INTRODUCTION 



B. SEPARATION OF THE GRADE PROTOZOA FROM THE 

 GRADE METAZOA. 



Formerly the name Protozoa was used for a sub-kingdom of the 

 Animal Kingdom equivalent in value to other sub-kingdoms which 

 were enumerated as the Coelentera, the Vermes, the Arthropoda, the 

 Echinoderma, the Mollusca, and the Vertebrata. In its earlier use 

 the great division " Protozoa " was made to include the SpongeS t 

 which we now assign to a divergent line of descent, the Parazoa, 

 opposed to the main line, the Enterozoa, in the higher grade of 

 animals called the Metazoa. The removal of the Sponges from 

 association with the Protozoa is chiefly due to the initiative of 

 Ernst Haeckel. By this step it became possible to give something 

 like a definite characterisation of the Protozoa and to mark them 

 off from all the higher animals. They are definitely characterised 

 by the fact expressed in the English name Cell-animals (Plasti- 

 dozoa), or less correctly unicellular animals, whilst all the higher 

 animals or Metazoa (inclusive of the Sponges) are Tissue-animals 

 (Histozoa). The fact indicated in these terms is that in 

 Protozoa a single cell or a colony of equi-pollent cells is the 

 organic "individual," whilst in the Metazoa the "individual" 

 is built up by cells which are differentiated into at least two 

 layers or tissues, the cells of each tissue being of like value 

 and origin with its fellow -cells of that tissue, but differing 

 essentially in structure, function, and origin from the cells of the 

 other tissue or tissues. These statements will be found on critical 

 examination to hold good in view of our present knowledge of both 

 Protozoa and Metazoa. Most of the Protozoa are unicellular, and 

 in those which form many-celled colonies, such as the Mycetozoa, 

 some of the Eadiolaria, Mastigophora, Ciliata, and Acinetaria, there 

 is no tendency for those cells to differentiate into groups of cells of 

 like structure and function to one another, but differing in structure 

 and function from another group or groups present in the same 

 colony. The only approach to an exception to this generalisation 

 is found in the specialisation of a cell here and there in the colony 

 as a reproductive cell; but, on the other hand, it is to be noted that any 

 cell in the colony is potentially a reproductive cell, and there is no 

 differentiation of a congeries or tissue of cells for reproductive pur- 

 poses in the general plan of the colonial structure. 1 It appears to 

 be the fact that we do not know of any forms at present existing 

 which furnish a transition from Protozoa to the Metazoa. There 



1 Though the existence of at least two "tissues" in the Metazoa suffices to dis- 

 tinguish them from all Protozoa, it may legitimately be contended that the congeries 

 of cells forming the colony of certain Protozoa (e.g. Volvox) is rather of the nature of 

 a "tissue" than of a merely loosely adherent association of cells which, as we see 

 in many Protozoan colonies, can and do separate freely and irregularly from such 

 association. 



