ZOOLOGICAL ARTICLES. 



PROTOZOA 



~T)ROTOZOA is the name applied to the lowest grade of 

 JtT the animal kingdom, and originated as a translation 

 of the German term "Urthiere." Whilst at first used 

 some forty years ago in a vague sense, without any strict 

 definition, so as to include on the one hand some simple 

 organisms which are now regarded as plants and on the 

 other some animals which are now assigned a higher place 

 in the animal series, the term has within the last twenty 

 years acquired a very clear signification. 



The Protozoa are sharply and definitely distinguished 

 from all the rest of the animal kingdom, which are known 

 by the names " Metazoa " or " Enterozoa." They are 

 those animals which are structurally single "cells" or 

 single corpuscles of protoplasm, whereas the Enterozoa 

 consist of many such units arranged definitely (in the first 

 instance) in two layers an endoderm or enteric cell-layer 

 and an ectoderm or deric cell-layer around a central 

 cavity, the enteron or common digestive cavity, which is 

 in open communication with the exterior by a mouth. 



The Protozoa are then essentially unicellular animals. 

 The individual or person in this grade of the animal king- 

 dom is a single cell ; and, although we find Protozoa which 

 consist of aggregates of such cells, and are entitled to be 

 called " multicellular," yet an examination of the details 

 of structure of these cell-aggregates and of their life- 

 history establishes the fact that the cohesion of the cells 

 in these instances is not an essential feature of the life of 

 such multicellular Protozoa but a secondary and non-essen- 

 tial arrangement. Like the budded "persons" forming, 

 when coherent to one another, undifferentiated " colonies " 

 among the Polyps and Corals, the coherent cells of a com- 

 pound Protozoon can be separated from one another and 

 live independently ; their cohesion has no economic signifi- 

 cance. Each cell is precisely the counterpart of its neigh- 

 bour ; there is no common life, no distribution of function 

 among special groups of the associated cells, and no cor- 

 responding differentiation of structure. As a contrast to 

 this we find even in the simplest Enterozoa that the cells 

 are functionally and structurally distinguishable into two 

 groups those which line the enteron or digestive cavity 

 and those which form the outer body wall The cells of 

 these two layers are not interchangeable ; they are funda- 

 mentally different in properties and structure from one 

 another. The individual Enterozoon is not a single cell ; 

 it is an aggregate of a higher order consisting essentially 

 of a digestive cavity around which two layers of cells are 



disposed. The individual Protozoon is a single cell; a 

 number of these individuals may, as the result of the pro- 

 cess of fission (cell-division), remain in contact with one 

 another, but the compound individual which they thus 

 originate has not a strong character. The constituent 

 cells are still the more important individualities; they 

 never become differentiated and grouped in distinct layers 

 differing from one another in properties and structure; 

 they never become subordinated to the individuality of 

 the aggregate produced by their cohesion ; hence we are 

 justified in calling even these exceptional aggregated 

 Protozoa unicellular. 



By far the larger number of Protozoa are absolutely 

 single isolated cells, which, whenever they duplicate them- 

 selves by that process of division common to these units 

 of structure (whether existing as isolated organisms or as 

 constituents of the tissues of plants or of animals), separ- 

 ate at once into two distinct individuals which move away 

 from one another and are thenceforward strangers. 



Whilst it is easy to draw the line between the Protozoa 

 and the Enterozoa or Metazoa which lie above them, on 

 account of the perfectly definite differentiation of the cells 

 of the latter into two primary tissues, it is more difficult to 

 separate the Protozoa from the parallel group of unicellular 

 plants. 



Theoretically there is no difficulty about this distinction. 

 There is no doubt that organisms present themselves to us 

 in two great series starting in both cases from simple 

 unicellular forms. The one series, the plants, can take up 

 the carbon, hydrogen, oxygen, and nitrogen necessary to 

 build up their growing protoplasm from mineral com- 

 pounds soluble in water, compounds which constitute the 

 resting stage of those elements in the present physical 

 conditions of our planet. Plants can take their nitrogen 

 in the form of ammonia or in the form of nitrates and 

 their carbon in the form of carbonic acid. Accordingly 

 they require no mouths, no digestive apparatus ; their 

 food being soluble in water and diffusible, they absorb at 

 all or many points of their surface. The spreading diffuse 

 form of plants is definitely related to this fact. On the 

 other hand the series of organisms which we distinguish 

 as animals cannot take the nitrogen, necessary to build up 

 their protoplasm, in a lower state of combination than it 

 presents in the class of compounds known as albumens ; 

 nor can they take carbon in a lower state of combination 

 than it presents when united with hydrogen or with 



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