46 THE SUB-KINGDOM PROTOZOA. 



microscopic organisms only which exhibit easily recognized animal charac- 

 teristics, some one or more tangible clues to such facile recognition have 

 to be enumerated. The primary basis for such distinction here selected is, 

 as in the case of the landmarks enumerated for the convenient subdivision 

 of the Protozoic sub-kingdom itself, associated with the phenomena of nutri- 

 tion. Excepting in a few small aberrant groups, distinguished mostly by 

 their endoparasitic mode of existence, and possessing in consequence an 

 abnormal and retrograde type of structure, all those forms referable to the 

 animal section of the series, exhibit in a conspicuous and readily verified 

 manner, their capacity to ingest solid particles of food, and their depend- 

 ence upon such solid food ingestion for the growth and display of their 

 various vital functions. Among the more highly organized Protozoa, 

 a special organ of ingestion or oral aperture forms a characteristic and 

 permanently recognizable structural feature, while in others there is no such 

 special organ, the food being incepted indifferently at any part of the 

 periphery, the actual process of its seizure and ingestion, or the recognition 

 of the presence of externally derived pabula within the substance of the 

 parenchyma or endoplasm being in these instances requisite for the satis- 

 factory determination of the question. In those low-organized unicellular 

 plants, on the other hand, which at first sight, on account of their closely 

 corresponding form and motile properties are apparently indistinguishable 

 from their animal congeners, the act of food injection is never witnessed, 

 nor is its presence to be detected within their inner substance. Nutrition 

 here, as among the higher ranks of the vegetable kingdom, is effected by 

 the absorption of the requisite pabulum in a purely liquid state. 



With relation to its chemical aspect, the composition of the nutrient 

 matter assimilated respectively by animal and vegetable organisms is found 

 again to be essentially distinct. All animal forms at present known are 

 absolutely dependent on other proteaceous, or, so to say, " ready manu- 

 factured" organic matter for their food supply. Plants, on the other 

 hand, while in a few exceptional cases, such as the so-called " insectivorous 

 species " and certain fungi, capable of sustaining life on similar formed 

 protein, manufacture this substance themselves out of the crude material 

 distributed in the liquid or gaseous condition in the fluids which they 

 imbibe. Plants thus fulfil the r61e of builders up or constructors from the 

 inorganic of organic materials, while animals are, without exception, the 

 consumers or breakers down of this same substance. 



Accessory to the very important nutritive feature of distinction above 

 submitted, there remain yet certain other characteristics that may be cited 

 as of supplemental though subordinate utility in predicating the animal 

 or vegetable nature of a given low-typed organism. Chief among these, 

 and having a psychological rather than a physiological bearing upon this 

 question, has to be mentioned the characters afforded by the respective 

 modes of locomotion exercised by the separate representatives of these two 

 groups in a like fluid medium. It is necessarily only between the fla^ellum- 



