Protozoa. Of the many functions which 

 make up the vital activities of Protozoa, that of irritability or response 

 to external stimuli undoubtedly stands nearest to the basis of con- 

 sciousness of the higher animals, and it may be expressed either by 

 the general protoplasm or by specialized " sensory " ectoplasmic 

 modifications. 



The other functions of Protozoa, notably those of nutrition and 

 excretion, can be treated with greater assurance, and can be more 

 readily compared with similar functions in higher animals. In the 

 present chapter we may first inquire how closely these more ele- 

 mentary functions agree with those of the higher forms, and then 

 consider some of the evidence upon which consciousness has been 

 attributed to these primitive forms. 



A. Intra-cellular Digestion in Protozoa 



A distinction must be made at the outset between the digestive 

 processes of most Metazoa and of Protozoa. In the former, with 

 some exceptions, the digestive fluids are poured out from the 

 epithelial cells which line the digestive tract, into the lumen of that 

 tract, and the food is digested in the stomach or intestine. In the 

 Protozoa and in some Metazoa {e.g. the Ccelenterata), on the other 

 hand, the food is taken directly into the cells and there digested. 

 The former method of digestion is said to be inter-cellular, the latter 

 intra-cellular. 



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